NMI TR 1
Basis of NML2003 Scale of Relative
Spectral Irradiance
Frank Wilkinson
First edition — December 2004
Bradfield Road, Lindfield, NSW 2070
PO Box 264, Lindfield, NSW 2070
Telephone: (61 2) 8467 3600
Facsimile: (61 2) 8467 3610
Web page: http://www.measurement.gov.au
© Commonwealth of Australia 2004
Contents
1
Outline .................................................................................................................... 1
2
Theory ..................................................................................................................... 1
3
The Filter Radiometry ............................................................................................. 2
3.1 The Filter Radiometers .................................................................................. 3
3.2 Spectral Responsivities of the Radiometers ................................................... 5
3.3 Sphere Configurations ................................................................................... 7
3.4 Effects of Varying Sphere-wall Target Areas ................................................ 8
3.5 Filter Radiometry with the Spectral Irradiance Lamps .................................. 8
4
The Blackbody ........................................................................................................ 9
4.1 Quality and Temperature Profiles .................................................................. 9
4.2 Power Circuit and Temperature Control ...................................................... 12
4.3 Window Transmittances .............................................................................. 13
5
Comparisons of Lamps with the Blackbody ......................................................... 14
5.1 Optical Systems ........................................................................................... 14
5.1.1 Gathering the Flux ........................................................................... 14
5.1.2 Comparing the Flux ......................................................................... 16
5.1.3 Beam Uniformity ............................................................................. 18
5.1.4 Target Stray-light Control and Monitors ......................................... 18
5.1.5 Corrections using the Monitors ....................................................... 19
5.2 Pyrometer Temperature Measurements ....................................................... 19
5.3 The Blackbody–Lamp Comparisons............................................................ 20
5.3.1 Spectral Ranges ............................................................................... 20
5.3.2 Measured Ratios – Raw Data .......................................................... 22
5.4 Blackbody Temperatures ............................................................................. 22
5.4.1 From the Filter Radiometers ............................................................ 23
5.4.2 Temperatures from the Optical Pyrometer ...................................... 26
6
Measured Spectral Power Distributions ............................................................... 27
6.1 Comparisons with Planckian Radiators ....................................................... 27
6.2 Mismatch of Lamp Spectra in Measurement Overlap Range ...................... 27
6.3 Differences in Calibrations from Different Reference Lamps ..................... 31
6.4 Differences between the NML2003 and NML1990 Scale Values .............. 34
6.4.1 UV-visible Spectral Range .............................................................. 34
6.4.2 IR Spectral Range ............................................................................ 35
6.5 Change in the NML Scale of Relative Spectral Irradiance .......................... 35
7
Uncertainties ......................................................................................................... 37
7.1 Uncertainties in Filter Radiometer Spectral Responsivities Ri(λ) ............... 39
7.1.1 Contribution from Calibration of the Reference
Detector Standards ........................................................................... 40
7.1.2 Uncertainties in Transfers from the Reference Detectors ............... 40
7.1.3 Radiometer Detector Non-linearity ................................................. 45
7.1.4 Amplifier Gain Ratios ..................................................................... 45
7.2 Uncertainties in the Blackbody–Lamp Comparisons .................................. 46
7.2.1 Random Transfer Uncertainties ....................................................... 46
7.2.2 Systematic Transfer Uncertainties ................................................... 47
7.2.3 Quality of the Blackbody Cavity ..................................................... 49
7.2.4 Blackbody Window Transmittances ................................................ 50
7.2.5 Optical Pyrometer Temperature Measurement................................ 55
7.2.6 Lamp Operating Current .................................................................. 55
7.2.7 Total Uncertainties in Calibrations of the Filter Radiometers
and Corresponding Temperature Uncertainties from Pairs of
Radiometers ..................................................................................... 56
7.3 Weighted Mean Blackbody Temperatures and Uncertainties ..................... 56
7.3.1 Temperature Uncertainties are Uncorrelated for Different
Lamp Types ..................................................................................... 56
7.4 Uncertainties in Lamp Relative Spectral Irradiances................................... 58
7.4.1 Uncertainties due to Blackbody Temperature Uncertainty ............. 58
7.4.2 Random Transfer Uncertainties ....................................................... 58
7.4.3 Uncertainties due to Blackbody Window Transmittance
Uncertainties .................................................................................... 58
7.4.4 Uncertainties due to Lamp Current Uncertainty.............................. 58
7.4.5 Wavelength Uncertainties................................................................ 59
7.4.6 Spectroradiometer detector non-linearity ........................................ 59
7.4.7 Summation of uncertainties in relative spectral irradiances ............ 59
8
Comparison of Temperature Uncertainties with Temperature Discrepancies ...... 62
8.1 Temperature Discrepancies .......................................................................... 62
8.1.1 Comparison with Discrepancies from Different Filter
Radiometers ..................................................................................... 62
8.1.2 Comparison with Spectral Overlap Temperature Discrepancies ..... 63
8.1.3 Comparison with Apparent Temperature Discrepancies in
Different Reference Lamp Calibrations .......................................... 63
8.1.4 Comparison with the Pyrometer – Filter-Radiometer
Temperature Differences ................................................................. 63
8.1.5 Discussion of Discrepancies ............................................................ 64
9
The NML2003 Scale of Spectral Irradiances ....................................................... 64
9.1 Calibration of Working Standards and Key Comparison Transfer
Standards ..................................................................................................... 64
10 Conclusions and Recommendations ..................................................................... 66
11 References ............................................................................................................. 66
1
OUTLINE
The NML2003 scale of relative spectral irradiance for the spectral range 240 to
2500 nm has been established by comparing a number of tungsten–halogen lamps
spectrally with a high-temperature blackbody. The spectral power distribution of the
blackbody has been obtained from the Planck spectral radiance equation using
temperatures obtained from filter radiometry. A cross-check of the temperatures was
done with temperatures measured with an optical pyrometer. The SPD of the
blackbody, combined with the ratios of the lamp and blackbody SPDs, provides the
relative spectral irradiances of the lamps. These are then normalised to absolute
spectral irradiances at particular distances by additional measurements of the lamp
illuminances at the required position.
The filter radiometry has been done using seven radiometers to measure the spectral
balance of incident radiation. These had spectral bandwidths of about 40 nm and peak
responses at wavelengths from 340 to 1540 nm. A small integrating sphere was made
to contain four of the radiometers at any one time. Using the current NML scale of
spectral response, the spectral responses of the radiometers including the sphere were
measured, for flux incident on the same area of the sphere wall as that to be irradiated
by the blackbody or lamps.
If the flux comes from the blackbody radiator, ratios of the radiometer signals can be
used to calculate the blackbody temperature. Another method is to measure the
radiometers signal ratios for flux from the spectral irradiance lamps themselves, then
to compare these lamps spectrally with the blackbody and then to calculate from the
two sets of measurements the blackbody temperature, thus leading to the detailed
lamp spectrum. As repeated calibrations and use of the radiometers were shown to be
necessary and the life of the blackbody is very short, this second approach was
adopted.
2
THEORY
The spectral radiances L(T) at wavelength  of a Planckian blackbody radiator at
thermodynamic temperature T are given by:
L   ( )c1n 5 exp c2 / n T   1
2
1
(1)
where () is the blackbody window transmittance, nλ is the refractive index of the
medium and c1 and c2 are constants with current values of 3.74183  10-16 W.m2 and
1.4388  10-2 m.K, respectively.
Two filter radiometers will respond to flux from a lamp with ratios of signals S1/ S2
given by:
S1 / S 2   E R1, d /  E R2, d
(2)
where E are the lamp spectral irradiances and R1, and R2, are the spectral responses
of the two radiometers.
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For such ratio measurements, the signal ratio is a function of the relative spectral
irradiances E():
S1 / S2   E()R1, d /  E()R2, d
(3)
Lamp relative spectral irradiances E() may be compared with the blackbody relative
spectral radiances ()ε(λ)L(,T) to obtain a spectral distribution of quotients ,
where:
    E   /    ( ) L , T 
(4)
() are the blackbody window transmittances and  are the cavity spectral
emissivities. The lamp relative spectral irradiances E() are then given by:
E( )   ( ) ( ) ( ) L(, T )
(5)
Substituting values for E() from eq. (5) into eq. (3):
S1 / S 2    ( ) ( ) ( ) L( , T ) R1 ( )d /   ( ) ( ) ( ) L( , T ) R2 ( )d
(6)
A value of temperature T is found that, when used with eq. (6), provides the measured
ratio S1/S2. Temperatures are found using different pairs of radiometers. Temperatures
from pairs of radiometers are selected that result in the lowest uncertainties. A
weighted-mean temperature is calculated from the selected results. Finally, using the
mean value of T, the lamp relative spectral irradiances are obtained using eq. 5.
3
THE FILTER RADIOMETRY
3.1
The Filter Radiometers
Some details of the sphere and filter radiometers are shown in Figs 1 to 3. The interior
of the Halon-coated sphere has a diameter of approx 50 mm. The input aperture has a
diameter of 12 mm. The four viewing apertures for the radiometers have diameters of
about 5 mm. The radiometers mount into tubes attached to the sphere and have outer
apertures of diameter 3.5 mm and inner apertures with diameters of from 3 to 5 mm,
depending on the detector. These apertures are built into the radiometer housings and
are placed on either side of the band-pass interference filter. The field of view of each
radiometer has a full angle of about 50º, being of the sphere side-wall, and excludes
the area irradiated by the incoming beams.
The areas of sphere wall irradiated by the monochromator for spectral response
calibrations (Fig 3(a)) and by the spectral irradiance lamps for signal ratio
measurements (Fig 3(b)) are closely matched. Sensitivity of the spectral responses and
the signal ratios to different areas has been studied and uncertainties have been
estimated for area mismatches.
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Figure 1 (left) and Figure 2 (right). Integrating sphere showing four filter radiometer
positions, section through radiometers and disassembled radiometers
Figure 3. Sections through sphere showing input beams
from monochromator (a), and from spectral irradiance lamp (b)
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There are seven radiometers, four of which can be mounted on the sphere at any one
time. Reproducibility of position is high, with the field-of-view of the radiometers
determined by their inbuilt apertures, and as they are viewing quite uniform spherewall radiances, removing and replacing them generally does not affect their response
within the uncertainties to be discussed further on. The interference filters have
spectral band-passes (FWHM) of 20 to 40 nm providing peak responses when
combined with the detectors at approx 340, 450, 550, 700, 940, 1300 and 1540 nm
(Fig 4). Si photodiodes are used in the radiometers with the five shorter peakwavelengths, and InGaAs detectors for the 1300 and 1540 nm peak responses. An
eighth radiometer with peak response near 304 nm has been made up but not been
included in the calibrations that have been described here. Some details of the
detectors and filters currently in use are given in Table 1. All measurement data and
analysis are recorded in the laboratory book1 and spreadsheet
SPHERERADMay04.xls.2
Calibrations and use with two configurations have been adopted: ‘config 6’ with peak
responses 340, 450, 550 and 700 nm, and ‘config 3’ with peak responses at 700, 940,
1300 and 1540 nm. Ratios of responses of each radiometer to that of the radiometer
#6N with a peak response near 700 nm are calculated and measured, thereby relating
all of the radiometers in the two configurations.
0.0012
Rad #4
Rad #3
Rad #11
400
600
800
Rad #6N
Rad #8
Rad #1
Rad #9
Response (A/W into sphere)
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
200
1000
1200
1400
1600
1800
Wavelength (nm)
Figure 4. Spectral responsivities of the radiometers in the sphere
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Table 1. Some specifications of filter radiometers used with integrating sphere
Peak
Radiometer
wavelength
#
(nm)
4
3.2
340
3
450
11
550
6N
700
8
940
1
1300
9
1540
Detector
type
Filter
type
Hamamatsu
Si S1227 –
66BQ
Andover
350FS40
Melles
as above
Griot
03FIV328
Si S1226 –
Andover
44BK
550FS40
Si S1227 –
Andover
66BQ
700FS40
Melles
Si S1337 –
Griot
66BQ
03FII121
Epitaxx
Melles
InGaAs
Griot
ETX3000T5 03FIL135
Telcom
Melles
Devices
Griot
InGaAs
03FIL139
35PD3MA
Spectral
Amplifier
Sphere
bandwidth
gain used
position
(nm)
(V/A)
32
4
109
36
3
108
41
2
3  107
39
1
107
41
4
107
20
2
107
21
3
3  107
Spectral Responsivities of the Radiometers
The most recently measured spectral responsivities of the radiometers in terms of
amperes of photocurrent per watt of monochromatic radiation incident on a defined
area of the sphere wall opposite the input aperture are shown with logarithmic scales
in Figs 5 to 7. These were measured over their main response bands multiple times
using a 2 nm wavelength interval and 2 nm spectral bandwidth. The main response
bands were typically 120 nm wide and chosen to include wavelengths responsible for
95 to 99% of the responses to the standard lamps.
Measurements were done of the broad out-of-band ‘wing’ responses using a 10 nm
wavelength interval and 2 nm SBW. The wing responses were measured on the short
wavelength side down to wavelengths were the product of the low radiometer response
and the low lamp emission added negligible contribution to the signal, and on the long
wavelength side up to the upper limit of the detector response. In Figs 5 to 7, some of
the responses are plotted at a constant lower level (typically 1E-09 A/W); very low
negative values were obtained in these spectral ranges. The level is indicative of the
uncertainty. In the case of the UV radiometer #4, which is not blocked in the spectral
region near 700 nm, separate measurements were made to determine this level, and the
difference in the integrated response between using this upper estimate and using zero
responses in the two spectral ranges involved was calculated and used in the assessment
of the uncertainty. For the other radiometers the contributions from the spectral ranges
where these negative values have been obtained are negligible (<0.01%). Uncertainties
in these measurements will be discussed in detail further on.
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1.0E-03
Rad #4
Rad #3
1.0E-04
Response (A/W)
1.0E-05
1.0E-06
1.0E-07
1.0E-08
1.0E-09
1.0E-10
200
300
400
500
600
700
800
900
1000
1100
1200
1300
Wavelength (nm)
Figure 5. Spectral responses of UV and blue sphere radiometers July 2003
1.0E-02
Rad #11
Rad #6N
1.0E-03
Response (A/W)
1.0E-04
1.0E-05
1.0E-06
1.0E-07
1.0E-08
1.0E-09
1.0E-10
1.0E-11
300
400
500
600
700
800
900
1000
1100
1200
1300
Wavelength (nm)
Figure 6. Spectral responses of green and red radiometers July 2003
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1.0E-03
Response (A/W)
1.0E-04
1.0E-05
1.0E-06
1.0E-07
Rad #8
Rad #1
Rad #9
1.0E-08
1.0E-09
400
600
800
1000
1200
1400
1600
1800
Wavelength (nm)
Figure 7. Spectral responses of the infra-red sphere radiometers in July 2003
Unfortunately, most of the responses are drifting at an excessive rate, up to 1% /
month when integrated over the main response range. This is attributed mainly to
degradation of the interference filters. Interference filters from a range of
manufacturers were sourced and enquiries were made about the supply of more stable
units. None were offered. The compact design of the sphere and radiometers has
required filters with diameters of 12.5 mm and it has become obvious that this is too
small to avoid the effect of diffusion of water and possibly other atmospheric
components through the paint sealing the edges of the filters into and degrading the
interference layers. The UV filters use more stable materials in the layer stacks and
the responses of those radiometers have proven much more stable.
For the final calibrations and uses of these radiometers the spectral responsivities of
each and the measurement of the signal ratios for each standard lamp have been done
for each of the two configurations within a period of 10 days.
3.3
Sphere Configurations
Configurations of the sphere with two sets of radiometers have been defined with one
common radiometer, #6N (700 nm peak), for measurements essentially of UV-visible
wavelengths or upper visible-NIR wavelengths. The two configurations are:
Config 6: #4 (340 nm), #3 (450 nm), #11 (550 nm), #6N (700 nm);
Config 3: #6N (700 nm), #8 (940 nm), #1 (1300 nm), #9 (1540 nm).
Ratios of signals are taken mostly between the other radiometers and #6N.
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3.4
Effects of Varying Sphere-wall Target Areas
In the first use of this sphere in 2000–013 (earlier and different geometries were tried
prior to 2000) the lamps irradiated the sphere aperture at a distance of about 780 mm.
The different lamp types: Ushio Electric 500 W with horizontal filaments, Sylvania
FEL 1000 W with vertical filaments, GEC 750W with vertical multi-stranded
filaments, and other types, irradiate different sized areas at the rear of the sphere. The
spectral response calibrations of the radiometers had been done using monochromatic
beams irradiating a particular area of the sphere rear wall, approx 12 mm high 
13 mm wide. Some of the lamp-irradiated areas were different from this: by up to
1 to 2 mm in each dimension. An improved match of this area with the lampirradiated areas was found by using slightly smaller apertures just in front of the
12 mm sphere aperture for the lamp measurements.
A new round of tests was started in early 2003. By this time it was decided to increase
the lamp–aperture distance to 2 m, to use only the 12 mm sphere aperture and to
measure the signal ratios for each lamp with the sphere in two orientations, rotating it
through 90º about the input axis. Beam profiles from the lamps would still be
somewhat elliptical due to elongated filament shape, so measuring with two
orientations would reveal some of the sensitivity to this. The dimensions of the fullyincluded areas of the sphere wall irradiated by each filament design are given in
Table 2.
The effects of using these different sphere wall areas on the ratios of the responses of
the radiometers are discussed in part 7 – Uncertainties.
Table 2. Beam dimensions at rear wall of integrating sphere for 12 mm sphere aperture
59 mm in front of wall, and lamp with given source dimension 2 m from sphere aperture
Lamp type
Filament or diffuse
envelope dimensions
(h  w, mm)
Beam dimension on sphere
wall 59 mm behind 12 mm
aperture (h  w, mm)
Ushio Electric 500 W
5  21
clear envelope
Ushio Electric 500 W,
15  40
diffused envelope
Sylvania FEL 1000 W
21  6
GEC 750 W
25  18
Measurement of radiometer spectral responses
3.5
12.5  13.0
12.8  13.5
13.0  12.5
13.1  12.9
12.0  13.0
Filter Radiometry with the Spectral Irradiance Lamps
Over the period 2 – 14 July 2003 a range of spectral irradiance lamps were measured
using the filter radiometers. The ratios of signals from the different radiometers to that
from radiometer #6N were recorded for two orientations of the sphere, rotated through
90 about its input axis in order to test the sensitivity of the ratios to the slightly
asymmetric beam orientation for each type of lamp. The differences are discussed in
§7.1.2.3 Table 15.
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Each lamp was operated at its calibration current and mounted in the 300 mmdiameter air-cooled cylindrical housings. Apertures or knife-edge screens were used
close to the lamps to define the source area. Details are given in the laboratory
workbook4. In the end, a 2.0 m distance was chosen between the sphere aperture and
the lamps, and the built-in aperture alone was used as the limiting aperture. This
dictated the areas of the sphere wall irradiated by the different lamps for the two
sphere orientations, and thus the mismatch between the area used for the spectral
response calibrations and that used for the ratio measurements.
The lamps that were tested (see Table 2) were:

five FEL types, nos. FEL1-5;

three FEL types, CCPR transfer standards, nos. BN-9101-196, -206, -247;

one FEL type, 1990 CCPR transfer standard, no. H148;

two GEC types, 1990 CCPR transfer standards, nos. E14, E18; and

three Ushio Electric types, nos. SI25, SI27 and U121.
Details of the measured radiometer signal ratios are recorded in spreadsheet
SPHERERADMay04.xls.
4
THE BLACKBODY
4.1
Quality and Temperature Profiles
The blackbody radiator5 6 is shown in Figs 8 and 9. A stainless steel housing with
fused silica windows contains a graphite cavity held between water-cooled copper
conductors maintained in contact with it by a spring under relatively high
compression. The graphite cavity is resistively heated by an ac current of up to 400A,
using voltages up to 15V. At temperatures less than about 2400K heating may be done
under a moderate vacuum (about 10-3 torr), but for higher temperatures the housing is
firstly evacuated and then filled with .99999 Ar at about 1.2 atmospheres.
The graphite cavities are approx 130 mm deep and 15 mm in diameter, with profiling
of the cross-section along the length to increase the temperature uniformity. The
temperature is maintained substantially uniform from the base of the cavity and along
the side wall about 70 mm towards the mouth. Temperature profiles measured by the
manufacturer for the two cavities used to set up this scale are shown in Fig 10.
Calculations by Mark Ballico of spectral emissivities for three cavities are shown in
Fig 11.
An area of the base of the cavity with a diameter of less than 2 mm was selected for
viewing by limiting apertures. The temperature profiles reported (Fig 10, along V
section of  axis) suggest temperature ranges of about 1K for the two cavities within
this diameter. The temperature that is determined at a particular time by the methods
described is the average for the area used.
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Figure 8. IKE graphite blackbody, power supply and control equipment,
showing one of the cavities (lower front)
Figure 9. Cross-section of IKE blackbody showing graphite cavity, graphite foam
insulation, fused silica windows and water-cooling circuits
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Figure 10. Manufacturer's temperature profiles for two graphite cavities
Figure 11. Spectral emissivity calculations for three cavities
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The analysis7 done by Mark Ballico shows effective emissivities of from 99.85% to
99.93% for cavity 16–44 from 250 to 1600 nm when a surface emissivity for the
graphite of 80% is assumed. This variation in emissivity is less than 0.1%. It is only
one-half of this variation if the surface emissivity is 85%.
For the second cavity that was used, no.16–45, the variation in effective emissivity
between 250 nm and 1600 nm is about 0.25% for a surface emissivity of 80%, and
about 0.21% if the surface emissivity is 85%. It has not been possible to measure the
surface emissivities, particularly their full directional characteristics and at the high
operating temperatures near 2900K.
4.2
Power Circuit and Temperature Control
At a temperature of about 2900K the cavity voltage and current are about 12.5V and
340A. The circuit used to run and control the blackbody is shown in Fig 12.
Figure 12. Blackbody power supply and temperature control feedback circuit
Two large Variac transformers supply a step-down 12V 600A transformer that is
connected directly to the ends of the graphite cavity. The output instability of this
circuit for new cavities and stable mains voltage is about 2 to 3%. The rear of the
cavity is viewed by a tele-radiometer with a narrow wavelength response near
450 nm. Amplified signals from this detector are compared with a selectable reference
voltage and the difference is used to generate a phase-sensitive ac signal that is
amplified with high gain and fed into a 500W power amplifier. 50Hz power from this
amplifier is fed into a toroidal transformer wrapped around the cavity feed line so that
up to 500 W of power in or out of phase with the main transformer power is used to
regulate the cavity output – at least to the monitor at the rear of the cavity.
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With this circuit operating the output instability from the front of the cavity at about
400 nm is generally now less than 0.5%. This improvement is good, but still not good
enough to provide comparisons over a reasonable spectral range with the blackbody
held at one temperature. The residual instability is handled by monitoring the cavity
output at different wavelengths during the cavity–lamp comparisons and correcting
for drift, as will be discussed later.
4.3
Window Transmittances
This blackbody is viewed through a Suprasil fused silica window. In this use of the
blackbody, a transfer of relative spectral irradiances is obtained, so only the relative
spectral transmittances of the need to be known.
94
Transmittance (%)
93
92
91
90
21-Aug-03
17-Oct-03
6-Nov-03
17-Nov-03
89
0
500
1000
1500
2000
2500
3000
Wavelength (nm)
Figure 13. Spectral transmittances of blackbody window BBW1 as measured by Cary
5 spectrophotometer on given date before and during the blackbody operation on 6
days between 10 Oct and 13 Nov 2003
The spectral transmittances of the window after cleaning with isopropyl alcohol were
first measured using the McPherson monochromator in January 2000. They were then
measured using the Cary 5 spectrophotometer, where some differences from the
McPherson measured values were noted8. Transmittances were remeasured using the
Cary spectrophotometer during the use of the blackbody9. These are shown in Fig 13.
The transmittance values that are shown, measured closest to the time the blackbody
was compared with lamps, were used in the calculations of the blackbody temperature
and the lamp relative spectral irradiances.
It should be pointed out that using the product ()L(,T) in eq. 6, page 2, small
errors in the slope of the transmittance profile () simply result in compensating
changes in the relative spectral radiance function L(,T) and a slightly different
temperature, T, the products of which have negligible errors.
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Following the comparisons of the blackbody with the lamps, uneven deposits,
probably carbon, were noticed on the inside of the window. The area of the window
used for the blackbody-lamp comparisons was slightly larger than that used for the
transmittance measurements, but both were a lot larger than that used by the optical
pyrometer. The possibility that the window transmittances also changed between
high-temperature operation under an argon atmosphere and subsequent exposure to air
has also been considered. These matters result in the uncertainties in the window
spectral transmittances being highly significant, which will be discussed in detail in
§7.2.4.
5
COMPARISONS OF LAMPS WITH THE BLACKBODY
5.1
Optical Systems
5.1.1 Gathering the Flux
A mirror optical system was constructed to allow comparison of spectral irradiances
of lamps with those from a part of the bottom of the cavity wall with a diameter of
about 1.5 mm. This would be done in such a way that the spectral reflection function
of the system was common to both sources and therefore did not have to be measured.
The mirror system is shown in Fig 14. An overhead view of the system is shown in
Fig 15. The two mirrors M1, M2 mounted on the rotary table RT have concentric
surfaces, forming a Schwarzschild imaging system10. The two sources BB or L
mounted in conjugate positions of the system may be imaged in the plane of aperture
A, with respectively 8  or 1/8  linear magnification free from chromatic or spherical
aberrations and coma.
Figure 14. Mirror system for comparing blackbody with spectral irradiance lamps
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Figure 15. Overhead view of the optical system for comparing the spectral
irradiance lamp (lower left, housing removed) with the blackbody (lower right, not
running) using the rotary table mounted mirrors and the McPherson double
grating-monochromator (background)
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5.1.2 Comparing the Flux
The beams from the two sources have quite different geometries (see Fig 14). Flux
from the blackbody passing through aperture A has reasonable spatial uniformity in
its spectral power distribution (depending on the temperature uniformity of the base of
the cavity), but the lamp image is very non-uniform. This consists of the image of a
filament with its range of temperatures, and in the infra-red, significant contributions
from a much larger envelope. Envelopes may reach 600 to 700C. Some
calculations11 showed that at 1500 nm more than 3% of irradiance may come from
envelopes of FEL-type lamps.
The initial thought was to direct the flux from both sources into a small integrating
sphere and view the side wall or a baffle with the monochromator. However, the
spectral radiances of the sphere side wall or baffle are not high enough, especially
with the blackbody at about 2900K, to allow comparisons down to 250 nm with
adequate signal/ noise levels.
Another idea was to use an integrating sphere and directly view the area of the sphere
wall on which both beams were incident. Of course, the spectral irradiances of the
area viewed would have to be adequately uniform and representative of the whole
lamp irradiance. Filter radiometers mounted in the sphere would simultaneously
measure the blackbody temperature.
The problem with this arrangement is that the sphere provides some amplification of
the target area radiances through inter-reflections, but this is not constant across the
spectrum due to varying reflectance efficiencies. For a given flux input at a particular
wavelength, the sphere adds a certain level of extra radiance to the target area. If the
beam is spread over say, twice the area, the same additional radiance is provided but
the radiance due to the incoming incident flux is now only one-half of its previous
value. Therefore, the fractional increase due to sphere amplification will be doubled.
But, the fractional increases vary across the spectrum. It would only be possible to
spectrally compare two beams in this way if they were perfectly uniform and targeted
the same area of the sphere wall. Perfect matching of the areas at the back of the
sphere irradiated by the lamps and the blackbody was found to be not possible.
The idea of using a sphere was abandoned in favour of using a plane diffuser, with the
monochromator viewing a small area over which the spectral irradiances from both
sources are uniform and, in the case of the lamps, representative of the emission from
the whole lamp. This optical system is shown in Figs 16 and 17. Flux passing through
aperture A from each source falls on a similar area of a Halon diffusing plate H that is
imaged by mirror M3 onto the monochromator slit S. As the targeted area of the
blackbody cavity has a diameter of about 1.5 mm and the chosen diameter of the
receiving aperture A is 12 mm, this allows lamp structures with diameters up to
96 mm to be accommodated.
NMI TR 1
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Figure 16. Plan of blackbody and lamp irradiance of Halon plate H. Lamp flux
(limit rays L) and blackbody flux (rays BB) both irradiate an area of diameter 12 mm
centred on Halon plate behind a 10  10 mm aperture. The monochromator slit S
views a 5  4 mm area of the plate. One of four filter radiometer monitors M that
surround the target is shown.
Figure 17. View of the aperture A1 (Fig 14) and the housing containing the Halon
plate showing the four filter radiometers that monitor the incident blackbody radiation
NMI TR 1
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5.1.3 Beam Uniformity
Having settled on the size of the aperture in the image plane, this determines the size
of the beam from the blackbody incident on the Halon plate. In order to match this
beam diameter (12 mm) at the plate with flux from the lamps, the plate needed to be
about 79 mm behind the aperture. A study was then necessary to determine the sizes
of the areas of uniformity of the beams from each type of lamp at this distance behind
the image plane.
The lamps are all tungsten halogen types with quite small filaments and envelopes.
The part of each lamp to be used as the source is isolated from other structures such as
the envelope supports and base by using an aperture or lower shield mounted close to
the lamp. The largest dimension used for the lamps is 80 mm, being for the GEC lamp
envelopes. For the FEL and Ushio Electric type lamps the largest dimension is
40 mm. It was decided not to use most of the results of comparisons involving the
GEC lamps.
The study12 showed that areas of uniformity varied from diameters of 5.7 mm for a
source dimension of 40 mm, up to 10 mm for a source dimension of 8 mm, at the
plane of the Halon target. By back-lighting of the target from the monochromator with
monochromator slits of 5  2 mm it was possible to limit its viewing area to 5  4 mm
in the centre of the uniform area.
5.1.4 Target Stray-light Control and Monitors
The Halon target needed to be shielded from stray room light including light escaping
from the lamp housing or indirect light from the blackbody. Another problem was to
try to account for instability of the blackbody by monitoring flux falling on the target.
An aluminium housing was made up to substantially enclose the Halon plate and
surround it by monitors comprising four of the filter radiometers described above.
This arrangement is shown in Figs 16 and 17. The performance of the monitors will
be discussed further on.
The housing interior was painted with Pascol EasySpray Flat Black pressure pack.
The aperture surrounding the exposed Halon area was painted with 3M matt black
paint (radiometry lab). The beams from both the blackbody and the lamps cover a
closely matched circular area of diameter 12 mm that slightly exceeds the area of the
aperture covering the Halon. There will be some flux inter-reflected between the
Halon and the surrounding black paint. The paints are spectrally non-selective well
into the infra-red region. The reflections from the target areas of these closely
matched beams are assessed to be very similar. Therefore, errors due to reflections
back onto the Halon plate have been assessed as negligible.
NMI TR 1
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5.1.5 Corrections using the Monitors
It was stated earlier that even with the feed-back circuit regulating the output of the
blackbody, the temperature and radiances were still not stable enough. It was found
necessary to monitor the output and correct for measured instability.
Four filter radiometers were chosen from the set used to measure the lamp spectra
with the integrating sphere, and mounted around the Halon target shown in Figs 16
and 17. Their peak wavelengths were 340, 550, 940 and 1530 nm. Calculations based
on Planck’s equation show that for small temperature changes in the blackbody, the
radiance varies inversely proportionally to the wavelength. In the running of the
measurement program13, signals from the four monitors are integrated simultaneously
with the readings taken by the spectroradiometer detector. It is assumed that variations
in the Halon plate spectral radiances from the blackbody radiation will be mainly due
to fluctuations in the blackbody temperature.
At the start of a comparison between the blackbody and a lamp, initial readings RN are
taken for the blackbody from each radiometer 1 to N which has an effective
wavelength λE,N At all subsequent spectroradiometer measurements of the Halon plate
radiance for the blackbody, if the monochromator measurement wavelength is λm and
the monitor signal has drifted by ΔR, the correction multiplying factor FBB,N to be
applied to the spectroradiometer measurement due to blackbody temperature drift, that
has been obtained from that monitor is given by:
FBB, N  (1  R / RN )E , N / m
(7)
The correction factors provided by the four monitors never differed from one another
by more than 0.1%, and generally agreed to within about 0.01%, even though the
effective wavelengths varied by a factor of 4.5. This gave significant confidence in the
assumptions that the variations in output were primarily temperature-related and that
corrections for fluctuations at the several-percent level could be made in this way.
5.2
Pyrometer Temperature Measurements
An optical pyrometer14 was used for independent measurements of the blackbody
temperature. It viewed an area at the base of the cavity with a diameter of approx
3 mm. A view of the arrangement used is shown in Fig 18. The outer lens of the
pyrometer is about 1.3 m from the base of the cavity.
Readings were taken with the pyrometer across the rotary table for the blackbody
before and after each set of measurements by spectroradiometer. These revealed the
blackbody temperature drift during the tests, whereas the spectroradiometer readings
were corrected for the drift according to the monitor signals. Over all of the blocks of
wavelengths measured in the lamp–blackbody comparisons, the average drift in the
blackbody temperature was 0.25K, with 53% of drift less than 1K and 7% exceeding
5K.
NMI TR 1
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Figure 18. The pyrometer views the blackbody across the
rotary table with the mirrors swung clear of the view path
5.3
The Blackbody–Lamp Comparisons
5.3.1 Spectral Ranges
In any integrations of responses of the filter radiometers with the blackbody spectra
and spectral ratios according to eq. 6 (§2), it is necessary to perform the summations
over spectral ranges wide enough to include all non-zero products E()R that make
any significant contribution to the sums. This means integrating up to at least 1300 nm
for the radiometers containing Si photodiodes, and up to at least 1800 nm for those
containing InGaAs diodes. It also means that to obtain a blackbody temperature via
lamp radiometer measurements and lamp-blackbody ratios, the latter must also be
measured over this extended wavelength range for a single blackbody temperature.
This temperature was chosen to be the temperature at the start of each comparison,
when the initial monitor radiometer readings were taken. All subsequent
measurements were corrected for temperature drift to what they should be if the
blackbody was at the initial temperature.
The lamp current was also monitored during the comparisons. For these lamps,
operated at distribution temperatures near 3100K, any drift in the lamp current, ΔI,
away from the calibration current, I, produces an empirically derived correction factor
FL for measurements at a wavelength λm given by:
FL  7  (1  I / I )  400 / m
(8)
A scaling factor of 7 has been found to apply to these lamps at 400 nm near DT
3100K. These corrections were made by the measurement program at the completion
of each comparison. Details of the systems used in the comparison are given in
Table 3.
NMI TR 1
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Lamps were compared with one of the two cavities as listed in Table 4. The cavity 1644 failed (collapsed) after approx 50 hours of operation at various temperatures near
or above 2800K.
Table 3. Details of systems used for the comparisons
Wavelength
range (nm)
MonoBlaze
Spectral
chromator wavelength bandwidth
(+ filter)
(nm)
(nm)
Detector
Amplifier, transimpedance
Double
300
2.5
Hamamatsu PMT
R562 #SA4965
AD515KH op amp
106 V/A
Single
+ 2412
300
5
Hamamatsu Si
S1337-1010BQ
AD515KH op amp
109 V/A
Double
+ 2412
1000
5
Hamamatsu Si
S1337-1010BQ
AD515KH op amp
109 V/A
1100–1800
Double
+ ITG9
1000
5
Telcom Devices
InGaAs # TD1
1500–2500
Single
+ ITG9
1850
10
NEP 10x5 mm 2
stage cooled PbS
AD515KH op amp
108 V/A
NEP DMC7 +
Ithaco 3 Dynatrac
lockin PSD
240–650
700–1100
(for UVvis)
650–1100
(for IR
tests)
Table 4. Comparisons of spectral irradiance lamps with the blackbody
Cavity and
temperature
Date
16/10/03
16/10/03
22/10/03
22/10/03
22/10/03
22/10/03
22/10/03
31/10/03
31/10/03
6/11/03
6/11/03
6/11/03
12/11/03
12/11/03
12/11/03
13/11/03
13/11/03
13/11/03
13/11/03
13/11/03
16-44 ~ 2850K
16-44 ~ 2850K
16-44 ~ 2900K
16-44 ~ 2900K
16-44 ~ 2900K
16-44 ~ 2900K
16-44 ~ 2900K
16-44 ~ 2810K
16-44 ~ 2810K
16-44 ~ 2900K
16-44 ~ 2900K
16-44 ~ 2900K
New cavity
16-45 ~ 2850K
16-45 ~ 2850K
16-45 ~ 2850K
16-45 ~ 2920K
16-45 ~ 2920K
16-45 ~ 2920K
16-45 ~ 2920K
16-45 ~ 2920K
NMI TR 1
Lamps
No of
tests
FEL3, FEL5, H148, U121
FEL3, FEL5
FEL3, FEL4, SI27
SI27
FEL4, SI27
FEL3
FEL3
FEL2, E18, U121
FEL2
E14
E18
E14, E18
1 each
2 each
1 each
1 each
1 each
once
once
1 each
once
once
once
1 each
Spectral range, interval
(normalisation at 550
nm unless stated)
650–1800, 50
1500–2500, 50
250–1300, 50
240–450 (or –670), 10
240–1110, 10
240–700, 10
240–318, 2 (360 = 1)
650–2500, 50
660–1750, 10
240 –318, 2 (360 = 1)
240–430, 10 (700 = 1)
250–1100, 50
FEL4
FEL4
H148, U121
FEL2, H148, SI25
FEL2
FEL2
SI25
H148, SI25
once
twice
3 each
1 each
once
once
once
1 each
650–1800, 50
1500–2500, 50
1500–2500, 50
250–1100, 50
240 –318, 2 (360 =1)
320–700, 10
440–700, 10
240–430, 10 (700 = 1)
21
5.3.2 Measured Ratios – Raw Data
Measured ratios of relative spectral power distributions of lamps and the blackbody at
various temperatures (see Table 4) are shown in Figs 19 and 20. A reasonable
absorption band in some lamp emission is obvious near 280 nm.
3
FEL2
FEL3
FEL4
H148
SI25
SI27
E18
Ratio (=1 at 700 nm)
2.5
2
1.5
1
0.5
200
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Figure 19. Comparison of given lamp with blackbody at UV-visible wavelengths
1.6
Ratio (=1 at 1050 nm)
FEL5
FEL3
U121
H148
1.4
1.2
1
0.8
0.6
600
800
1000 1200 1400 1600 1800 2000 2200 2400 2600
Wavelength (nm)
Figure 20. Comparison of given lamp with blackbody at IR wavelengths
NMI TR 1
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5.4
Blackbody Temperatures
5.4.1 From the Filter Radiometers
After comparison of each lamp with the blackbody, the spectral distributions of the
ratios of their relative spectral power were spline interpolated to 5 nm wavelength
intervals. These ratios were then used with iterations of blackbody temperatures to find
the temperature required to solve the measured signal-ratio eq. (6) for each lamp and
various filter radiometer pairs. The temperatures are given in Table 5. These
temperatures from different lamps are unrelated, being measured on different occasions.
Table 5. Blackbody temperatures measured from lamp-blackbody comparison and
signal ratios measured for the lamp using different pairs of filter radiometers
Lamp
FEL2
FEL3
FEL4
SI25
SI27
E14
E18
H148
Lamp
FEL2
FEL3
FEL4
FEL5
H148
U121
Temperatures from ratios of radiometers with peak wavelengths (K)
Range ± (excluding
340/450 450/550 550/700 700/940 450/700
340/450 temp)
2929.0
2942.1
2938.9
2950.8
2940.2
6.0
2900.0
2914.8
2908.2
2919.6
2910.8
5.7
2898.6
2913.0
2905.9
2917.0
2908.8
5.5
2932.6
2944.7
2937.3
–
2940.2
3.7
2903.2
2914.2
2909.9
–
2911.6
2.2
2907.0
2916.6
2907.6
2920.3
2910.9
6.3
2905.0
2913.7
2908.4
2917.8
2910.4
4.7
2931.7
2940.2
2938.5
2951.2
2939.2
6.3
Temperatures from ratios of radiometers with peak wavelengths (K)
Range ±
(excluding
700/940 940/1300 1300/1540 700/1300 700/1540 940/1540
1300/1540
temp)
2820.8
2810.6
2856
2816.3
2821.1
2821.4
5.1
2830.2
2811
2853
2821.6
2825.5
2821
9.6
2863.7
2854.2
2881
2859.5
2862.2
2860.7
4.8
2831.6
2821.4
2862
2827.1
2831.3
2831
5.1
2833.6
2819.6
2856
2827.4
2831
2828.5
7
2819.1
2811.2
2866
2815.6
2821.7
2824.2
6.5
In Table 5, temperatures resulting from the use of ratios from the UV radiometer #4
(340 nm) and the blue response radiometer #3 (450 nm) are significantly (12 to 15K)
lower than those from use of the visible and NIR response radiometers. They are in
fact lower for pairing of this UV radiometer with any of the longer peak-wavelength
units. This, taken together with higher uncertainties in the transmittances of the
blackbody window – of their measurement and their possible changes with
contamination, has resulted in rejection of temperatures obtained from the use of the
UV radiometer in these tests. However, as it was regarded as more stable in its
spectral responses than the other radiometers due to more stable interference filter
transmittances, its use in future for tests that do not involve a blackbody window
should be strongly considered.
NMI TR 1
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Also in Table 5, temperatures from the ratios #1 / #9 (1300 / 1540 nm) are
considerably higher than the temperatures from the other combinations. This is
because changes in the ratios of the integrated responses of these radiometers are
relatively insensitive to temperature changes. The combined uncertainty in the
response of these two radiometers is 0.52%, and this corresponds to a temperature
uncertainty of 32K. For this reason, temperatures from this pair have been left out of
the calculation of the range of temperatures obtained from the different radiometer
pairs in Table 5.
Certain combinations of radiometers are not available; none of the shorter
wavelength-response radiometers can be ratioed with the IR radiometers, for example.
In order to interpret the meaning of the ranges of temperatures presented in the above
table and to decide on choices of either ‘local’ or average temperatures it is necessary
to examine the uncertainties in the filter radiometer ratios. The temperature
uncertainties resulting from the uncertainties in the ratios of the integrated radiometer
responses depend inversely on the separation of the effective wavelengths of the
radiometers. Temperatures derived from ratios involving common radiometers have
uncertainties that are correlated. The sign of this correlation is important in selecting
any results that might be averaged.
The standard uncertainties in the absolute integrated responses of each filter
radiometer are given in Table 6, as they have been finally calculated and given in the
uncertainty budget in Table 19. It can be seen from their random and systematic
components that their uncertainties will be virtually uncorrelated (radiometer #4 with
the higher systematic uncertainty was in the end not used). Uncertainties in the
estimated calculated values of the ratios ∫ Lλτ(λ)ρ(λ)R1,λdλ / ∫ Lλτ(λ)ρ(λ)R2,λdλ due to
the uncertainties in the measured spectral ratios ρ(λ) are also given in Table 6 together
with the corresponding temperature uncertainty for a blackbody operated near 3000K.
Table 6. Estimated uncertainties in the calculated ratios
∫ Lλτ(λ)ρ(λ)R1,λdλ / ∫ Lλτ(λ)ρ(λ)R2,λdλ due to uncertainties in the measured values of the
radiometer spectral responsivities RN,λ and in the measured ratios ρ(λ) (ratios that were
used to obtain weighted mean temperatures and their uncertainties are shown in bold)
Random
Systematic
Uncertainty (%) in
Peak uncertainty in uncertainty in For ratio of
Radiocalculated ratio
response integrated
calculated
radiometer
meter
∫ Lλτ(λ)ρ(λ)R1,λdλ /
(nm)
response
integral
pair
∫ Lλτ(λ)ρ(λ)R2,λdλ
∫ RN,λdλ
∫ ρ(λ) dλ
#4
340
0.20
0.28
#4 / #3
0.38
#3
450
0.20
0.10
#3 / #11
0.27
#11
550
0.18
0.06
#11 / #6N
0.24
#6N
700
0.15
0.03
#3 / #6N
0.26
#8
940
0.14
0.01
#6N / #8
0.21
#1
1300
0.39
0.02
#8 / #1
0.42
#9
1540
0.20
0.02
#1 / #9
0.44
#6N / #1
0.42
#8 / #9
0.25
NMI TR 1
Equivalent
temperature
uncertainty
near 3000K
(K)
3.1
3.9
3.5
1.9
3.4
8.7
24
3.8
3.7
24
If all of the radiometers in one configuration have comparable response uncertainties,
the lowest temperature uncertainty is going to result from ratio measurements using
the radiometers with the greatest effective wavelength separation. For measurements
over the shorter wavelength region 240 to 1300 nm, which involved the ‘config 6’ set
of radiometers, the most widely spaced were #3 (450 nm) and #6N (700 nm). Spectral
measurements up to 1300 nm also allowed for temperature measurements based on a
‘config 3’ ratio #6N / #8 (700 / 940 nm). Temperature uncertainties from these two
pairs of radiometers, which share the common radiometer #6N (700 nm peak), are
negatively correlated. Weighted mean temperatures were calculated using both pairs
and are highlighted in Table 7. The uncertainty in the weighted mean temperature will
be discussed in §7.
This selection left out the use of the radiometer #11 (peak at 550 nm). If this were to
be included it would have to be paired with either #3 (450 nm) or #6N (700 nm). A
pairing with radiometer #8 (940 nm) was not possible in the way the configurations
were set up. If temperatures from #3 / #6N, #3 / #11 and #11 / #6N are averaged, there
is considerable correlation in the uncertainties, and the combined uncertainties are
also higher because of the closer peak wavelengths. It will be shown in the discussion
of uncertainties (part 7) that the weighted mean temperatures obtained from the
chosen ratios #3 / #6N and #6N / #8 are reasonably consistent with temperatures
obtained from the radios #11 / #6N.
If a simple average is taken of the temperatures obtained from the radiometer ratios:
#3 / #11, #11 / #6N, #3 / #6N and #6N / #8, as given for each lamp in Table 4, the
average temperatures differ from the weighted mean temperatures by not more than
0.8K across eight lamps, with an average difference of 0.4K. For this set of
radiometers, excluding radiometer #4 (340 nm), it makes little difference how the
mean temperature is calculated, but as discussed later the selection of a weighted
mean temperature results in a lower uncertainty.
For temperature measurements using radiometers with peak responses in the range
700 to 1540 nm, the choice was dictated by using pairs that gave the lowest
uncertainties and with minimal correlation. These were the pairs #6N / #1
(700 / 1300 nm), and #8 / #9 (940 / 1540 nm). These choices are also highlighted in the
above table.
For each pair of temperatures so obtained, a weighted mean temperature is calculated
from the two temperatures according to the equation:
TAV   (Ti / U i ) /  (1 / U i )
2
2
(9)
where Ui is the uncertainty in the temperature from one of the pairs of radiometers
that has been selected. Weighted mean temperatures were obtained for each of the
tests involving the lamps listed in Table 5 and are given in Table 7.
NMI TR 1
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5.4.2 Temperatures from the Optical Pyrometer
Blackbody temperatures were measured using the optical pyrometer at the start and
finish of each comparison, and during the comparisons after measurements over each
block of wavelengths during the switching of the mirror system between the
blackbody and the lamp. Thus, many temperatures were measured during each
comparison which extended over a number of wavelength regions using different
monochromator and detector configurations.
The pyrometer-given temperatures, which typically numbered about 20, were assessed
in terms of the average corrections given by the monitor radiometers at the time for
the drift in the blackbody temperature. The pyrometer-given temperatures taken near
the monitor measurements that resulted in corrections closest to unity were averaged
to provide a ‘best estimate’ of the pyrometer-given temperature applicable to the
‘initial conditions’ of the blackbody at the start of each comparison. These were the
conditions on which the filter radiometer temperature measurements were also based.
These best estimates of the pyrometer-given temperatures have standard uncertainties
due to the distributions of the readings assessed of typically ±0.5 K. They are
compared with the ‘weighted mean’ temperatures from the filter radiometry in
Table 7.
Table 7. Comparison of calculated ‘weighted mean’ filter radiometer temperatures (K)
of the blackbody with ‘best estimate’ temperatures measured by the optical pyrometer
for different lamps and the two configurations of the filter radiometers
Lamp
FEL2 FEL3 FEL4 H148 SI25
SI27
E14 E18
Comparisons using filter radiometers 340, 450, 550 and 700 nm for range 240 to 1100 nm
Weighted mean temp (K) 2943
2913
2911
2942
2941
2912 2913 2912
Pyrometer temperature
2931
2904
2905
2930
2929
2905 2912 2910
Comparisons using filter radiometers 700, 940, 1300, 1550 nm for range 650 to 2500 nm
FEL2 FEL3 FEL4 FEL5 H148 U121
E18
Weighted mean temp (K) 2819
2821
2860
2829
2828
2820
2806
Pyrometer temperature
2812
2815
2847
2817
2817
2811
For these measurements, the pyrometer calibration included allowance for the
blackbody window transmittance at 650 nm, the peak of the response band of the
pyrometer. A spectral transmittance obtained from the Cary 5 spectrophotometer at a
time closest to (either before or after) the blackbody measurement was used. As later
discovered, this did not allow for spatial non-uniformity in the window transmittance.
The average difference between the calculated ‘weighted mean’ temperature from the
measurements involving the shorter-wavelength ‘config 6’ radiometers and the
pyrometer-measured temperature is 7.6 K (+4.5K, –6.5K). For measurement
involving the longer-wavelength ‘config 3’radiometers the average difference is 9.7 K
(+3.3K, –3.7K). This discrepancy will be discussed in §7 – Uncertainties.
NMI TR 1
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6
MEASURED SPECTRAL POWER DISTRIBUTIONS
6.1
Comparisons with Planckian Radiators
Reference lamp relative spectral irradiances were calculated from these comparisons
using the calculated ‘weighted mean’ blackbody temperatures as discussed above. In
order to show differences between these SPDs and regular blackbody spectral power
distributions, ratios of lamp SPDs and blackbody SPDs at about the same distribution
temperatures have been calculated and are shown in Figs 21, 23. For comparison, the
NML1990 scale calibrations values for these lamps have been compared with similar
blackbody SPDs as shown in Figs 22, 24.
Several persistent features are apparent in Fig 21. There is an absorption band near
280 nm, of variable depth. This is also seen in Fig 22. There are two weaker emission
lines, one near 310 nm and the other near 590 nm. Note that the spectral resolution
used in these comparisons was 2 nm but the data interval above 400 nm is only
10 nm.
The ratios shown in Fig 23 above 1100 nm are much noisier, so persistent departures
from Planckian distributions are not obvious. The optical path-lengths used for the
blackbody and the lamps were equal in this comparison system, so atmospheric water
vapour IR absorption should be the same and cancel in the comparison, although
short-term fluctuations in levels may have increased the random noise in the
comparisons.
6.2
Mismatch of Lamp Spectra in Measurement Overlap Range
Five lamps were compared with the blackbody using blackbody temperatures
obtained from the UV–visible response radiometers at wavelengths up to 1100 nm,
and separately using temperatures obtained from the IR response radiometers at
wavelengths starting as low as 700 nm. Therefore, lamp spectral distributions have
been obtained from these two separate groups of radiometers and measurements
overlap over the wavelength range 700 to 1100 nm.
The ratios of the relative spectral distributions of the two sets of results that were
obtained for the five lamps are shown in Fig 25. The sets for the different lamps have
been spread out by varying the normalisation values. The ratios at 1100 nm have been
left out as these were obtained using the Si photodiode and its linearity of response at
this wavelength is considered to be unreliable.
NMI TR 1
27
1.10
Ratio SI27 to blackbody at 3080 K
Ratio FEL2 to blackbody at (K) 3195
Ratio SI25 to blackbody at 3090 K
Ratio FEL3 to blackbody at (K) 3160
Ratio H148 to blackbody at 3025 K
Ratio FEL4 to blackbody at (K) 3160
Ratio (=1 at 550 nm)
1.05
1.00
0.95
0.90
0.85
200
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Figure 21. Ratios of measured lamp relative spectral irradiances with spectral power
distributions of Planckian radiators at about the same distribution temperatures
1.10
SI27
E14
FEL2
SI25
FEL3
H148
FEL4
E18
Ratio (=1 at 550 nm)
1.05
1.00
0.95
0.90
0.85
200
300
400
500
600
Wavelength (nm)
700
800
900
1000
Figure 22. Ratios of lamp NML1990 scale spectral irradiance calibrations with
spectral power distributions of Planckian radiators at about the same distribution
temperatures
NMI TR 1
28
1.02
Ratio (=1 at 1100 nm)
1.00
0.98
0.96
0.94
0.92
0.90
0.88
0.86
600
Ratio FEL5 to blackbody at 3330 K
Ratio FEL3 to blackbody at 3320 K
Ratio U121 16 Oct to BB at 3250 K
Ratio FEL2 to blackbody at 3320 K
Ratio FEL4 12 Nov to BB at 3330 K
Ratio H148 to blackbody at 3165 K
Ratio U121 31 Oct to BB at 3250 K
800
1600
1000
1200
1400
1800
2000
2200
2400
Wavelength (nm)
Figure 23. Ratios of IR relative spectral irradiances of lamps measured in
Oct-Nov 2003 using the blackbody with relative spectral irradiances of
Planckian radiators at about the same distribution temperatures
Ratio normalised at 850 or 1100 nm
1.02
1.00
0.98
0.96
0.94
0.92
0.90
0.88
0.86
600
FEL5
FEL4
FEL3
H148
U121
E18
E14
E36
800
1000
1200
1400
1600
1800
2000
2200
2400
Wavelength (nm)
Figure 24. Ratios of lamp relative spectral irradiances based on NML1990
scale calibrations with spectral power distributions of
Planckian radiators at about the same distribution temperatures
NMI TR 1
29
1.015
FEL2
Linear (FEL2)
FEL3
Linear (FEL3)
FEL4
Linear (FEL4)
H148
Linear (H148)
E18
Linear (E18)
Ratio (arbitrary scale)
1.01
1.005
1
0.995
0.99
0.985
650
700
750
800
850
900
950
1000
1050
1100
Wavelength (nm)
Figure 25. Ratios of relative spectral irradiances of lamps obtained from
UV-visible range lamp-blackbody comparisons and comparisons at overlapping
IR wavelengths using blackbody temperatures from different filter radiometers,
shown with linear trend-lines
Linear ‘trend lines’ have been fitted to the ratios for the purpose of estimating the
approximate blackbody temperature mismatches that would result in these spectral
differences. Blackbody IR temperatures were adjusted until the trend-lines were
horizontal. The temperature differences that were determined in this manner are given
in Table 8. They show discrepancies of –5K to +2.7K, with an average of
–1.0K and a standard deviation of 4K. These differences will be considered in the
analysis of the uncertainties in estimating the blackbody temperatures in §7.
Table 8. UV/vis test range and IR test range blackbody
temperature differences calculated for given lamps
Lamp
E18
H148
FEL2
FEL3
FEL4
Mean
SD
NMI TR 1
Temp difference UV/vis
range – IR range (K)
3.8
–2.4
–5.0
2.7
–4.2
–1.0
4
30
6.3
Differences in Calibrations from Different Reference Lamps
Different types of lamps have been included in setting up this new scale in order to
reveal some of the systematic errors that may result from non-uniformities in beam
spectral irradiances and the way these affect the responses of the filter radiometers in
the sphere, or the spectroradiometer viewing the plane Halon target plate. These will
be discussed in detail in the following section on Uncertainties.
For the lamp-blackbody comparisons in the spectral range 240 to 1100 nm, three
different types of reference spectral irradiance standard lamps were used: two FEL
types (FEL3 and H148) from different manufacturers with slightly different vertical
filament and envelope geometries and mounting, and an Ushio Electric lamp (SI25)
with a horizontal filament and envelope. These were subsequently used to calibrate
other NML secondary standards and three CCPR key comparison transfer standards.
The calibrations of these secondary and transfer standards were done using the normal
laboratory spectral irradiance comparison transfer optics15. This system involves the
direct irradiation by both lamps of a BaSO4 plate in a symmetric arrangement and the
interchange of lamp positions, so systematic optical effects due to differences in the
lamp geometry are considered to be negligible compared with those that are possible
with the use of the mirror-imaging system for comparing the blackbody and the
reference lamps.
Discrepancies in the calibrations of the reference lamps therefore show up as
consistent differences in the calibrations of these secondary lamps, as shown in
Figs 26 and 27, with variations between them being an indication of the noise in the
transfers. Normalised distributions of ratios of blackbody SPDs assessed as likely to
account for most of these differences have been added as trend lines. A temperature
difference of 0.7K would largely account for the differences in calibration values
obtained from the reference lamps SI25 and FEL3. A larger temperature difference of
about 3.5K is necessary to account for the trend in the differences from the reference
lamps SI25 and H148.
For the IR transfers to other lamps, three FEL lamps were chosen as the reference
lamps. It had become obvious that these were both the most stable and powerful of the
range of lamps chosen, and with the results in the PbS spectral range above 1700 nm
being quite noisy, these measurements would most benefit from the use of these 1 kW
lamps. Similar comparisons of transfers from the three references to 3–4 other lamps
are shown in Figs 28 and 29. The similarities in the curves are interpreted as due to
lower noise levels involved when comparing these test and reference lamps than when
comparing the reference lamps with the blackbody. Any systematic differences are
largely hidden by the higher levels of random noise in the blackbody-lamp transfers.
Temperature differences of the order of 3K appear to be required to account for the
larger trends in these differences.
NMI TR 1
31
1.01
Ratio (=1 at 550 nm)
1.005
1
0.995
BN-9101-196
FEL5
0.99
200
300
400
BN-9101-206
FEL6
500
BN-9101-247
0.7K temp diff
600
FEL1
700
800
Wavelength (nm)
Figure 26. Ratio of spectral irradiances of given lamp from reference lamps
FEL3 and SI25, normalised to unity at 550 nm
1.02
Ratio (=1 at 550 nm)
1.01
1
0.99
BN-9101-196
FEL5
BN-9101-206
FEL6
BN-9101-247
3.5K temp diff
FEL1
0.98
200
300
400
500
600
700
Wavelength (nm)
800
900
1000
1100
Figure 27. Ratio of spectral irradiances of given lamp obtained from reference lamps
H148 and SI25, normalised to unity at 550 nm
NMI TR 1
32
1.02
BN-9101-196
BN-9101-206
BN-9101-247
FEL6
2K temp diff
Ratio (=1 at 1100 nm)
1.01
1
0.99
0.98
1000
1200
1400
1600
1800
2000
2200
2400
Wavelength (nm)
Figure 28. Ratio of spectral irradiances of given lamp obtained from reference lamps
FEL3 and FEL5, normalised to unity at 1100 nm
1.02
BN-9101-196
BN-9101-206
BN-9101-247
3K temp diff
Ratio (=1 at 1100 nm)
1.01
1
0.99
0.98
1000
1200
1400
1600
1800
2000
2200
2400
Wavelength (nm)
Figure 29. Ratio of spectral irradiances of given lamp obtained from reference lamps
FEL4 and FEL5, normalised to unity at 1100 nm
NMI TR 1
33
6.4
Differences between the NML2003 and NML1990 Scale Values
6.4.1 UV-visible Spectral Range
Most of the reference lamps used for the shorter wavelength UV-vis comparisons and
some of the lamps calibrated from them were previously calibrated about June 2002 in
terms of the NML1990 spectral irradiance scale as then maintained. This means that
it is expected that the scale may have drifted somewhat since its establishment in
1990. When the new calibration values of these lamps (normalised at 550 nm) are
compared with their NML1990 scale values the results are as shown in Fig 30.
1.05
FEL4
FEL3
SI27
E18
E14
FEL2
H148
SI25
1.04
1.03
Ratio (=1 at 550 nm)
1.02
1.01
1.00
0.99
0.98
0.97
0.96
0.95
200
300
400
500
600
700
800
Wavelength (nm)
Figure 30. Ratios of spectral irradiances of given lamp based on NML2003 spectral
irradiance scale and values based on the NML1990 scale as maintained in 2002,
normalised to unity at 550 nm
The curves for most of the lamps suggest that the NML2003 scale values represent
slightly higher temperatures than those of the former scale. Changes in the
calibrations of the newer FEL lamps and the Ushio Electric lamps (SI25, SI27) are
reasonably similar. The two GEC lamps, E14 and E18 have apparently reduced their
UV irradiances. The older type FEL lamp, H148, that was used in the 1990 CCPR key
comparison, appears to have increased its temperature relative to it previous
calibration. Changes in the NML spectral irradiance scale have been estimated by
averaging the changes that have occurred for the lamps SI25, SI27, FEL2, FEL3 and
FEL4 for the spectral range 250 to 850 nm.
NMI TR 1
34
6.4.2 IR Spectral Range
The change in the NML scale in the IR spectral range is less clear. Most of the lamps
that represented the former NML1990 scale in that range have since become unstable,
failed altogether or have been discounted from the current work as unsuitable on
account of their size. Lamp H148 is the only lamp left originally calibrated against the
NML1990 scale, with lamp U121 calibrated against this scale more recently. The new
calibration values for these two lamps are compared with their NML1990 scale values
in Fig 31. It appears that their relative temperatures have drifted apart. On account of
the larger short-wavelength changes in the calibration values of lamp H148, its higher
operating temperature and its greater use than lamp U121, it is assumed here that lamp
U121 has remained the more stable of the two. The changes for this lamp are being
used as a measure of the changes in the NML scale between 1990 and 2003 for
wavelengths above 850 nm.
1.04
H148 16-Oct-03
U121 16, 31 Oct, 12 Nov
Ratio (=1 at 700 nm)
1.02
1.00
0.98
0.96
600
800
1000
1200
1400
1600
1800
2000
2200
2400
Wavelength (nm)
Figure 31. Ratios of IR spectral irradiances of the lamps H148 and U121 based
on their NML2003 calibrations to their former NML1990 scale calibrations,
normalised to unity at 700 nm
6.5
Change in the NML Scale of Relative Spectral Irradiance
The multiplying factors to be applied to the NML1990 scale to produce spectral
irradiance units based on the NML2003 scale when normalised to unity at 550 nm are
shown in Fig 32 and given in Table 9. (Differences between the IR curve for lamp
U121 in Fig 31 and the IR curve in Fig 32 are due to differing numbers of data points
and different normalisation wavelengths).
NMI TR 1
35
1.03
Multiplying factor
1.02
1.01
1.00
0.99
0.98
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
Wavelength (nm)
Figure 32. Multiplying factors to be applied to the former NML1990 scale of relative
spectral irradiance to produce irradiances based on the NML2003 scale when
normalised to unity at 550 nm
Table 9. Multiplying factors to be applied to NML1990 scale values as maintained in
June 2002 to obtain NML2003 relative spectral irradiance scale values
(normalised at 550 nm)
Wavelength
Multiplying
Wavelength
Multiplying
(nm)
factor
(nm)
factor
250
1.0104
700
1.0015
260
1.0033
750
0.9944
270
0.9977
800
1.0094
280
1.0107
850
1.0157
290
1.0024
900
1.0149
300
1.0036
950
1.0154
310
1.0109
1000
1.0164
320
1100
1.0104
1.0126
330
1.0134
1200
1.0076
340
1.0091
1300
1.0058
350
1.0038
1400
1.0010
360
1.0079
1500
1.0135
370
1.0095
1600
1.0090
380
1.0088
1700
1.0007
390
1.0057
1800
0.9989
400
1.0029
1900
0.9990
450
0.9995
2000
1.0168
500
1.0033
2100
1.0120
550
1.0000
2200
1.0183
600
1.0020
2300
1.0205
650
1.0007
2400
1.0103
NMI TR 1
36
For comparison with these changes, the results of the last CCPR key of spectral
irradiance units in 1990 are shown in Fig 33. Note that the subsequent NML1990
scale became based on the KCRVs (mean international units) resulting from that
comparison for the UV and IR wavelengths.
15
NIST
10
NML
ETL
Difference (%)
5
INM
IOM
0
NIM
NPL
-5
DPT
NRC
-10
OMH
PTB
-15
VNIIOFI
-20
200
400
600
800 1000 1200 1400 1600 1800 2000 2200 2400
Wavelength (nm)
Figure 33. 1990-91 CCPR key comparison of units of spectral irradiance:
differences of participants from grand mean values (used as KCRVs)
7
UNCERTAINTIES
The final uncertainties in the relative spectral irradiances of the lamps are determined by:

the estimated uncertainty in the blackbody temperature;

the quality of the blackbody, including spectral emissivities and window
transmittances;

the uncertainties in the spectral comparison of the lamp with the blackbody;

uncertainty in lamp operating current;

spectroradiometer detector non-linearity; and

spectroradiometer wavelength uncertainty.
For the additional step of normalising the scale of relative spectral irradiance to an
absolute scale of spectral irradiance in SI units, which involves to measurements of
lamp illuminances, there are additional uncertainties due to:

NML1997 photometric scale (cd) uncertainty;

lamp distance;

lamp current;

stray light;

photometer colour correction factor;

photometer linearity; and

photometric integration for the illuminance.
These uncertainties will be addressed in turn.
NMI TR 1
37
The method that was used to obtain the blackbody temperature was not a direct use of
the filter radiometers with the blackbody, rather their use with each lamp and then the
spectral comparison of the lamp with the blackbody. The results of these two
measurements were combined and as was shown in §2 Theory eq. 6, the blackbody
temperature T was found that satisfied the relationship:
S1 / S 2    ( ) ( ) ( ) L( , T ) R1 ( )d /   ( ) ( ) ( ) L( , T ) R2 ( )d (10)
where R1(λ) and R2(λ) were the radiometer responses relative to one another, and
S1/S2 was the radiometer signal quotient measured for the lamp.
The uncertainties in the components making up the products in the above integrals
have both random and systematic parts. The systematic uncertainties are mainly due
to wavelength uncertainties in both the calibration of the NML reference standard
detectors (Si and InGaAs) and when transferring calibrations from these to the
radiometers. These systematic uncertainties must be added according to the type of
correlation that is expected in these uncertainties for each pair of radiometers that is
used to measure the blackbody temperature.
For each radiometer, each input variable x has an uncertainty due to a systematic
wavelength uncertainty at wavelength  of value u(,x). For example, the relative
uncertainty in the signal due to the effect of the systematic wavelength uncertainties in
determining the radiometer spectral responses RI(,x) is given by:
us , x ( Si )   F   /  Ri  , x  /  F  Ri  , x 
where:
F ( )   ( ) ( ) ( ) L(,T )
(11)
(12)
is the weighting function generating the signal as given in eq. 10.
The individual uncertainties in the sums are then added linearly taking into account
their signs and correlations:
(13)
u s (Si )   u s , x
x
Note that the systematic uncertainties uS,x(Si) may be correlated or uncorrelated
depending on the nature of the wavelength uncertainties in the calibrations of the
reference detector and radiometer spectral responses and other spectral measurements.
Further, the combined uncertainties uS(Si) in the signals from two radiometers may
also be correlated or uncorrelated through common wavelength errors, so correlations
and signs need to be carefully managed in the calculations.
For the random (uncorrelated) wavelength errors, and using the example of their
effect on the radiometer spectral responsivities, the corresponding uncertainty
components are:
u R ( Si )    F   /  Ri  y,    /   F  Ri  y,  
2
NMI TR 1
(14)
38
and treating these as uncorrelated inputs, the sum of these uncertainties is given by:
u R (Si ) 
u
2
R, y
(15)
(Si )
y
For the ratio of the signals from two radiometers, the variance in the ratio S1/S2 will
be given by:
2
2
2
2
(16)
u rel ( S1 / S 2 )  u rel ,S S1   u rel ,S S 2   u rel , R ( S1 )  u rel , R ( S 2 )
The uncertainties in the ratios for various pairs of radiometers have been calculated16
and are given in Table 19 following discussion of the various component uncertainties
forming the inputs to eq. 12.
The sensitivity coefficients relating the uncertainties in the blackbody temperature to
the uncertainties in the ratios for the various pairs of radiometers have been calculated
for blackbody temperatures near 2900K. They are given in Table 10.
Table 10. Changes in blackbody temperature (K) near 2900 K resulting from one
percent change in signal ratios from radiometers with indicated effective wavelengths
For effective
wavelengths
(nm)
340 / 450
450 / 550
550 / 700
700 / 940
450 / 700
7.1
Change in blackbody
temperature for change in
ratio (K / %)
8.1
14.5
15.0
16.2
7.4
For effective
wavelengths
(nm)
940 / 1300
700 / 1300
1300 / 1540
700 / 1540
940 / 1540
Change in blackbody
temperature for change
in ratio (K / %)
20.7
9.1
53
7.8
14.9
Uncertainties in Filter Radiometer Spectral Responsivities Ri(λ)
These responsivities were measured in absolute units as photocurrent / unit spectral
flux entering the sphere at each test wavelength. As they refer to a single geometry
common to all of the radiometers (and only to this geometry) they will henceforth be
treated on the same scale as relative spectral responsivity units.
Other contributing uncertainties include sensitivity to beam geometric conditions (as
these might affect different radiometers differently), temporal changes, detector nonlinearity and amplifier gain ratios. The radiometer spectral responses have been
integrated (weighted by the lamp spectrum) for assessment in terms of relative
responses to the lamps and uncertainties in these are added to the uncertainties in the
gain ratios, temporal changes etc to make up the budget totals in Table 19.
For spectral response calibrations using the ‘standard’ geometry and at a particular
time, the uncertainties in the spectral responses include random components due to the
random uncertainties in the reference detector responses and random transfer
uncertainties. The systematic uncertainties contain components from the calibration of
the reference detectors and the transfers each regarded as type B uncertainties and
each due to possible monochromator wavelength errors.
NMI TR 1
39
7.1.1 Contribution from Calibration of the Reference Detector Standards
7.1.1.1 Random Uncertainties
The reference detectors that were used to calibrate the filter radiometers were Si
diodes H5W, H6W, H7W and H8W, and InGaAs diode TD1. These reference
detectors were calibrated for their relative spectral responsivities by comparison with
a group of bolometers. Absolute (spectral flux) responsivities were obtained by
normalisation of the relative responses at various visible wavelengths for the Si
diodes, and at 1297 nm for the InGaAs diode, by reference to the NML cryogenic
radiometer. Bolometer signal noise dominates most of these transfers but there is a
minor component of uncertainty due to possible random wavelength errors.
Random wavelength standard uncertainties of 0.03 nm for the Si reference detectors
and 0.06 nm for the InGaAs detector were assessed. Considering all of the input
uncertainties, the combined fractional standard uncertainties in the spectral responses
(which are mostly of ~ 0.2 to 0.3%) were integrated with the weighting function as
shown in eq. 14 to calculate the standard uncertainties in the integrated responses.
These are calculated in an Excel spreadsheet17 and given in Table 19.
7.1.1.2 Systematic Uncertainties in Reference Detector Calibrations due to
Wavelength Errors
The relative spectral responses of the reference detectors were measured by reference
to bolometer standards requiring wider spectral bandwidths for the comparisons.
Standard wavelength uncertainties of 0.12 nm have been assessed for calibration of
the reference silicon detectors and 0.14 nm for the InGaAs detector. The systematic
component is expected to be an offset in the same direction at all wavelengths, that is,
the errors are expected to be positively correlated. The uncertainties in the spectral
responses resulting from these uncertainties have been calculated taking into account
their signs, and then integrated with the weighting function as shown in eq. 11 to
calculate the standard uncertainty in the integrated response for each radiometer. They
are given in Table 19. They are similarly correlated between most of the radiometers
and their signs are taken into account when calculating the combined uncertainties for
each pair of radiometers as shown in eq. 16.
7.1.2 Uncertainties in Transfers from the Reference Detectors
7.1.2.1 Random Transfer Uncertainties
Calibrations of radiometer spectral responses were done multiple times. At the shorter
wavelengths, four different Si reference detectors were used so the transfer uncertainties
include differences in the calibrations of these reference detectors as well.
Some statistics are given in Table 11 of the effective wavelengths of the radiometers
when used with lamps at distribution temperatures of about 3200K, the wavelength
ranges identified as the main response bands and over which the responses were
measured at 2 nm intervals, and the fraction of the signal calculated for the adopted main
response band. Also given in Table 11 are the number of such tests used in the final
calculations, the variation in the calculated integrated responses for a representative lamp,
and the estimated standard uncertainties in the responses calculated for each radiometer
as determined by the random measurement inputs and excluding systematic uncertainties
due to wavelength errors, sphere spatial non-uniformity and temporal response drift.
NMI TR 1
40
The uncertainties used in Table 19 have been taken directly from values given in
Table 11. Where three or more repeated tests have been done an experimental
standard deviation of the mean value of the integrated response has been calculated.
Where only two tests were done the difference in the integrated response was used
conservatively as the semi-range of what was adopted as a rectangular distribution.
Hence the large degrees of freedom given in Table 19.
Table 11. Some characteristics of the sphere filter radiometers,
the spectral response test results and their standard uncertainties
Radiometer
Effective wavelength (nm) for
lamp at D.T. ~ 3200K
Wavelength range of main
response (nm)
% of response in main range
No. final tests over main range
ESDM of integrated response to
lamp in main response range (%)
No. final tests of wing responses
Uncertainty in integrated
responses over wings (% of total)
#4
#3
#11
#6N
#8
#1
#9
348
460
547
701
940
1294
1550
300–
400
99
7
400–
520
89
4
480–
620
99
4
640–
780
97
4
850–
1010
99
2
1230–
1360
95
6
1400–
1600
76
3
0.09
0.02
0.02
0.02
0.03
0.06
0.05
2
2
2
3
3
5
3
0.07
0.14
0.02
0.02
0.03
0.28
0.09
7.1.2.2 Long-wavelength Out-of-band Responses
The lamp irradiances peak near 1 μm, as do the Si photodiode responses. Therefore,
for the UV and visible radiometers with peak responses up to 700 nm, the out-of-band
responses on the long wavelength side are the most significant. The signal
contributions from these, calculated for different lamps, can be checked by measuring
the effective transmittances of long-pass glass filters. The calculated and measured
transmittances17 are compared in Table 12. The differences are used as a check on the
levels of uncertainties that have been calculated for the measured upper wing
responses as shown in Table 11. They are considered to be in reasonable agreement.
Table 12. Comparison of directly measured transmittances of cut-off glass filters with
transmittances calculated using measured filter spectral transmittances, radiometer
spectral responses and lamp relative spectral irradiances based on the NML1990 scale
Rad # tested,
peak w.l.,
(nm)
Filter
number
Glass type
4 (340)
4 (340)
3 (450)
3 (450)
11 (550)
6N (700)
6N (700)
FYG132
FYG130
FYG164
ITG9
FRG107
ITG9
ITG30
Corning 3389
Chance OY8
Corning 3484
Corning 2540
Corning 2403
Corning 2540
Schott RG830
NMI TR 1
Thickness
(mm)
3.2
4.9
5.7
3.0
3.0
3.0
5.9
Approx
50% cut-off
wavelength
(nm)
430
465
555
990
645
990
840
Transmittance (%)
calculated
measured
1.31
1.40
9.9
1.75
1.02
1.28
2.54
1.21
1.30
10.1
1.78
1.05
1.27
2.56
41
These results for the long-wavelength wing-responses of the UV and visible response
radiometers give some additional confidence in the measurements of the longwavelength wing-responses of the IR radiometers and the short-wavelength wingresponses of all of the radiometers. Unfortunately, there were no suitable filters
available for similar checking of the radiometers peaking at 940, 1300 and 1550 nm,
nor short-pass long-wavelength-blocking filters for checking any of the radiometers.
7.1.2.3 Input Beam Geometry and Sphere Non-uniformity
Different types of spectral irradiance lamps produce varying input beam geometries to
the integrating sphere housing the radiometers. To allow for mismatch of the area of
the sphere wall used for the radiometer calibrations and that irradiated by each lamp
type, additional spectral responsivities were measured using a range of target areas of
the sphere rear wall. These measurements were done over the main response bands
and the responses were convolved with some standard lamp spectra (using the current
NML1990 spectral irradiance scale) to determine the variation of the spectrallyintegrated responses. The results18 are given in Table 13.
As the temperature measurements are based on filter radiometer ratio measurements,
what is required is the departures of the ratios of responses to different target areas
from those for the calibration area of 12  13 mm. These are given in Table 14 for the
pairs of radiometers that have been used as the basis of the blackbody temperature
measurements.
The areas of sphere wall irradiated by the filaments of the different lamps are given in
Table 2 and are generally close to 12.5  13 mm. If IR emissions from the envelopes
of these lamps are considered to be significant, then the areas irradiated by flux from
the envelopes need also to be considered. These have been calculated19 as having
dimensions up to about 14.2  13 mm. This is still within the range of areas assessed,
given in Table 14.
Table 13. Ratios of calculated spectrally-integrated response of filter radiometers for
different sphere-wall incident beam-areas to that of dimensions 12 mm  13 mm (h  w)
Radiometer
#
Peak
wavelength
(nm)
4
3
11
6N
8
1
9
340
450
550
700
940
1300
1550
NMI TR 1
Spectrally integrated response to a quartz halogen lamp
at DT ~ 3100K for given sphere wall area to response
for incident area 12  13 mm (height  width, mm)
10  10 or
13  14 or
11  12
12  13
10  11
13.5  14.5
–
1.0006
1
1.0005
1.0053
1.0014
1
0.9973
1.0032
1.0022
1
0.9985
1.0015
1.0003
1
0.9993
1.0029
1.0015
1
0.9980
0.9981
0.9976
1
1.0001
1.0024
1.0018
1
0.9995
42
As the areas of the sphere wall that were irradiated in both the response calibrations
and the lamp signal ratio tests were all slightly oval, additional tests were done of the
changes in the radiometer signal ratios for each lamp when the sphere was rotated
through 90º about its input axis. The changes in the responses are given in Table 15,
expressed as mean % differences and standard deviations of % differences in response
for the larger groups of similar lamps, or mean and ranges of % differences in the case
of tests involving only two lamps.
Considering the differences given in Table 15 that were measured for these two
positions, and the variations for different beam sizes given in Table 14, the standard
uncertainties in the calculated ratios of the radiometer response-weighted spectral
irradiances for the lamps with clear envelopes, when using radiometer spectral
responses measured for the standard 12  13 mm sphere wall area, are estimated to be:
±0.12% for the ratios #4 / #6N, #3 / #6N, #11 / #6N, and #8 / #6N, #9/ #8;
±0.23% for the ratio #1 / #6N.
These spatial response uncertainties have been included in the uncertainty budget in
Table 19. They may be considered to be uncorrelated between different types of
lamps, whereas the uncertainties in the radiometer spectral responses are partially
correlated. This will be discussed further when the uncertainties are compared with
apparent temperature discrepancies between different types of lamps.
Table 14. Calculated ratios of responses of given radiometers to flux from a quartz
halogen lamp for different dimensions of beam incident on sphere rear wall
Calculated value of (ratio of responses rad#/rad6N for
given beam target area) relative to (ratio for beam target
Radiometer
Peak
area of 12  13 mm) for areas (mm, height  width)
ratio
wavelengths
#/#
(nm)
10  10 or
13  14 or
11  12
12  13
10  11
13.5  14.5
4 / 6N
340 / 700
–
1.0002
1
1.0012
3 / 6N
450 / 700
1.0038
1.0011
1
.9980
11 / 6N
550 / 700
1.0017
1.0018
1
.9992
8 / 6N
940 / 700
1.0014
1.0011
1
0.9987
1 / 6N
1300 / 700
0.9966
0.9972
1
1.0008
9/8
940 / 1550
1.0005
0.9996
1
0.9986
Table 15. Differences between the ratios of radiometer signals
measured using two sphere orientations for three lamp types
Mean differences and standard deviations or ranges in
differences of ratios for sphere shaft vertical–horizontal for lamp
For ratio
type and number of lamps (%)
rad # /rad #
(peak wavelengths)
Ushio Electric
FEL 1000W
GEC 750 W
500W (4, 2 lamps)
(6 lamps)
(2 lamps)
4 / 6N (340/700)
–0.01, 0.11
0.04, 0.16
–0.10, ±0.15
3 / 6N (450/700)
<0.01, 0.06
0.04, 0.10
–0.03, ±0.12
11 / 6N (550/700)
–0.01, 0.04
0.02, 0.06
<0.01, ±0.10
8 / 6N (940/700)
0.10, ±0.02
–0.03, 0.11
0.08, ±0.04
1 / 6N (1300/700)
0.06, ±0.01
<0.01, 0.05
0.07, ±0.02
8 / 9 (940/1540)
<0.01, <±0.01
–0.01, 0.05
0.04, ±0.02
NMI TR 1
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7.1.2.4 Systematic Wavelength Errors in Transfers
During the calibrations of the filter radiometers #3, #11, #6N, #8, #1 and #9 using the
tungsten halogen lamp, the monochromator wavelength calibration was considered to
have a systematic standard uncertainty of ±0.1 nm. It was considered to have an
uncertainty of ±0.2 nm using the xenon arc lamp for the wavelength range 240 to
1010 nm for calibration of the UV radiometer #4 (non-uniform image of arc at
entrance slit).
Different pairs of gratings were used for the calibrations of radiometer #4, the group
#3, #11, #6N and #8, and the pair #1 and #9. However, it is considered that the main
source of systematic error in wavelength arises when the Hg lamp source is
substituted by the continuum sources such as the tungsten filament lamp or the xenon
arc. Between the calibrations of the radiometers #3, #11, #6N and #8, and the IR pair
#1 and #9, the gratings were changed but the tungsten filament lamp was not moved.
It is considered that any systematic wavelength offset will be similar (in direction and
magnitude) for both of these groups. Therefore, any wavelength offsets are treated as
100% correlated for calibrations of all of the radiometers within these two groups but
uncorrelated with the calibration of radiometer #4.
The uncertainties due to these possible wavelength errors have been calculated
according to eq. 11 and are given in Table 19. They vary from 0.28% near
340 nm to 0.002% near the peak of the lamp emission spectrum near 940 nm. These
standard uncertainties have to be added or subtracted according to which radiometers
are paired and whether their effective wavelengths are on the same side or opposite
sides of the peak of the lamp spectrum. The uncertainties that have been calculated for
various ratios of radiometer measurements are also given in Table 19.
7.1.2.5 Temporal Response Drifts
Changes of the responses of the different radiometers have varied considerably with
time. Generally, the peak responses dropped, whilst the responses of the long wings
increased. Some representative changes are given in Table 16.
Table 16. Some representative changes in integrated responses of sphere radiometers
Period
139 days
61 days
62 days
38 days
46 days
55 days
62 days
Radiometer
#4
#3
#11
#6N
#8
#1
#9
Wavelength (nm)
340
450
540
700
940
1300
1550
% change
+0.45
–0.7
–2.0
–0.8
–0.3
–1.1
+0.2
For the comparisons given in Table 16, the responses over the main response band (or
whole response band where reliably available) were convolved with a representative
lamp spectrum and the change in the integral was measured. Note that in all cases
where the peak response drops the wing responses increase, by a smaller amount, so
that the change in the integral over the whole response range is smaller than the drop
in the peak response.
NMI TR 1
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Some of the decreases in response may be due to changes in the sphere gain due to
reflectance changes, but the changes are not uniform with wavelength and they would
be expected to be greater in the UV region, which is not necessarily true. The changes
are blamed on the deterioration in the interference filters. However, the UV filter in
rad #4 includes a Schott UG11 long-wavelength blocking glass and other materials in
the stack that are known to be more stable and moisture resistant than those used at
longer wavelengths.
Due to the above changes, all final measurements of the signal ratios with the lamps
and the spectral responses of at least the main response bands of the radiometers were
done within 5 to 10 days. The intervals for the radiometers with the larger rates of
change were: 5 days for #11, 5 days for #6N and 8 days for #1. The standard
uncertainties in integrated responses that have been calculated19 based on conservative
estimates of semi-ranges equal to twice the pro-rata previously-measured changes in
responses. The corresponding standard uncertainties for these changes vary from
0.02% to 0.12%, and are included in the uncertainty budget in Table 19.
7.1.3 Radiometer Detector Non-linearity
Detector linearities were tested20 at photocurrent levels close to those used for both
the spectral response calibrations and the signal ratio tests with the spectral lamps,
which were up to twenty times higher.
No non-linearity was detected in any of these detectors, but the uncertainties in the
measurements over each 5:1 signal range varied from 0.01% to 0.06%. Allowing for
the total range of signals and their relative contributions to integrated signal levels, the
standard uncertainties for possible non-linearity of these radiometers have been
assessed as
±0.03% for rad #4;
±0.05% for rad #3, #11, #6N, #8;
±0.04% for rad #1;
±0.1% for rad #9.
These uncertainties have been used in summary Table 19.
7.1.4 Amplifier Gain Ratios
The gains of the amplifiers used for measuring the signal ratios for the spectral
irradiance lamps were measured in 1998, 2000 and 2002 (and some earlier). The
RATIOS of these gain factors were measured in June 2003 using one of the photodiodes
as the current source, a stable lamp and the voltmeters that were used for the ratio
measurements of the lamps, and are compared with the 2002 ratios in Table 17.
Table 17. Comparison of amplifier gain factors from tests done in
Sept 2002 and June 2003
Ratio to gain of PF1
For
Nominal
Difference
Amplifier
radiometers
gain (V/A)
(%)
Sept 2002
June 2003
6N
PF1
107
1
1
1, 11
PF2
3.0085
3.0084
<0.01
3  107
8
3, 9
Amp 6
10
9.6680
9.6718
+0.04
8
Amp 7
107
0.99095
0.99083
+0.01
9
4
Amp 7
10
87.630
87.703
+0.08
NMI TR 1
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The uncertainty in the measured gain ratios was assessed at the time21 as ±0.02% (1σ).
The109 ohm feedback resistor used in Amp 7 was replaced in 1998 and has drifted
down in value since then by about 0.5%. However, as the other resistors used in these
amplifiers are much more stable, the result obtained in June 2003 for this high-gain
range of Amp 7 suggests a higher resistance than that existent in 2002.
The conclusion reached here is that the standard uncertainties in the gain ratios used
should be:
±0.02% for amplifiers used with radiometers #1, #3, #8, #9, #11; and
±0.1% for Amp 7 109 Ω range used with radiometer #4
relative to the gain of amplifier PF1 used with radiometer #6N. These uncertainties
are used in Table 19.
7.2
Uncertainties in the Blackbody–Lamp Comparisons
7.2.1 Random Transfer Uncertainties
Multiple comparisons were made between the blackbody and some of the lamps. These
were from 2 to 4 in number. For comparisons numbering three or more, the ESDMs (%)
of the set have been calculated, otherwise the range is used; the values obtained are
shown in Figs 34 to 35. Only the values for the PbS spectral range are significant, as it
will be shown that systematic uncertainties will dominate most of these results. Note that
these measurements include the effects of monochromator random wavelength
uncertainties, which are in any case estimated to not exceed about 0.03 nm.
In a manner similar to the calculations of the random uncertainties due to random
wavelength uncertainties as discussed in §7 and calculated according to eq. 14, the
uncertainties u(SI) in the spectrally-integrated radiometer responses to the random
uncertainties in the spectral ratios  are given by:
u  ( Si )   F   /      /  F    
2
(17)
where the weighting function:
F ( )   ( ) ( ) L( , T ) Ri ( )
(18)
with components LTRi as defined earlier in §7. The uncertainties
range from 0.02–0.07%, as given in Table 19.
NMI TR 1
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0.5
FEL3
H148
SI25
FEL2
FEL4
E18
Range or STDEV (%)
0.4
0.3
0.2
0.1
0.0
200
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Figure 34. Range or SD (%) of values of ratios of given lamp SPD to blackbody SPD
for spectral comparisons in the range 250–1100 nm, normalised to unity at 550 nm
3.0
FEL2
FEL3
FEL4
FEL5
E18
H148
U121
Range or STDEV (%)
2.5
2.0
1.5
1.0
0.5
0.0
500
1000
1500
2000
2500
Wavelength (nm)
Figure 35. Range or SD (%) of values of ratios of given lamp SPD to blackbody SPD
for spectral comparisons in the range 650–2500 nm, normalised to unity at 1050 nm
NMI TR 1
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7.2.2 Systematic Transfer Uncertainties
7.2.2.1 Wavelength Uncertainty
The spectroradiometer wavelength uncertainty for the UV-visible range transfers up
to 1300 nm was estimated to be 0.1 nm, and for the higher IR wavelengths 0.2 nm.
Considering that the spectra of the lamps and the blackbody being compared were
similar, this level of uncertainty is estimated to have resulted in negligible
uncertainties in the blackbody temperature estimates or the resulting lamp relative
spectral irradiances.
7.2.2.2 Optical Transfer Uncertainties
The Spectroradiometer viewed a 5 × 4 mm area of a plane Halon plate onto which
flux from the blackbody or the lamp was directed by the rotary table-mounted mirror
system. Errors in the comparison of these sources would occur if the plate area viewed
was not covered by flux that was sufficiently spectrally uniform and in the case of the
lamps, representative of the spectral power distribution of the whole lamp as it is
normally viewed. The effects of these non-uniformities on the comparison will now
be discussed.
The blackbody is imaged onto a 12 mm-diameter aperture (aperture A in Fig 16) in
front of the Halon plate with 8 times magnification. This geometry produces an
almost collimated beam through the aperture so the image falling on the plate is only
just out of focus. The plane of the source has been set about 30 mm in front of the
cavity base so that, for a 12 mm image aperture, the aperture receives flux from an
area of the cavity base with a diameter of up to 2 mm.
The temperature across this viewed area is not quite uniform, with variations of 1 to
2K for different cavities as reported by the supplier. The affect of this and the
effective spectral emissivities of the cavity will be discussed in a following section. At
issue here is whether any non-uniformity in the flux covering the viewed area of the
Halon plate may result in errors in the blackbody-lamp spectral comparison. The
answer is no. An ‘effective’ or average blackbody temperature is calculated for
whatever flux is used by the spectroradiometer. Except for the use of the optical
pyrometer, there is no separate measurement of the blackbody temperature then
followed by use of the blackbody in perhaps a slightly different viewing geometry for
comparison with the lamps.
The lamps must be calibrated for their spectral irradiances from the whole lamp
normal to a given area on a specified viewing axis. Under conditions of normal use
there is no imaging of the lamp involved. However, in order to avoid the need to
calibrate the spectral transmission (reflection) function of the mirror system used to
image the blackbody, the choice was made to image the lamp as well as the blackbody
using the same optical system but in reverse, thus cancelling the transmission
function.
NMI TR 1
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The Halon plate irradiation conditions for the lamp are far more critical and difficult
than for the blackbody. The lamp image is, of course, very non-uniform. The bright
filament image is surrounded by a larger image of the envelope and perhaps some
supporting structures that have been shown to contribute appreciable amounts of IR
irradiance above about 1200 nm. Therefore, a plane behind the image has to be
chosen in which the image is completely defocused and the irradiances produced by
all significant radiating structures have uniform areas that intersect over at least the
viewed area of the plate (5 × 4 mm). The spectral power distribution of the flux within
this common area of uniformity should be the same as that from the lamp in a nonimaged field viewed on the same axis and collected over the same solid angle (in this
case a cone of full angle 1.4º).
The target plane was chosen to be 79 mm behind the plane of the lamp image and
assessed22 as far enough back to give an area of uniform irradiance with a diameter of
about 5.7 mm for structures with diameters up to 40 mm. This is the length of the FEL
lamp envelope. Filament sizes are much smaller, typically 20 × 6 mm, which produce
larger areas of irradiance uniformity in the measurement plane. It is necessary to keep
the beam size in the measurement plane as small as possible as this relates directly to
the radiance of the plate that will be viewed by the spectroradiometer. This must be as
high as possible to obtain adequate signal levels for transfers down to about 240 nm.
Provided that the above assessments are correct there should be no errors in
comparing the blackbody radiation with that from the lamps arising from beam nonuniformities at the Halon target plate for the spectroradiometer. No additional
uncertainties have been allowed for this.
7.2.3 Quality of the Blackbody Cavity
There are two aspects to this: the temperature uniformity of the area viewed and the
effective spectral emissivities of the base of the cavity given the temperature
distribution of the larger part of the cavity. The temperature distributions measured
and provided23 by the supplier have been used for these assessments (see Fig 10).
The maximum variation in temperature over the 2 mm-diameter area of the base of
either of the cavities that were used is 1K. The departures from a true Planckian
spectral distribution attributed to a mean temperature for the area near 2900K of the
average of a 1K range of Planckian SPDs when normalised at 550 nm are <0.01%
from 240 to 2500 nm.
The cavities that were used have been modelled24 for the effective emissivities at the
cavity apex based of several assessments of graphite surface emissivity and the cavity
spatial temperature distributions provided by the supplier. The values assessed are
shown in Fig 11 and given in Table 18.
NMI TR 1
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Table 18. Assessed spectral emissivities for two graphite cavities operated near
2700K using two levels of emissivity for plane graphite
and temperature distributions supplied by the cavity manufacturer
Cavity
Surface emissivity
Wavelength (nm)
250
550
850
1250
1600
16–44
16–45
0.80
0.85
0.80
0.85
Cavity apex emissivity at given wavelength
.9986
.9990
.9959
.9970
.9990
.9993
.9978
.9984
.9991
.9994
.9984
.9988
.9992
.9994
.9987
.9990
.9993
.9995
.9988
.9991
The blackbody temperatures are obtained by filter radiometer ratios – for the
wavelength range 250 to 1100 nm mainly by using weighted mean values obtained
using radiometers with peak responses near 450 nm and 700 nm, and near 700 nm and
940 nm. These ‘ratio’ temperatures largely take into account the roll-off in the
emissivities at shorter wavelengths. The measurements result in ‘ratio’ or ‘effective’
temperatures that are slightly lower than the thermodynamic temperature of the
cavity, and the departures of the spectral emissivities from those of a Planckian
radiator at the same ratio temperature over this wavelength range 250 to 1100 nm are
estimated to be not more than 0.05%.
For the temperature measurements applicable to the lamp–blackbody comparisons in
the IR range 700 to 2500 nm the effective temperature was a weighted mean of ratios
using peak response wavelengths near 700 nm and 1300 nm, and 940 nm and
1540 nm. The variations in the spectral emissivities of the two cavities over this
wavelength range are much smaller than for the visible and UV range and are
expected to reduce even further at wavelengths up to 2500 nm. Again, as the effective
temperature takes these variations largely into account, the departures of the spectral
emissivities from those of a Planckian radiator at the same ratio temperature over this
wavelength range 700 to 2500 nm are estimated to be not more than 0.05%. This
uncertainty was used to calculate uncertainties for this component that are given in
Table 19.
7.2.4 Blackbody Window Transmittances
Following problems with the Cary 5 spectrophotometer measurements in the late
1990s the spectral transmittances of the blackbody window and other fused silica
windows were measured with a specially constructed optical system using the
McPherson monochromator in January 200025. Since then the Cary 5 has been
serviced and used to remeasure the blackbody window during 2001 and at the
beginning and through its use in October to November 2003. The transmittances26
measured are shown in §4.3 Fig 13.
Comparing the 2003 transmittance values with those measured mainly by the
McPherson system in 2000 (Fig 36), slight changes in the slopes in the curves are not
important, as much of the change is compensated by a change in the difference
between the effective and actual temperature of the blackbody.
NMI TR 1
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1.005
Ratio of transmittance
1
0.995
0.99
0.985
21-Aug-03
6-Nov-03
2K change
Linear (17-Oct-03)
0.98
0
500
1000
17-Oct-03
17-Nov-03
Linear (21-Aug-03)
Linear (6-Nov-03)
1500
2000
2500
Wavelength (nm)
Figure 36. Ratios of spectral transmittances of blackbody window measured with the
Cary5 spectrophotometer on indicated day during the main period of operation of the
blackbody to values measured in 2000, predominantly with the McPherson
monochromator. Also shown with three of the ratio curves are linear trend lines, and a
curve representing a change in the spectral emission of a blackbody for a 2K change
in temperature near 2900K.
Looking at the smaller structures within these curves, there are slight differences in
the spectral transmittances measured by the Cary 5 and the McPherson system
throughout the spectral range 250 to 2400 nm, of the order of 0.1%. For the lastmeasured transmittances on 17 Nov 2003 there is a larger drop in the transmittances
in the UV range.
The gradual drop in the level of the curves in Fig 36 is also unimportant to this ratio
filter-radiometry method of determining effective temperature, but it is important to
temperature measurement using the optical pyrometer. Changes in the transmittances
at 650 nm, the wavelength used by the pyrometer, were significant and corrections
were made for the changes at this wavelength.
Following comparisons of the blackbody with the lamps and after the last spectral
transmittance measurements on 17 Nov 2003, it was noticed that there were uneven
scattering deposits on the inside of the window that were more pronounced towards
the middle. The regular transmittance of the window was spatially scanned using a
narrow-band 650 nm interference filter with the detector, a beam diameter of 1 mm
and a 2 mm step interval. The transmittances relative to that at the centre of the
window27 are shown in Fig 37. There is an overall gradient across the window with a
dip in the middle that is about 5% lower in transmittance than that of the outer areas
near the edges. The gradient superimposed on this dip is about 1%/mm.
NMI TR 1
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Figure 37. Relative regular transmittances of blackbody window (central region of
figure) at 650 nm with a 1 mm diameter beam and 2 mm data spacing, after ~ 65
hours of exposure to the graphite cavity at temperatures above 2500K
The blackbody imaging system (Fig 14a) views the base of the cavity through a
circular area of the window with a diameter of 19 mm. The area that was tested for
spectral transmittances using the Cary 5 spectrophotometer was about 12 × 7 mm.
These areas were well centred on the middle of the window. The area through which
the pyrometer viewed the cavity was a circle of diameter about 3 mm. The centre of
this area is estimated to have an uncertainty of ±2 mm with respect to the centre of the
window.
The difference in the transmittances of the areas used by the imaging system and by the
spectrophotometer is calculated as about 1% at 650 nm. If the difference is constant
across the spectrum, or has a slight but regular slope, it will not be very significant for
this type of ratio filter-radiometry measurement and blackbody-lamp comparison.
However, it is possible that the non-uniformity increases at shorter wavelengths, so this
mismatch in areas used and measured may result in significant errors.
When the average transmittance over the area used by the pyrometer at the centre of
the window is compared with that measured by the Cary spectrophotometer, it is
found to be about 0.4% lower. At least the final measurements made by the pyrometer
need to be corrected for this change in tranmittance, which amounts to a correction of
the temperatures of about +1.5K.
If the area used has an uncertainty in its position with respect to the centre of the
window of up to ±2 mm, the uncertainty in the transmittance may be up to ±2%.
It was also speculated that the window deposits might include a film responsible for
the increased UV absorption seen in the lower curve in Fig 13. If this is present it may
have been oxidised after exposure to air. Prior to this, and protected by the Ar shield
gas, such a film may have had quite different spectral absorptances. Therefore,
following the window transmittance uniformity test, the central area of the inside
NMI TR 1
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surface was scanned by XPS (Specs Sage 150, by Phil Martin) for traces of metals or
elements other than those expected. The only elements identified were O, C and Si, as
were expected.
The cause of the increased UV absorption remains unknown, as does the state of the
window absorption after high-temperature graphite exposure and before any oxidation
on air exposure. The measurement of the cavity temperature through the window
under these conditions is considerably less certain using single channel optical
pyrometers and absolute radiance measurements. Future measurements using the
pyrometer should only be considered if the blackbody can be operated without a
window.
Any oxidation of the window deposit that changed its transmittance would certainly
have a significant affect on the accuracy of the pyrometer measurements, but its
affects on the measurements of lamp relative spectral irradiances by ratio-radiometry
are difficult to predict. If the transmittance changes are spectrally neutral there is no
effect. If they have a slight but regular slope there is minimal effect, as the changes
are largely compensated by the calculation of a slightly different effective
temperature. But, if they occur, they may well affect the UV spectral region to a much
greater extent.
In the study of the window transmittances in 200026, the window was cleaned with
cotton wool and isopropyl alcohol and then exposed to about 8 hours of operation of
the blackbody at ~ 2400K in a high-purity argon atmosphere and about 3 hours at ~
2800K. Following this, the window was closely examined for deposits but none were
visible. Its transmittances were then remeasured. They were close (within ~ 0.1%) to
those measured earlier through the visible and IR wavelengths but up to 0.6% higher
in the UV range. After repeat cleaning with alcohol the UV transmittances were
lowered again. A probable explanation was the deposition of a very thin antireflection layer that was easily removed. There is no information here about possible
oxidation effects that may have changed the UV transmittances even more. This
report serves merely to suggest the probability of such effects. As they are too hard to
quantify they are best avoided by dispensing with a window altogether.
In summary we have:

differences in transmittances measured with the Cary5 and McPherson
monochromator systems;

measured changes in transmittances during operation of the blackbody;

mismatches in areas used and measured;

non-uniformity in the window deposits;

the possibility that this non-uniformity will vary spectrally;

the fact that this was not identified and quantified earlier during the
comparisons; and

the possibility of changes in transmittances through oxidation of the deposits.
These conditions suggest that systematic uncertainties in the window relative spectral
transmittances, as they affect the ratio-method of temperature measurement and
relative spectral irradiance calculations, should be reasonably high in the UV spectral
range. The uncertainty in the transmittance near 650 nm, as it affects the pyrometer
measurement of temperature, should also be reasonably high.
NMI TR 1
53
7.2.4.1 Random Transmittance Uncertainties
Random uncertainties have effects both on the estimates of the blackbody effective
temperatures and then on the relative spectral irradiances of the lamps, as these are
obtained directly from individual spectral transmittance values as shown by eq. 5 in §2.
Their effects on the spectrally integrated radiometer responses have been estimated
based on their differences from linear trend lines drawn for results measured on 21
August and 6 Nov 2003 (see Fig 36). The standard deviation of the differences for
both dates was 0.026% between 250 nm and 2400 nm. The maximum was 0.13%. A
standard random uncertainty of 0.03% has been adopted and used to calculate the
uncertainties in the spectrally integrated radiometer responses that are given in Table
19. These are all less than 0.01%.
Random uncertainties in transmittances more directly affect the spectral irradiance
uncertainties. Differences between transmittance values and the trend lines that have
been examined are up to 0.08% through the range 250 to 2400 nm, except at 250 nm
and between 800 nm and 840 nm where they are up to 0.13%. These values have been
adopted as standard uncertainties in the transmittance values in the uncertainty budget
of Table 20.
7.2.4.2 Systematic Transmittance Uncertainties
A systematic uncertainty in the transmittance at 650 nm between of ±2% has been
estimated for the optical pyrometer measurements, based mainly on the uncertainty in
the position of the viewing axis of the pyrometer through the window and its
interaction with the non-uniformity of the window contamination. The affect of this
will be discussed in §7.2.5. There are no grounds at present for further increasing the
transmittance uncertainty on account of extra speculative changes due to oxidation of
deposits on the window.
Considering now the effects on the ratio filter radiometry, systematic uncertainties will
be assumed to be highest in the UV spectral region but reducing substantially into the
visible and IR regions. It will also be assumed that there will be positive correlation of
any errors within each spectral region. This will have different affects on the
uncertainties in the effective blackbody temperatures, and on the relative spectral
irradiances of the lamps that are calculated using these temperatures, the measured
lamp/blackbody spectral ratios and the window transmittances.
Blackbody temperatures from ratios using the UV radiometer #4 were ruled as
unreliable in §5.4.1 due to discrepancies with temperatures from the other radiometers,
the cause of which was unknown but could include higher changes in the window UV
transmittances. Temperature uncertainties due to systematic uncertainties in UV
spectral transmittances therefore do not have to be considered. As the changes in the
visible and IR spectral ranges shown in Figs 13 and 36 are reasonably neutral or can
be accommodated by changes in the effective blackbody temperatures, it will be
assumed that temperature uncertainties due to systematic transmittance uncertainties
are negligible.
However, the same cannot be said for their affect on the spectral irradiance
uncertainties. The relative spectral irradiances are calculated on the basis of only two
blackbody effective temperatures. The first covers the wavelength range 250 to 700 nm
NMI TR 1
54
and is estimated using radiometer signal ratios with effective wavelengths between
450 and 850 nm. The second temperature is for the range 710 to 2500 nm and is
obtained from ratios using effective wavelengths in the range 700 to 1540 nm. Any
departures of the blackbody emission through the window from the spectral power
distribution given by that effective temperature over the extended wavelength ranges
represent errors. The errors will typically increase into the blue and UV spectral region
if there is too much short wavelength selectivity in the window absorption, as is
apparent in the transmittances measured on 17 November 2003 (Fig 36).
Mainly on the basis of the changes in the window spectral transmittances seen in
measurements made on 6 November 2003, standard uncertainties have been estimated
to be 0.05% at and above 500 nm, increasing linearly with wavelength to 0.1% at
400 nm, to 0.2% at 350 nm, to 0. 3% at 300 nm and to 0.5% at 250 nm. These
values have been used in the uncertainty budget in Table 20.
7.2.5 Optical Pyrometer Temperature Measurement
The optical pyrometer was calibrated by the Temperature project28 with an estimated
uncertainty of about 2K. The calibration was done with the blackbody window to
allowed for its transmittance, using the NML reference standard pyrometer HTSP.
The stability of this pyrometer is reported to be to within ±0.5K (at 2σ) over a period
of 3 months.
The combined uncertainty in these temperature measurements by the filter radiometry
and the optical pyrometer, without allowing for window non-uniformity, is ±3.2 K for
the short wavelength calibrations29. With the additional uncertainty in the window
transmittance, of ±2% as discussed in the previous section, the uncertainty in the
pyrometer temperature measurement at 650 nm and a temperature near 2850K
increases to ±7.5 K30. The combined uncertainty in the pyrometer and filter
radiometer temperatures is then ±7.9 K. The possibility of changes in the window
transmittance due to oxidation of deposits was discussed earlier, but its occurrence is
speculative and no extra uncertainty for this is included here.
7.2.6 Lamp Operating Current
The lamps were operated using the same current shunts and digital voltmeters for the
measurements of the filter radiometer signal ratios and the comparisons of the lamp
with the blackbody. Therefore, the effect of any small systematic errors in the current
measurement cancel out as far as they affect the determination of the blackbody
temperatures. The effects of lamp current errors (departures from target current) are
reduced to negligible levels by monitoring the lamp current during the
spectroradiometer measurements and correcting for the small errors as described in
§5.3.1. The uncertainties after making these corrections are assessed as negligible.
NMI TR 1
55
7.2.7 Total Uncertainties in Calibrations of the Filter Radiometers and
Corresponding Temperature Uncertainties from Pairs of Radiometers
The various random uncertainties in the calibrations of the filter radiometer responses
to spectral flux inputs that have been discussed above have been added in quadrature
and are given in Table 19 together with the assessed degrees of freedom.
The systematic uncertainties have been left separate, as when calculating the total
uncertainty in the ratios for two radiometers they need to be combined according to
the type of correlation between the wavelength error-related uncertainties for each
radiometer.
Total uncertainties for the radiometer combinations are given in Table 19 together
with the temperature uncertainties that they would represent for a blackbody operated
at a temperature of about 3000 K.
7.3
Weighted Mean Blackbody Temperatures and Uncertainties
As indicated in §5.4.1, weighted mean temperatures were calculated from two
temperatures using two pairs of radiometers according to the equation:
TAV   (Ti / U i ) /  (1 / U i )
2
2
(19)
where Ui is the uncertainty in the temperature from one of the pairs of radiometers
that has been selected.
The uncertainty in the weighted mean temperature is given by:
U AV  (1 /  (1 / U i )
2
(20)
Using the uncertainties for each radiometer from Table 19 the standard uncertainties
in the weighted mean temperatures for measurements using the filter radiometers #3,
#6N and #8 (450 to 940 nm) are calculated31 as ±1.7 K, and for measurements using
the radiometers #6N, #8, #1 and #9 (700 to 1540 nm) are ±3.2 K.
The uncertainties in the spectral irradiances of the lamps that were compared with the
blackbody, normalised to unity at 555 nm, have been calculated on the basis of these
uncertainties in weighted mean temperatures and are given in Table 20.
7.3.1 Temperature Uncertainties are Uncorrelated for Different Lamp Types
The uncertainties given in Table 19 for the spectral response calibrations of the filter
radiometers are correlated when it comes to comparing temperatures measured via
different lamps. However, if the lamp types are different, temperature discrepancies
may arise due to sphere non-uniformity component uncertainties that are uncorrelated.
The uncertainties in temperature due to random transfer uncertainties plus
uncertainties due to radiometer sphere spatial non-uniformity as this affects the
measurement of ratios from different types of lamps have been assessed32 as:

for weighted mean temperatures from the use of radiometers (peak wavelengths)
450 / 700 nm and 700 / 940 nm: ±1.1 K; and

for use of radiometers 700 / 1300 nm and 940 / 1540 nm: ±1.5 K.
NMI TR 1
56
Table 19. Assessed uncertainties in the integration of the lamp spectral flux by each filter radiometer due to uncertainties in the blackbody
window, blackbody emissivity, radiometer calibrations and variations in the lamp and radiometer-sphere input geometry
Source of uncertainty
(Reference
paragraph)
Standard Uncertainties and degrees of freedom for given radiometer (%)
Radiometer
Effective wavelength (nm)
#4
340
DOF
#3
450
DOF
#11
550
DOF
#6N
700
DOF
#8
940
DOF
#1
1300
DOF
#9
1530
DOF
Uncorrelated uncertainties due to
§7.2.1
ratios of lamp/blackbody
§7.2.4
transmittances of BB window
§7.2.3
emissivity of blackbody
§7.1.1.1 calibration of reference detector
§7.1.2.1 integrated main response band
§7.1.2.1-2 integrated wing response
§7.1.2.3 sphere geometric factors
§7.1.2.5 temporal drift
§7.1.3
non-linearity of detector
§7.1.4
amplifier gain ratio
Total uncorrelated uncertainty
%
0.034
0.007
0.011
0.030
0.091
0.067
0.120
0.017
0.030
0.100
0.201
1000
1000
1000
1000
5
1000
1000
1000
100
100
110
%
0.019
0.006
0.009
0.022
0.018
0.144
0.120
0.029
0.050
0.020
0.201
1000
1000
1000
1000
3
1000
1000
1000
100
100
2189
%
0.019
0.006
0.009
0.011
0.020
0.023
0.120
0.115
0.050
0.020
0.179
1000
1000
1000
1000
3
1000
1000
1000
100
100
2074
%
0.018
0.005
0.009
0.015
0.015
0.020
0.120
0.058
0.050
0.020
0.148
1000
1000
1000
1000
3
2
1000
1000
100
100
1251
%
0.017
0.005
0.008
0.017
0.029
0.030
0.120
0.029
0.050
0.020
0.143
1000
1000
1000
1000
1000
2
1000
1000
100
100
610
%
0.066
0.007
0.011
0.035
0.058
0.277
0.230
0.115
0.040
0.020
0.392
1000
1000
1000
1000
5
4
1000
1000
100
100
16
%
0.049
0.004
0.006
0.024
0.052
0.086
0.120
0.058
0.100
0.020
0.203
1000
1000
1000
1000
2
2
1000
1000
100
100
53
Correlated uncertainties due to
§7.1.1.2 wlgth error in ref det calibration
§7.1.2.4 wlgth error in transfer to test det
Total correlated uncertainty due to wavelength
0.003
-0.279
-0.276
1000
1000
1000
0.035
-0.063
-0.028
1000
1000
1000
0.025
-0.038
-0.012
1000
1000
1000
0.019
-0.012
0.007
1000
1000
1000
0.009
0.002
0.011
1000
1000
1000
0.010
0.008
0.018
1000
1000
1000
-0.011
0.009
-0.002
1000
1000
1000
Taking into account correlations and signs:
Combined uncertainties for ratios of
(peak wavelengths - nm)
Uncertainty in ratio of integrated response (%)
Equivalent temperature uncertainty
(K)
NMI TR 1
#3 / #6N
(450 / 700)
0.252
1.9
#11 / #6N
(550 / 700)
0.233
3.5
#6N / #8
(700 / 940)
0.206
3.4
#6N / #1
(700 / 1300)
0.420
3.8
#8 / #1
(940 / 1300)
0.418
8.7
# 1 / #9
(1300 / 1540)
0.442
24
#8 / #9
(940 / 1540)
0.249
3.7
57
7.4
Uncertainties in Lamp Relative Spectral Irradiances
The uncertainties in the lamp relative spectral irradiances result mainly from the
blackbody temperature uncertainties but at each wavelength must also include random
transfer uncertainties from the blackbody to the lamp, blackbody window
transmittance uncertainties, lamp current uncertainty, effects of wavelength
uncertainties, spectroradiometer detector non-linearity etc. These uncertainties are
combined in an Excel spreadsheet33 and are given together with the combined values
at representative wavelengths in Table 20.
7.4.1 Uncertainties due to Blackbody Temperature Uncertainty
These have been calculated32 for the temperature uncertainties for the different
spectral ranges as given in §7.3, and are given in Table 20.
7.4.2 Random Transfer Uncertainties
These were shown earlier in Figs 34 and 35. They become significant below 300 nm
and in the IR above about 1100 nm. The calculated standard uncertainties from these
multiple measurements are included in Table 20.
7.4.3 Uncertainties due to Blackbody Window Transmittance Uncertainties
These include both random and systematic uncertainties. They were discussed at
length in §7.2.4.1 and §7.2.4.2 and values are given at each wavelength in Table 20.
7.4.4 Uncertainties due to Lamp Current Uncertainty
Small random errors in current have been dealt with by taking simultaneous
measurements of current with the filter radiometer readings or the spectroradiometer
readings. In this way the levels of uncertainty due to the random current uncertainties
are assessed as negligible (<0.01%).
The contribution of lamp current systematic error to the blackbody temperature
uncertainties was dismissed as negligible earlier, as the same current measurement
system was used for both the filter radiometer signal ratio measurements and during
the spectral comparisons of the lamps with the blackbody. As future uses of these
lamps may involve different circuit components (current shunt, voltmeter), allowance
must be made for the uncertainty in the absolute current at the time of calibration.
The estimated standard uncertainty in the resistance of the current shunt is 0.01%. The
standard uncertainty in the current measurement by the voltmeter is also assessed as
0.01%, resulting in a combined uncertainty of 0.014%. The affect of this depends on
the lamp wavelength and corrections are made as described in §5.3.1, according to the
normalisation wavelength of the relative spectral distribution being measured. The
uncertainties calculated for this combined current uncertainty of 0.014% are given in
Table 20.
NMI TR 1
58
7.4.5 Wavelength Uncertainties
The monochromators were calibrated using multiple Hg lamp spectral lines for each
setup and comparison. The uncertainties in the wavelength settings are assessed as
0.1 nm over the range 240 to 1100 nm and 0.2 nm over the longer IR wavelength
range. The wavelength reproducibility was however much better than this with
uncertainties estimated not to exceed 0.05 nm.
The spectra of the sources being compared are quite similar, being tungsten halogen
lamps running at distribution temperatures of about 3000 to 3200K compared with
one another or with the blackbody at between 2800 and 2950K. Errors in the
measured spectral irradiance ratios when normalised at 550 nm are estimated not to
exceed 0.05% at UV wavelengths and 0.02% at visible and IR wavelengths as given
in Table 20. These uncertainties expected to each have a large DOF: set to 1000.
7.4.6 Spectroradiometer detector non-linearity
The detectors used with the spectroradiometer were a Hamamatsu R562
photomultiplier, a Hamamatsu S1337 Si photodiode, a Telcom Devices 35PD10M
InGaAs photodiode and an NEP type D2 10x5 mm PbS detector with DMC7 controller.
These have all been tested for linearity, and no non-linearity greater than ~ 0.02% for
the PMT and photodiodes, or ~ 0.2% for the PbS detector, for 5:1 signal ranges was
found. However, for these comparisons the irradiance levels for the target plates from
the blackbody and the lamps, or the primary and secondary standards being compared,
were well matched and were rarely more than 50% different. Therefore the uncertainties
for this source of error given in Table 20 are assessed as 0.02%.
7.4.7 Summation of uncertainties in relative spectral irradiances
The total uncertainties from Table 20 for the relative spectral irradiances of the lamps
compared directly with the blackbody are shown from 240 to 2500 nm in Fig 38.
3.0
Standard uncertainty (%)
2.5
2.0
1.5
1.0
0.5
0.0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
Wavelength (nm)
Figure 38. Standard uncertainties in the primary reference lamp spectral irradiance
relative to that at 555 nm
NMI TR 1
59
Table 20. Estimated uncertainties in the spectral irradiances of the primary standard lamps calibrated using the blackbody
and filter radiometers in establishing the NML2003 spectral irradiance scale, relative to the irradiance at 555 nm
Wavelength
(nm)
Type
240
250
260
270
280
290
300
310
320
330
340
350
360
370
380
390
400
450
500
550
555
600
650
NMI TR 1
Uncertainties at 1 S.D. of spectral irradiance values relative to that at 555 nm due to
uncertainty in
Transfers
Lamp
SpectroBlackbody window blackbody current Wavelength
NML2003
error
and
radiometer
transmittance
blackbody
to
for
reproducdetector
temperature
reference spectral
ibility
nonlinearity
random systematic
lamps
tests
B (%)
A (%)
B (%)
A (%)
B (%)
B (%)
B (%)
0.680
0.15
0.6
2.8
0.113
0.05
0.02
0.632
0.13
0.5
0.8
0.105
0.05
0.02
0.588
0.03
0.45
0.5
0.097
0.05
0.02
0.547
0.03
0.4
0.45
0.091
0.05
0.02
0.508
0.03
0.35
0.4
0.084
0.05
0.02
0.473
0.03
0.32
0.3
0.078
0.05
0.02
0.440
0.03
0.3
0.2
0.073
0.05
0.02
0.409
0.03
0.28
0.15
0.068
0.05
0.02
0.380
0.03
0.26
0.15
0.063
0.05
0.02
0.353
0.03
0.24
0.15
0.058
0.05
0.02
0.327
0.03
0.22
0.15
0.054
0.05
0.02
0.303
0.03
0.2
0.15
0.050
0.05
0.02
0.280
0.03
0.18
0.15
0.046
0.05
0.02
0.258
0.03
0.16
0.15
0.043
0.05
0.02
0.238
0.03
0.14
0.15
0.039
0.05
0.02
0.219
0.03
0.12
0.15
0.036
0.05
0.02
0.200
0.03
0.1
0.1
0.033
0.02
0.02
0.121
0.03
0.075
0.1
0.020
0.02
0.02
0.057
0.03
0.05
0.1
0.009
0.02
0.02
0.005
0.03
0.05
0.1
0.001
0.02
0.02
0
0
0
0
0
0
0
0.039
0.03
0.05
0.1
0.006
0.02
0.02
0.075
0.03
0.05
0.1
0.013
0.02
0.02
(continued)
Total standard uncertainty
A (%)
2.80
0.81
0.50
0.45
0.40
0.30
0.20
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.10
0.10
0.10
0.10
0
0.10
0.10
B (%)
0.92
0.81
0.75
0.69
0.63
0.58
0.54
0.50
0.47
0.43
0.40
0.37
0.34
0.31
0.28
0.26
0.23
0.15
0.08
0.06
0
0.07
0.10
A+B (%)
2.95
1.15
0.90
0.82
0.74
0.65
0.58
0.53
0.49
0.46
0.43
0.40
0.37
0.35
0.32
0.30
0.25
0.18
0.13
0.12
0
0.13
0.14
D.O.F.
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
60
Table 20 (continued) Estimated uncertainties in the spectral irradiances of the primary standard lamps calibrated using the blackbody
and filter radiometers in establishing the NML2003 spectral irradiance scale, relative to the irradiance at 555 nm
Wavelength
(nm)
Type
700
750
800
850
900
950
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
NMI TR 1
Uncertainties at 1 S.D. of spectral irradiance values relative to that at 555 nm due to
uncertainty in
Transfers
Lamp
SpectroBlackbody window blackbody current Wavelength
NML2003
error
and
radiometer
transmittance
blackbody
to
for
reproducdetector
temperature
reference spectral
ibility
nonlinearity
random systematic
lamps
tests
B (%)
A (%)
B (%)
A (%)
B (%)
B (%)
B (%)
0.107
0.03
0.05
0.1
0.018
0.02
0.02
0.160
0.03
0.05
0.1
0.022
0.02
0.02
0.207
0.13
0.05
0.1
0.026
0.02
0.02
0.248
0.03
0.05
0.1
0.030
0.02
0.02
0.284
0.03
0.05
0.1
0.033
0.02
0.02
0.317
0.03
0.05
0.1
0.036
0.02
0.02
0.346
0.03
0.05
0.1
0.038
0.02
0.02
0.395
0.03
0.05
0.3
0.042
0.02
0.02
0.436
0.03
0.05
0.3
0.046
0.02
0.02
0.470
0.03
0.05
0.3
0.049
0.02
0.02
0.499
0.03
0.05
0.35
0.052
0.02
0.02
0.524
0.03
0.05
0.4
0.054
0.02
0.02
0.545
0.03
0.05
0.4
0.056
0.02
0.02
0.563
0.03
0.05
0.4
0.058
0.02
0.02
0.579
0.03
0.05
0.4
0.059
0.02
0.02
0.594
0.03
0.05
0.4
0.061
0.02
0.02
0.606
0.03
0.05
0.4
0.062
0.02
0.02
0.618
0.03
0.05
0.5
0.063
0.02
0.02
0.628
0.03
0.05
0.5
0.064
0.02
0.02
0.637
0.03
0.05
0.5
0.065
0.02
0.02
0.645
0.03
0.05
0.5
0.066
0.02
0.02
0.653
0.03
0.05
0.5
0.067
0.02
0.02
Total standard uncertainty
A (%)
0.10
0.10
0.16
0.10
0.10
0.10
0.10
0.30
0.30
0.30
0.35
0.40
0.40
0.40
0.40
0.40
0.40
0.50
0.50
0.50
0.50
0.50
B (%)
0.12
0.17
0.22
0.26
0.29
0.32
0.35
0.40
0.44
0.48
0.50
0.53
0.55
0.57
0.59
0.60
0.61
0.62
0.63
0.64
0.65
0.66
A+B (%)
0.16
0.20
0.27
0.28
0.31
0.34
0.37
0.50
0.54
0.56
0.62
0.66
0.68
0.70
0.71
0.72
0.73
0.80
0.81
0.81
0.82
0.83
D.O.F.
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
61
8
8.1
COMPARISON OF TEMPERATURE UNCERTAINTIES WITH TEMPERATURE
DISCREPANCIES
Temperature Discrepancies
Four types of temperature discrepancies have been identified in these measurements:

temperature differences resulting from the use of different pairs of radiometers in the
same sphere configuration;

temperature-related mismatch between spectra measured in an overlap range within the
two separate spectral ranges of lamp–blackbody comparisons;

differences in calibrations of secondary lamps obtained from different reference lamps
that appear to be directly temperature-related; and

differences between pyrometer-measured temperatures (corrected for window nonuniformity) and filter radiometer weighted-mean temperatures.
From results given in §5.4.1, 5.4.2, 6.2 and 6.3, these differences are compared in Table 21
with the weighted-mean temperature uncertainties given above.
Table 21. Comparison of blackbody weighted-mean temperature uncertainties from filter
radiometer calibration uncertainties with temperature discrepancies found in the
measurements
Values (K) for
Differences
Temperature
Temperature
Temperature
between
differences
differences
semi-range from 4
radiometer
corresponding to corresponding to
Spectral to 5 different pairs
weighted-mean
spectral mismatch differences in
range (nm) of radiometers
values and
in overlap range
calibrations
across 6 to 8
pyrometer
700 to 1100 nm for from different
lamps
measured
same lamp
reference lamps
(from Table 5)
temperatures
(from Table 8)
(from §6.3)
(from Table 7)
240–1100
±2.2 to ±6.3
Calculated
uncertainty
due to
radiometer
calibration
uncert. (K)
–0.7 to +3.5
+1 to +12
±1.7
–2 to +3
+6 to +13
±3.2
–5 to +4
700–2500
±4.8 to ±9.6
8.1.1 Comparison with Discrepancies from Different Filter Radiometers
For the shorter spectral range calibrations, the 1.7K uncertainty in weighted mean temperature
compares with temperature semi-ranges of from 2.2K to 6.3K, with a mean semi-range of
±5.1K. This assessed uncertainty does not adequately cover this range, although the combined
uncertainties for some of the other radiometer pairs such as #3 / #11 (450 / 550 nm)
correspond to temperature uncertainties in excess of ±5K.
For the longer wavelength calibrations, the 3.2K uncertainty compares with semi-ranges of
from 5.1K to 9.6K, with a mean of ±6.4K. Again, the assessed uncertainty in the weighted
mean temperature does not appear to cover the discrepancies adequately. The temperature
uncertainties corresponding to the combined uncertainties in each of the filter pair ratios
(excluding #1 / #9: 1300 / 1540 nm) vary from 3K to 11K with an average uncertainty of
5.8K. This is close to the mean semi-range.
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8.1.2 Comparison with Spectral Overlap Temperature Discrepancies
When comparing results from a lamp for the short wavelength range and the IR range over the
overlapping spectral range, the temperature uncertainties for each range need to be added in
quadrature for comparison with a mismatch in the spectra that appears to be mainly due to a
temperature discrepancy. The combined uncertainty in temperature is ±3.6 K.
This standard uncertainty is considered to adequately cover the range of the differences
measured, from –5 K to +4 K and a mean value of –1K.
8.1.3 Comparison with Apparent Temperature Discrepancies in Different Reference
Lamp Calibrations
For apparent differences in calibration temperatures of secondary lamps from different
reference lamps (as seen in Figs 26 to 29), figures are only available from the three lamps
used, representing three types with different filament and envelope geometries. For the
shorter-wavelength calibrations up to 1100 nm, the three temperature differences are –0.7K,
+3.5K and +4.2K. For any two lamps, the combined uncertainty in the temperature using only
the random and geometric component uncertainties in the radiometer calibrations is ±1.6 K
(refer to §7.3.1). This uncertainty is considered somewhat low to cover the differences of up
to 4.2 K found in the measurements.
For the longer-wavelength calibrations the three temperature differences are –2 K,
+3 K and +5 K. The combined temperature uncertainty for two lamps for the radiometers used
is ±2.1 K. This uncertainty is considered to marginally adequately cover the temperature
discrepancies from the IR range measurements.
8.1.4 Comparison with the Pyrometer – Filter-Radiometer Temperature Differences
The average difference between the calculated temperatures based on the filter radiometers
and the pyrometer-measured temperatures, for five lamps compared with the blackbody just
before the final window transmittance tests on 17 Nov 2003, was 11.5 K + 1.5 – 2.5K.
In the discussion of the blackbody window transmittances in §7.2.4 it was concluded that
there should be a correction of the pyrometer-measured temperatures of about +1.5 K for
mismatch of the window area used by the pyrometer and the area measured by the
spectrophotometer and the non-uniformity of the contamination. That was on the assumption
that the area that was used was at the centre of the window. That correction reduces the
average discrepancy between the filter radiometer temperatures and the pyrometer
temperature to about 10 K.
In §7.2.4 there was further discussion about the possible mis-alignment of the pyrometer
viewing axis with the centre of the window, of up to ±2 mm. This corresponds to possible
transmittance variations of ±2% which in turn corresponds to temperature uncertainties of
±7.5 K. Therefore, the discrepancy of about 10 K could be reduced down to 2.5K using this
standard uncertainty. This remaining difference is about the same as the uncertainty in the
filter radiometer measurement.
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8.1.5 Discussion of Discrepancies
The consistency of the measurements using the filter radiometers is variable, but differences
are generally close to the assessed uncertainties and certainly within 2σ values.
For the discrepancies with the optical pyrometer, the large uncertainty in the window
transmittance dominates and effectively obscures smaller systematic uncertainties of the order
of 1% that may be present.
9
THE NML2003 SCALE OF SPECTRAL IRRADIANCES
This report is primarily about the establishment of the NML2003 scale of relative spectral
irradiance. This is because it has been continuing practice at NML to measure the lamp
spectral power distributions only in relative units, and then by measurement of spectrallyintegrated irradiance at a specific distance to obtain the lamp absolute spectral irradiances in
SI units.
The spectrally-integrated irradiance is obtained by measuring the lamp illuminances. This step
has several advantages. In the current NML1997 scale of illuminance34, the uncertainty in the
NML base unit, the candela, is quite low at 0.17% (k = 1), and this has been demonstrated as
having remained very stable over many years. Transfers from this scale can be made very
quickly and accurately compared with the transfer times and uncertainties associated with
spectral measurements. Spectral irradiance lamps can be checked, at least for their
illuminance, on a more regular basis and with more accuracy than their spectral irradiances. If
changes are observed then possible changes in lamp temperature and spectrum can also be
investigated.
The uncertainties associated with this luminous intensity scale and its transfer to the spectral
lamps have been added to the uncertainty budgets for the calibrations of the spectral
irradiance key comparison transfer lamps, which will now be discussed.
9.1
Calibration of Working Standards and Key Comparison Transfer Standards
Calibrations of the NML working standard lamps and the key comparison transfer standard
lamps are done in the same way and incur similar uncertainties, so only the calibrations of the
transfer standards will be discussed here.
The illuminances of all lamps are measured35 individually by reference to a group of luminous
intensity standards. The NML2003 scale of relative spectral irradiance has been transferred36
to these lamps by comparing each with three primary lamp standards that had been calibrated
from the blackbody. In doing this and by averaging the results, the resulting uncertainties
associated with both random and systematic type errors in the calibrations of the references
are reduced, but countering this improvement is the extra mostly random transfer uncertainty
at each wavelength in this additional step.
The uncertainties in the relative spectral irradiances of the transfer standards that have now
been assessed based on the primary reference standard lamp uncertainties given in Table 20
have been added to the same additional uncertainties incurred in the illuminance
measurements to obtain total uncertainties in spectral irradiances that are regarded as the
current best estimates. These32 are given in Table 22 also shown from 250 to 2500 nm.
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Table 22. Uncertainties in spectral irradiances of CCPR key
comparison transfer standards using best estimates of uncertainties
in relative spectral irradiances based on the NML2003 scale as
assessed in November 2004
Standard uncertainty (%) in
Standard uncertainty (%) in
Luminous
Relative
Luminous
Relative
intensity
spectral
Total
intensity
spectral
Total
WaveWaveunit and
irradiance uncertainty
unit and
irradiance uncertainty
length
length
transfers of units and in spectral
transfers of units and in spectral
(nm)
(nm)
illuminance transfers
irradiance
illuminance transfers
irradiance
to lamps
to lamps
to lamps
to lamps
240
250
260
270
0.24
0.24
0.24
0.24
5.14
1.42
0.98
0.88
5.15
1.44
1.01
0.91
700
750
800
850
0.24
0.24
0.24
0.24
0.16
0.20
0.27
0.28
0.29
0.31
0.36
0.36
280
290
300
310
320
330
340
350
360
370
380
390
400
450
500
550
555
600
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.79
0.66
0.58
0.53
0.50
0.46
0.44
0.41
0.38
0.35
0.33
0.30
0.26
0.18
0.14
0.12
0.00
0.13
0.82
0.70
0.63
0.58
0.55
0.52
0.50
0.47
0.45
0.42
0.40
0.38
0.35
0.30
0.27
0.26
0.24
0.27
900
950
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.31
0.34
0.37
0.50
0.55
0.57
0.63
0.69
0.69
0.70
0.72
0.73
0.75
0.81
0.85
0.88
0.86
0.96
0.39
0.42
0.44
0.56
0.60
0.62
0.67
0.72
0.73
0.74
0.75
0.77
0.79
0.85
0.88
0.91
0.89
0.99
650
0.24
0.15
0.28
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10
CONCLUSIONS AND RECOMMENDATIONS
1.
The blackbody window turned out to present dominant uncertainties in the optical
pyrometer measurements and made significant contributions in the UV spectral range to
the uncertainties in the lamp spectral irradiances.
2.
There are discrepancies between the blackbody temperatures measured by the filter
radiometers and those measured by the optical pyrometer, of about 10 K and varying
according to the type of lamp involved in the transfer. At present the most likely cause
is probable errors in the blackbody window transmittance.
3.
The spectral responses of the filter radiometers were far from ideal due to instability of
the filters. The filters themselves are too small to prevent atmospheric penetration of the
sealed edge extending too quickly throughout the area of the filter and affecting both the
peak transmittance and wing and blocking transmittances of the filter.
4.
Although the mirror optical system is assumed to have worked correctly to obviate the
need to know its relative spectral reflectances, there may be sufficient spatial separation
of images of different parts of the lamps, and even the blackbody, to cause some
spectral non-uniformity in the images that are probed by the spectroradiometer – leading
to small systematic errors.
5.
The uncertainties may in the future be lowered by viewing the blackbody cavity and the
lamps directly with filter radiometers. Details of the viewing conditions will not be
canvassed here. The filter radiometers could be scaled up in size and mounted in a
larger integrating sphere, or irradiated directly and in turn, but each method has its own
drawbacks.
6.
If possible, the blackbody should in future be used without a window. Apart from the
possible transmittance changes at 650 nm as used by the optical pyrometer, the UV
transmittances have even higher uncertainties due to contamination and possible
changes after air exposure. For the work reported here these uncertainties have
prevented the use of measurements made with the UV filter radiometer which probably
had the highest stability of the radiometers that were used.
11
REFERENCES
1
Spectroradiometry: Blackbody Project book B LLN/0359, pages 108-114.
Spreadsheet H:\Sp irradiance project\Final documents/SPHERERADMay04.xls
3
‘Sphere radiometer measurements of standard lamp spectral irradiances in September 2000’.
F Wilkinson, 21 June 2001 F15/7-2001/3
4
Spectroradiometry: Blackbody Project book B LLN/0359, pages 107-111.
5
Supplied by Institut für Kernenergetik, Universität Stuttgart, Germany.
6
‘A new graphite cavity radiator as blackbody for high temperatures’, Groll M and Neuer G
in Temperature, its Measurement and Control in Science and Industry, Vol 4, part 1, 449-456
(1978)
7
M.J. Ballico, ‘Modelling of the effective emissivity of a graphite tube blackbody’,
Metrologia 32, 259-265 (1995/96)
8
Spectral transmittances of IKE blackbody window – a preliminary report, F Wilkinson, 4
June 2001, F15/7-2001/2
2
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9
Spreadsheet H:/Sp irradiance project/BB window/BB window 2003/BBwindow
transmittances Nov 2003.xls
10
Optical design provided by Dr W H Steel in private communications.
11
Spectroradiometry: Blackbody Project book C LLN/0529, page 14.
12
Spectroradiometry: Blackbody Project book C LLN/0529, page 10–18.
13
Program BBSCAN.BAS ver FW 16/09/03.
14
Timax #12631, LN68125
15
Test Method 15 in Quality System
16
Spreadsheet haea\Sp irradiance project\Final documents/SPHERERAD Uncert Nov04.xls
17
Spreadsheet haea\Sp irradiance project\Final documents/SPHERERADAug03.xls
worksheet ‘rad #4’ cells CY131…
18
Spreadsheet haea\Sp irradiance project\Final documents/SPHERERADMay04.xls
worksheet ‘rad #3’ cells AF71…
19
Workbook ‘Spectral irradiance development – Blackbody project book C’ page 52.
20
Spectroradiometry: Spectral irradiance scale development/Blackbody project LLN book C
pages 1-5.
21
Radiometry: Internal calibrations. Book 1. LLN/0182. Pages 17224-6.
22
Laboratory workbook ‘Blackbody project’ book C pages 13-15.
23
Temperature profile graphs provided by IKE, Germany accompanying supply of cavities
nos. 16-44, 16-45, 16-46, September 1992.
24
‘Analysis of F Wilkinson’s high temperature graphite blackbody’, M Ballico, private
communication, 16 August 1999.
25
‘Spectral transmittances of IKE blackbody window – a preliminary report’, F Wilkinson,
June 2001, F15/7-2001/2.
26
Spreadsheet haea\Sp irradiance project\Final documents/BB window transmittances Nov
2003.xls.
27
Spreadsheet H:\Sp irradiance project\Final documents/Window uniformity Feb04.xls.
28
File GB33/NML-1/22
29
Spreadsheet haea\Sp irradiance project\Final documents/SPHERERADMay04.xls
worksheet ‘FEL3 ratios’ cells H145…
30
Spreadsheet H:\Sp irradiance project\Final documents/SPHERERADMay04.xls worksheet
‘FEL4 ratios’
31
Spreadsheet haea\Sp irradiance project\Final documents/SPHERERADMay04.xls
worksheet ‘FEL2 ratios’ cells E80…
32
Spreadsheet haea\Sp irradiance project\Final documents/SPHERERAD May04.xls
worksheet ‘FEL2 ratios’ cells E131 …
33
Spreadsheet haea\Sp irradiance project\Final documents/NML2003 sp irrad scale
uncertainties rev Nov2004.xls
34
J. L. Gardner, D. J. Butler, E. G. Atkinson and F. J. Wilkinson, ‘New basis for the
Australian realisation of the candela‘ Metrologia 35, 235-239(1998).
35
NML Quality System Test Method PM-RAD-TM4 ver. 2.2
36
NML Quality System Test Method PM-RAD-TM15 ver. 2.2
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