HSE
Health & Safety
Executive
Investigation of a measurement technique to determine
the apparent source size for light emitting diodes
Prepared by National Physical Laboratory and Europtics Ltd
for the Health and Safety Executive 2005
RESEARCH REPORT 345 HSE
Health & Safety
Executive
Investigation of a measurement technique to determine
the apparent source size for light emitting diodes
Simon Hall
Laura Crane
David Gibbs
National Physical Laboratory
Hampton Road
Teddington
Middlesex
TW11 0LW
Brooke Ward
Europtics Ltd
Current ocular safety standards associated with the application of light emitting diodes (LED), and
other intermediate sources, cite the angular subtense of the apparent source as an essential quantity
for optical hazard assessment. Under these standards, the angular subtense parameter is calculated
from the apparent source size of the LED package and the specified most hazardous viewing distance.
However, an international standard for the measurement of the apparent source size parameter does
not yet exist.
This report describes the results of a study that provide rigorous practical support for a technique
proposed for the measurement of apparent source size when observed from the most hazardous
viewing distance. The results of this study allow, for the first time, an estimate of the potential optical
hazard of LEDs and other intermediate sources, in accordance with current safety standards. This is a
significant step in reducing the ambiguity that currently exists in the application of these optical safety
standards. The results also verify earlier numerical modelling of an improved method for the estimation
of the critical angular subtense parameter for extended sources, such as LEDs and intermediate
sources.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its
contents, including any opinions and/or conclusions expressed, are those of the authors alone and do
not necessarily reflect HSE policy.
HSE BOOKS
© Crown copyright 2005
First published 2005
ISBN 0 7176 6108 3
All rights reserved. No part of this publication may be
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Applications for reproduction should be made in writing to:
Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to hmsolicensing@cabinet-office.x.gsi.gov.uk
ii
ACKNOWLEDGEMENTS
We would like to acknowledge the software development expertise provided by Oxford
Framestore Applications Ltd.
iii
iv
CONTENTS
Executive Summary......................................................................................................... v ii
1
Introduction .............................................................................................................. 1
2
Theory....................................................................................................................... 2
2.1
Angular subtense .............................................................................................. 2
2.2
Beam measurements ......................................................................................... 5
2.3
The optical system ............................................................................................ 6
3
Description of Apparatus.......................................................................................... 7
3.1
Initial System Design ....................................................................................... 7
3.2
8-Bit System Design......................................................................................... 7
3.3
12-Bit System Design....................................................................................... 7
4
Measurement Procedure ......................................................................................... 11
4.1
Preparation for measurement.......................................................................... 11
4.2
Calibration of CCD array and associated equipment ..................................... 11
4.3
LED beam width measurement ...................................................................... 11
4.4
Transform Validation Experiment.................................................................. 14
5
Results .................................................................................................................... 15
5.1
Initial results ................................................................................................... 15
5.2
8-Bit Transform Validation Experiment results ............................................. 17
5.3
12-Bit Transform Validation Experiment results ........................................... 20
5.4
Yellow LED - Ligitek LUY 3833/A29 .......................................................... 25
5.5
Blue LED - Nichia NSPB500 Rank WS ........................................................ 27
5.6
Green LED - Nichia NSPG500 Rank GS ....................................................... 29
5.7
Red LED - Kingbright L-53SRC/E ................................................................ 31
5.8
White LED - Nichia NSPW500 Rank BS ...................................................... 33
5.9
Orange LED - Toshiba TLOH190P................................................................ 35
5.10 High Power Blue LED - Luxeon Star............................................................. 37
6
Uncertainty Analysis .............................................................................................. 39
7
Conclusions ............................................................................................................ 46
7.1
Future directions ............................................................................................. 50
Appendix 1: Second moment, azimuth and principle width derivation ......................... 51
Appendix 2: Design And Technical Specification For A Facility To Determine The Apparent Source Size Of Light Emitting Diodes ........................................................... 53
Appendix 3: LED techinical data sheets......................................................................... 58
References ...................................................................................................................... 59 Glossary .......................................................................................................................... 61
v
vi
EXECUTIVE SUMMARY
The work detailed in this report was commissioned to allow the optical hazard level of light
emitting diodes (LEDs), and more laser-like intermediate sources, to be quantified. The
dramatic increase in the use of superbright LEDs for consumer, medical and industrial
applications necessitates a responsible assessment of the hazard presented by these devices.
The International Electrotechnical Committee (IEC) and Commission Internationale de
l'Eclairage (CIE) cite that the angular subtense of the apparent source is an essential quantity for
the assessment of optical hazard. Under current optical hazard safety standards the angular
subtense parameter is calculated from the apparent source size and a specified most hazardous
viewing distance. However an international standard for the measurement of the apparent
source size parameter does not exist.
The aim of this current study is to provide rigorous practical support for a technique proposed
for the measurement of apparent source size when observed from the most hazardous viewing
distance. Development of the practical technique required the recognition of the apparatus
limitations and the development of strategies to overcome these limiting factors. Both an 8-bit
and 12-bit system were tested. The 12-bit systems’ superior dynamic range and cooled array
highlighted the effect of stray light and noise. This demonstrated the need for a large dynamic
range in the measurement facility to measure second moment beam diameters effectively.
A validation experiment suggested by the International Standards Organisation (ISO/TC 172/SC
9) comprehensively verified the suitability of the technique. It is therefore proposed that the
results of this work should be used to underpin the adoption of this methodology within
international standards for the assessment of the optical hazard potential of LEDs and other
intermediate sources.
The report highlights the following:
x Technical specification of the critical components and the design of a facility for the
measurement of apparent source size of LEDs and intermediate sources.
x Verification of an 8-bit and a 12-bit apparent source size measurement facility. This was
achieved by computerized processing of images of spatial beam profile using a
converging second moment method.
x High level of agreement between the propagation parameters derived through the 8-bit
and 12-bit methods using the IR LED. This was an unexpectedly good correlation
between results, considering the dynamic range limitations of the 8-bit camera.
x Evaluation of the astigmatic state of the beam by analysis of the change of azimuth as
the beam propagates. This was carried out by azimuth determination of the beam by the
comparison of the second moment widths in perpendicular axes.
x Measurement of a selection of 8 LEDs with differing peak emission wavelengths,
construction and beam propagation characteristics.
x Visualisation of real beam propagation using a montage of beam images and spatial
profiles related to the propagation envelope for one of the LEDs.
x Effective demonstration that the point in the beam envelope where a sharp image of the
electronic structure of the LED is obtained does not necessarily correspond to the beam
waist or location of the apparent source.
vii
x Identification of general astigmatism (as opposed to simple astigmatism) in the output
beam from one of the LEDs.
x Populated angular subtense contour plot with results from this work. This plot enables
the easy estimation of the angular subtense of real LEDs and intermediate sources from
the measured beam propagation characteristics.
x Verification of the technique using a test suggested by the International Standards
Organisation (ISO/TC 172/SC 9) identifying that this method can be applied
successfully to the analysis of beam propagation parameters and hence the apparent
source size determination for stigmatic and simple astigmatic beams from LEDs.
x Development of this technique would allow the assessment of generally astigmatic
beams in line with ISO 11146-2 ‘Lasers and laser-related equipment. Test methods for
laser beam widths, divergence angle and beam propagation ratio. Part 2: General
astigmatic beams’.
The results of this study allow, for the first time, the effective characterisation of the optical
hazard of LEDs and other intermediate sources, in accordance with the IEC and CIE standards.
This is a significant step in reducing the ambiguity that currently exists in the application
of these optical safety standards. The results also verify earlier numerical modelling of an
improved method for the estimation of the critical angular subtense parameter for extended
sources, such as LEDs and intermediate sources.
viii
1 INTRODUCTION The assessment of the optical hazard associated with beams from sources of light intermediate
in quality between a laser and light emitting diodes (LED)1 has been a challenging problem for
the international standards community for a large number of years.
This report has been produced to contribute to the international debate regarding the optical
hazard due to LEDs. The current requirements for the classification of LEDs follows IEC 60825
3
and requires a measurement of “apparent source size and its location”. The CIE publication
CIE S 009/E:2002 “Photobiological Safety of Lamps and Lamp Systems” cites apparent source
size as part of the methodology to calculate angular subtense and hence Retinal Hazard.
However a procedure for establishing apparent source size and location is not described.
The apparent source size of an LED is a critical parameter used in the assessment of the ocular
viewing hazard of these devices under ISO 60825-1 ‘Safety of laser products. Equipment
classification, requirements and user’s guide’. Under the committee draft IEC 60825-13
‘Measurements for the classification of laser products’ a proposed measurement method is
described to determine the apparent source size of LEDs. The validity of this method has been
questioned at a national and international level and continues to be debated within the various
standards bodies such as IEC, ISO and CIE. Specifically, the applicability of propagation
models to low divergence beams from LEDs has been challenged.
Previously the validity of these models has not been demonstrated through physical
measurement of LED devices. This project aimed to resolve this situation through the
construction of a suitable measurement facility and by performing an assessment of a range of
commercially available LED sources.
1
2 THEORY
Figure 1 is a schematic diagram of the proposed measurement method for the determination of
the apparent source size and beam characteristics of LEDs. A CCD diode array camera system
is placed on a movable carriage in front of the LED source. The relay lens of the camera system
allows the CCD to capture a spatial intensity profile of the beam at a particular plane. The beam
width is then calculated using a modified second moment technique. It is necessary to ensure
that enough of the beam power has been captured to allow an accurate determination of the
beam width. To address this problem a self-converging width measurement technique is used to
estimate the beam width at each measurement plane and represent the true value to an
acceptable level of uncertainty. This measurement is repeated at a number of locations along the
test beam axis until sufficient data points have been obtained to allow the fitting of a maximum
likelihood hyperbola using a least squares fitting technique. The coefficients of the fitted
hyperbola allow the derivation of the beam propagation parameters of the source.
A’
B’
v
u
LED
CCD
do
A
B
AA’ – plane of beam waist
BB’ – plane of transformed beam waist
u - distance from beam waist to lens
v – distance from lens to transformed beam waist
do – beam waist diameter
Figure 1 Proposed methodology to determine apparent source size of LEDs
If the beam waist is not accessible for direct measurement then using an aberration-free
focussing system, or transform lens can create an artificial waist. This may be necessary if, for
example, the beam waist is formed within the LED package or there is insufficient space to
perform the required number of measurements either side of the waist. The position and
diameter of this artificial waist can then be used, along with the known properties of the
transform lens, to calculate the location and size of the original beam waist. The equations used
to calculate the location and size of the original beam waist using this procedure are given in
Section 6 as part of the uncertainty derivation process.
2.1
ANGULAR SUBTENSE
The angular subtense of an apparent source of radiation in the 400 nm to 1400 nm wavelength
range is required by current laser safety standards 3 to permit calculation of the relaxation factor
C6, for thermal retinal damage from extended sources. It is the ratio of the angular subtense of
the source in question to that of a source that would form the realistic minimum spot size on the
retina (1.5 mrad). Classification or assessment of the thermal hazard from a source requires that
both the angular subtense (see Figure 2) and location of an extended source be known before
there can be a relaxation of the maximum permissible exposure (MPE). The location of the
source is required so that the angular subtense can be calculated for viewing this from the
2
minimum conceivable eye accommodation distance of 100 mm (in IEC standards) 3. It should be
noted that this latter assumption may not describe the full range of potential hazards. It is
possible that some large divergence sources, when held closer than 100 mm from the eye, might
produce a significant thermal hazard in a blurred retinal spot even though the eye cannot
achieve a sharp focus.
Optical
Source
Image of
Optical
Source
Į
Angular
Subtense
Eye
Figure 2 Classical representation of Angular Subtense
It is a simple matter to measure the physical size of the chip of a LED that has a Lambertian
radiation pattern but it is more difficult to know or measure the location or size of the apparent
source with low divergence beams from a LED. Such beams can have a near planar wavefront,
which would imply that the apparent source is located at infinity with an unknown angular
subtense. However, recent advances in the characterization of optical beams, both coherent and
incoherent, enable prediction of their propagation envelopes 2,16,17. It is now possible to assess
the intrabeam-viewing hazard by using known beam characteristics to estimate the angular
subtense of an extended source that would present the greatest hazard to a retina 3.
The level of the thermal hazard to the retina is defined here as the power or energy per
millimeter of beam diameter falling on the retina 19. The process of calculation of the size of the
beam formed on a retina and the fraction of incident power passing through the pupil has been
performed for a wide range of feasible conditions. The calculations assume that the beam has a
divergence of less than 30° and has a power density profile that produces the greatest peak
irradiance on the retina (i.e. a Gaussian profile).
Measurements of the enclosed power envelope of beams from lasers have confirmed that they
propagate with a hyperbolic profile, the constants of which are modified when passing through a
lens. The new constants can be used to estimate the location of the waist of the new hyperbola
and its Gaussian beam diameter as a function of propagation distance. In this way it is possible
to determine the spot size on the retina formed by a beam after passing through the lens of the
eye.
d01 – beam waist diameter of input beam
L1 – waist to lens distance
Zr1 – Rayleigh length of input beam
fe – focal length of lens
L2 – lens to transformed waist distance Zr1 – Rayleigh length of output beam
d02 – beam waist diameter of output beam
dr – beam diameter on retina
Lr –transformed waist to retina distance
Figure 3 Calculation of spot size (dr) on the retina of the eye.
3
For a given set of beam propagation constants (waist diameter and divergence say) it is possible
to predict the hazard level (P/d) at the retina. The hazard level results from calculations of the
fraction of beam power that passes through the 7mm iris of the eye as a function of both the
strength of the eye lens (assumed to vary anywhere between 14.5 mm and 17 mm) and the
distance of the incident beam waist from the eye. The maximum hazard occurs when the eye
accommodates itself at the most hazardous viewing distance. In the interests of simplicity, IEC
60825-1 assumes that this most hazardous viewing distance is 100 mm but this is not always
found to be the case.
Previous calculations (numerically verified by workers in Austria and the UK) have
concentrated on determining the spot size on the retina at the most hazardous viewing condition
as a function of the two beam propagation parameters, beam waist diameter and far-field
divergence. Knowing the spot size at the retina and by assuming the eye to be 17 mm "long",
the artifact of the angular subtense of the apparent source has been estimated over the most
relevant range of incident beam parameters. The values of angular subtense can be displayed as
contours in the two-dimensional map of waist diameter and divergence. Further calculations
based on the measured values of these parameters will also reveal the location of the apparent
source. If the Rayleigh length of the beam is significantly less than 50 mm then the source can
be assumed to coincide with the measured beam waist location.
While some rather extreme conditions have been assumed when modeling the beam (e.g. a
Gaussian beam profile), the procedure for estimating angular subtense from beam parameter
measurements is thought to offer an unambiguous and non-subjective result. While the
procedure may over-estimate the hazard level it can permit a greater relaxation of the MPE level
than simply assuming that C6=1.
A CCmap showing the range of angular subtense values as contours against the Beam waist
width and the beam divergence was produced from these calculations (Fig 4)2.
Contour of
equal angular
subtense in mrad
Beam waist
diameter and
divergence of LED
Angular subtense,
D, of LED
Figure 4 Theoretical plot of beam waist diameter (width) vs. beam divergence
showing contours of angular subtense
4
The contours show equal values of angular subtense in milliradian. To use the contour plot, the
waist diameter and the divergence of the LED beam are measured. The results are plotted on the
graph and the value of the angular subtense, D, is then read from the contour just below the
measured point.
The objective of this investigation was to demonstrate that it is possible to determine the
propagation characteristics of the beam produced by a LED. This information could then be
used to estimate the size of the image formed on the retina and from this the angular subtense of
the apparent source at the eye at a given distance. These results then enable the population of a
theoretical contour map of the computed angular subtense as a function of the measured beam
characteristics of LEDs. The angular subtense for all beam types can then be determined by
measuring the beam waist diameter and the divergence.
2.2
BEAM MEASUREMENTS
Measurement of the optical constants of the propagation envelope of a beam has been the
subject of considerable research over the last ten years. A consequence of this work is the
evolution of ISO standards for the measurement of the diameter and divergence of a beam. ISO
11146:1999. “Test methods for laser beam parameters: beam widths, divergence angle and
beam propagation factor” 6 is the current draft standard being reviewed by ISO. The procedures
and techniques that are described here for the determination of the diameter and location of the
apparent source of a beam are based on the principles underlying the ISO standards7 for
stigmatic and simple astigmatic beams. The proposed methods are applicable to beams whose
full divergence angle is less that 30°. Relaxation of the laser safety criteria should not be applied
to a beam displaying general astigmatism.
There are a number of methods available for measurement of the diameter of a beam as well as
its far-field divergence. The basic principles for those methods have been established in an ISO
standard. They are applicable to laser beams with a relatively small beam propagation ratio, M2.
Recent research has demonstrated that adequate steps have to be taken to counter the effects of
noise and offset errors when measuring the transverse irradiance distribution of a beam. When
these steps are taken, the propagation behaviour of incoherent broadband beams as well as highquality laser beams can be predicted reproducibly with considerable precision.
To accurately measure the second moment beam diameter both the number of pixels and the
level of digitisation of the signal received on each pixel has to be considered. For beams with a
rapidly changing beam diameter the number of bits in the digitisation process becomes more
critical. Noise on the image acquired by the camera both from electrical and optical sources
must be removed by setting a discrimination level. This effectively reduces the dynamic range
of the camera and this favours cameras with an inherently large dynamic range due to a larger
number of bits available on the digitisation electronics.
The methods leading to estimates of the diameter of a beam use a procedure known as the
Converging Second Moment diameter or width measurement (CSM). The schematic of this
method is shown in Figure 5. These methods are being defined in the revision of ISO 11146 that
is currently in preparation.
5
Figure 5 Schematic of converging second moment iteration
The preferred method for measuring all the propagation characteristics of a beam is to perform
CSM diameter measurements at a number of locations either side of the beam waist. The
calculation of second moment width is described in Appendix 1.
2.3
THE OPTICAL SYSTEM
The beam measurement process consists of using a CCD sensor to image the irradiance profile
at a minimum of ten measurement locations either side of the beam waist. The proposed optical
system contains variable magnifying optics that are designed to facilitate imaging the transverse
irradiance profiles to occupy approximately one quarter of the sensor screen height. Other
components are included in the system to attenuate the beam power to avoid sensor saturation
and to provide spatial calibration of the pixel array of the sensor.
6
3 DESCRIPTION OF APPARATUS
3.1
INITIAL SYSTEM DESIGN
An initial specification of the 12-bit measurement system was written and can been found in
Appendix 2. This specification details the required elements to measure apparent source size of
LEDs.
3.2
8-BIT SYSTEM DESIGN
Both an 8-bit and 12-bit camera systems were used for measurement. The final system design
for the 8-bit system was identical to that described in Section 3.3, except for the camera and
zoom lens. The details of the 8-bit camera are given in Section 3.2.1. The details of the
associated zoom lens are presented in Section 3.2.2.
3.2.1
8-Bit Camera System
The 8-bit system consisted of a analogue CCD interline transfer camera connected to an 8 bit
frame grabber card. A Leica Monozoom optic was used to adjust the size of the image of the
propagating beam from the test LED. An 8-bit system would imply a digitised dynamic range of
28 =256 bits. This takes no account of noise or camera processing. The dynamic range in these
measurements was assumed at the start to be one of the greatest limiting factors of the
measurement. This assumption was later shown to be true by adjusting discrimination levels and
plotting the effect against the measured second moment values for identical camera frames.
3.2.2
Leica Monozoom 7
The camera zoom lens used for the 8-bit system was a 1:7 par-focal microscope zoom. During
zooming the focus could be maintained, whilst providing a wide field of view and a long
working distance. The zoom did not include an integral iris and the zoom setting could not be
locked. The latter meant that special care was required to ensure that the zoom was not
disturbed during measurements, otherwise the dimensional calibration would be lost. The
shortcomings of this zoom prompted the acquisition of a higher specification zoom system to
form part of the 12-bit set-up.
3.3
3.3.1
12-BIT SYSTEM DESIGN
Electrical Measurements
The LED sources were operated at a constant current using a power supply stabilised to 0.02%.
Setting a constant voltage is also possible, although this is more likely to be affected by
differences in contact potential. To measure the current to the LED, a standard resistor was
placed in series with the power supply and the LED source. The potential across the standard
resistor was measured using a calibrated digital voltmeter. Using this value, the current to the
LED was calculated and recorded. This ensured that the same electrical conditions were used
for each LED measurement.
7
3.3.2
Transform Lens
Following a survey of commercially available products, it was identified that a single large
diameter achromatic lens of sufficient power and quality for the measurements was not
available. Two high quality achromats were combined to provide an equivalent effect. The large
diameter was required to provide effective coupling of the LED output to the camera input to
reduce vignetting. Optics of large diameter also allows the inner portion of the lens to be used
which introduces less aberration to the measurement process. It is critical that the geometry of
the lens is known accurately, so that the lens transformation properties can be calculated (see
Figure 6 and Figure 7).
Figure 6 Schematic diagram of achromat showing critical measurements needed to allow
the lens transformation properties to be calculated (all dimensions in mm).
A description of the parameters used can be found in the glossary.
The distance between the two lenses and the distance from the LED required calculation to
ensure that the beam would not overfill the aperture of the camera zoom. In addition, the
transformed waist diameter must not be too small as to cause measurement problems due to the
camera resolution. Additionally, the Raleigh length (distance for the beam diameter to increase
by —2) should be long enough to allow accurate distance measurement to be carried out. High
quality lens mounts with yaw and tilt adjustments were purchased to allow uniaxial alignment of
the measurement system.
Figure 7 Scale drawing of the two transform achromats showing some of
the calculated measurement distances, definitions of parameters are
given in section 6 (all dimensions in mm)
8
3.3.3
Optical Rail
A 2-metre cast iron optical rail was used as the primary bench for the mounting of the optical
components. A second, machined, aluminium rail was used to mount the LED and the achromat
lenses. This secondary rail was mounted on a roller carriage on the primary bench. All carriages
and benches carried vernier scales to ensure accurate measurement readings.
3.3.4
LED Mount
A stable LED clamp which could in turn be mounted on a 3 axis gimbal mount with height and
transverse adjustment was required. A commercially available solution was unavailable so a
clamp was designed and produced by the NPL workshop. This was mounted on a high stability
goniometric mount with height and transverse adjustment provided by two other stages. This
provided a low vibration mount with high resolution and repeatable displacement.
3.3.5
Beam Attenuation
Neutral Density (ND) optical filters were used to attenuate the light input to the camera. Critical
attributes were spectral neutrality and spatial uniformity. Tests were made on NPL’s primary
Zygo Fizeau interferometer to inform the purchase of a high quality set of filters with low
wavefront aberration. The filters were placed in a mount that allowed stacking of filters with an
adjustment for variable tilt to reduce inter-reflection. The proposed initial system did not include
a scatter screen and thus the use of an iris with the zoom system would have caused vignetting.
Later adoption of the scatter screen allowed this option.
3.3.6
Rotating diffuser
It was found to be necessary to use a rotating frosted scatter screen to allow visualisation of the
beam profile at the focal point of the zoom system. Measurements made without this system
caused vignetting problems. The rotating diffuser had the added advantage that it allowed the
use of the integral iris in the zoom lens to attenuate the LED light. Several measurements were
made to ensure that the real beam diameter was not greater than the diameter obtained by the
use of the screen.
3.3.7
Graticule
A photoetched transparent graticule with traceable calibration was used to calibrate the imaging
system (zoom in combination with the CCD array and analysis software).
3.3.8
12-Bit Camera System
The 12-bit system utilized a superior zoom system that had a larger input optic and a greater
magnification range. The CCD detector used produced a digital 12 bit output and incorporated a
two stage peltier cooler to both reduce the temperature of the array and the level of noise
acquired. The dynamic range of the system was 212=4096 bits, the software used a
“discriminator level” which allowed the baseline for detection to be raised above the ambient
noise level. The zoom system incorporated an iris which allowed the light levels to be reduced
within the range afforded by the detector integration time adjustment.
Calculations were performed to ensure that the LED and lens(es) were located so that the beam
could converge to a waist and re-expand within the travel range of the optical bench system. At
the same time the anticipated diameter of the beam at the transforming lens(es) was examined to
9
ensure that the beam size was not large enough to introduce significant aberration or vignetting
effects.
3.3.9
Leica Z16 Zoom
Two Leica zoom microscope systems were assessed, the Z6 and Z16 models. The Z16 was
found to have a greater focal range and would allow the measurement of a greater range of LED
types. The Z16 is an apochromatic zoom system with central beam path. A planapochromatic
0.5X objective was used and the zoom range was 0.57× – 9.2×. This high quality optic has a
similar field of view to the Monozoom 7 but has significantly lower aberrations. The inbuilt iris
can also be employed to attenuate light levels when used in conjunction with a scatter screen.
The lockable zoom setting allows calibration at a particular fixed zoom level.
3.3.10 Final 12-bit System Design
Figure 8 shows the final components used in the 12-bit system for the measurement of angular
subtense.
LED
Achromats
Rotating
diffuser
ND Filter and
Filter holder
CCD camera
Planofocal
Zoom Lens
Secondary
optical bench
Primary optical
bench
Figure 8 Final optical arrangement of the 12-bit system for the measurement of
apparent source size
10
4 MEASUREMENT PROCEDURE
4.1
PREPARATION FOR MEASUREMENT
The optical arrangement for the measurement of apparent source size is detailed in the
schematic diagram, Figure 8. Prior to measurement the components must be carefully aligned to
ensure that the LED beam is parallel to the optical axis of the primary and secondary bench.
The following points detail the steps required to align the components used with the optical axis
of the bench.
a) Establish a reference He-Ne beam parallel tothe optical bench by the use of at least two
movable irises or apertures.
b) Align the centre of the Zoom lens with the HeNe beam and use the imaging software to
ensure that the suitably attenuated beam is in the centre of the CCD array field of view.
c) Introduce the transform achromats one at a time and centre them on the beam. Ensuring
that the emerging beam is still creating a centred image in the camera.
d) Place the viewing screen at the focal point of the zoom lens by utilizing a reference grid
that can be resolved by the imaging software and can be coincident with the frosted side
of the screen.
e) A microscope, focused on the optical axis of the system, is used to locate and record the
positions of the components along the optical bench. This facility is used to set
accurately the appropriate distances between the LED and the achromat(s).
4.2
CALIBRATION OF CCD ARRAY AND ASSOCIATED EQUIPMENT
a) A reference grid or graticule is inserted in place of the frosted screen with the reference
grid plane coincident with the plane of the frosting, as determined using a telescope. A
CCD frame of the reference grid is recorded and analysed by the software to derive the
calibration factor (pixels/mm) to be used to convert subsequent beam pixel
measurements into linear dimensions.
b) All equipment used to measure the electrical characteristics of the LED were calibrated
and traceable to national standards, as is essential for such a system.
4.3
LED BEAM WIDTH MEASUREMENT
Once the system is aligned and the calibration procedures performed, the following steps are
required to predict the position of the beam waist from the vertex of LED.
a) A combination of ND filters and the iris of the Zoom lens are used to attenuate the
beam irradiance so that the full dynamic range of the CCD system is used. This is done
by locating the position of maximum irradiance, then placing filters in the beam path so
that the signal is just about saturating the CCD pixels. As readings are taken either side
11
of the maximum, the iris of the zoom lens and the exposure time of the camera can be
adjusted to maintain the signal level at the full dynamic range of camera. The Zoom is
also set so that approximately a quarter of the CCD field of view appears to be filled by
the largest diameter that is to be measured;
b) The image acquisition software is used to capture at least 10 equidistant beam images
either side of the beam waist. Each image has an associated image of background
optical noise captured at the same time by blanking out the LED with a black felted
beam stop;
c) The background frame is subtracted from the beam image frame before the digital width
analysis process is performed;
d) The corrected image is processed using the convergent second moment (CSM) method
to limit the dimensions of the CCD window that is subsequently analysed and hence
reduce noise contribution to the second moment evaluation. The CSM values of the
beam in the laboratory (CCD array) vertical and horizontal axes are calculated. A crossmoment of the beam distribution in the converged window is used to calculate the
azimuth of the principal axes of potentially non-circular distributions. This figure
enables the calculation of the dimensions of the beam along its principal axes. The ratio
of the principal dimensions (ellipticity), the azimuth angle of the principal axes relative
to the laboratory axes; and the calibrated linear magnitude of the principal dimensions
are recorded. The convergence of the 2nd moment calculations can be seen in Figure 9
The program then outputs the final 2nd moment measurements in the X and Y axes;
33.71
Figure 9 CSM software illustrating the calculation of the second moment values.
e) A least-squares (maximum probability) process is used to discover the best fitting
hyperbolic envelope to the propagating beam in each of its principal planes. The
coefficients of the hyperbolas are processed to reveal: the locations of the beam waists
relative to the vertex of the LED; the transverse dimensions of the waists; the values of
the Rayleigh Lengths of the beam along their principal planes; and the far-field
divergences in those planes;
12
f) If the beam is found to be astigmatic (i.e. the ellipticity of the beam is found to be
greater than 1.15 or less than 0.83) and there is a monotonic variation in the azimuth of
the principal planes of the propagating beam (twist) then the beam is deemed to suffer
from general astigmatism and no further investigation or relief of the thermal hazard
factor C6 can be justified without a more detailed analysis procedure;
g) If the beam is identified as stigmatic or simple astigmatic the determined values of the
beam waist widths and far-field divergences can be placed on the angular subtense
contour map (Figure 4) and the contour below the lowest uncertainty ellipse can be used
to identify the angular subtense to be used to determine the appropriate value of the
thermal hazard relaxation factor C6.;
h) If the Rayleigh Length in the least divergent principal plane is less than 50 mm then the
location of the apparent source can be regarded as the location of the beam waist in that
plane. If the Rayleigh Length is greater than 50 mm then the possible error in hazard
assessment can be greater than 5% and the location of the centre of curvature of the
wavefront arriving at the most hazardous viewing distance should be used to identify
location of the apparent source.
Figure 10 shows the required optical elements of the system to measure the angular subtense of
an LED.
Transform achromats
LED in
goniometric mount
Rotating diffuser
Zoom lens
ND filters
Enclosure to reduce
scattered light
Figure 10 Experimental apparatus
13
Cooled
Camera
CCD
4.4
TRANSFORM VALIDATION EXPERIMENT
To validate the suitability of the proposed measurement method for determining the beam
propagation parameters a Transform Validation Experiment can be undertaken. This technique
uses the beam propagation parameters to predict the size of a beam waist produced when a
known lens is inserted into the beam. This prediction is then verified by using the measurement
technique to measure the true diameter of the new beam waist with the lens inserted. The aim is
to achieve 10% (1 sigma) agreement between the predicted value and the measured diameter of
the beam waist. The transform validation experiment is shown in Figure 10 and schematically in
Figure 11. The points below detail the steps required in the transform validation method.
x Measure the transformed waist and estimate original waist
x Use the estimated waist to predict new waist formed by inserted lens
x Measure and estimate new waist for comparison with step 2. If the estimate and the
prediction agree sufficiently well the validation is complete
Measure
LED
CCD
Step 1
Estimate
Predict
LED
Step 2
Compare
Measure
LED
CCD
Step 3
Figure 11: The three steps of the primary ISO Transform Validation Experiment 6
The results of the Transform Validation Experiment are presented in Section 5.2.
14
5 RESULTS
5.1
INITIAL RESULTS
The ideal methodology to measure the LED beam would be through direct imaging. Some
difficulties were encountered due to vignetting of the beam by the zoom lens. This effect can be
seen in the asymmetry of the hyperbolic plot produced from the second moment analysis, Figure
12.
IR LED measurement showing Vignetting effect
5.000
4.500
2
2nd moment Beam radius
y = 0.0003x - 0.0142x + 3.4009
2
R = 0.9827
4.000
Horizontal width (mm)
Vertical width (mm)
Poly. (Vertical width (mm))
Poly. (Horizontal width (mm))
2
3.500
y = 0.0003x - 0.0146x + 3.297
2
R = 0.9854
3.000
2.500
-60.0
-40.0
-20.0
0.0
20.0
40.0
60.0
80.0
100.0
Distance from Beam Waist
Figure 12 Skewed fit of second moment values obtained showing vignetting effect of
zoom aperture
Noise effects from the intereflections between the filters used to attenuate the light from the
LED were found to be a particular problem. The differences in measured second moment
diameter caused by different filter combinations can be seen on Figure 13. The stray light noise
levels on the camera were very high and a discriminator level of 50 was required to produce the
analysis.
For the final measurements the procedure was adapted to only utilise the minimum number of
filter elements by manually finding the camera position that resulted in the greatest local
irradiance. The integration time of the camera and/or the iris in the zoom lens were then reduced
as much as possible to reduce the signal output from the camera pixels to a point where a ND
filter would reduce the signal levels to just below saturation. This was to ensure that the greatest
dynamic range of measurement was employed.
15
IRED through 2 lens transform - X-axis (Aug 26 Disc 50)
20.0
Converged 2M beam width (mm)
18.0
LSq Fit Hyperbola
4 filter set A
16.0
2 filter set B
1 filter C
1 filter D
14.0
4 filter set E
3 filter set E
12.0
2 filter set F
10.0
8.0
280
285
290
295
300
305
310
Di sta nce past vertex (mm)
Figure 13 IRED Led measurements demonstrating filter effects
Initial measurement work concentrated upon the confirmation of earlier work using an Osram
IR LED 2. Details of this LED can be found in Appendix 3. Early evaluations were pursued with
a 50 mm focal length singlet lens to examine the field of view required for the experiment.
16
5.2
8-BIT TRANSFORM VALIDATION EXPERIMENT RESULTS
5.2.1
Direct Measurements of IR LED
The 8-bit system using the Cohu CCD camera and Leica monzoom 7 lens was used for the
initial work to confirm the method for measurement of angular subtense. Figure 14 details the
required apparatus to measure the beam waist of an LED directly. Measurements of the beam
width were made using the procedure in Section 4.3.
Rotating
diffuser
LED
ND Filter and
Filter holder
CCD camera
Planofocal
Zoom Lens
Secondary
optical bench
Primary optical
bench
Figure 14 Optical set-up for direct apparent source size measurement
Converged 2M beam
width (mm)
SFH 400 IRED (950 nm) @ 50 mA Raw Beam (5 Aug)
6.5
5.5
4.5
3.5
2.5
0
5
10
15
Distance past vertex (mm)
20
25
Figure 15 Plot of converged beam width for IRED LED
The results, shown in Figure 15 and Table 1, indicate that the IR LED produces a beam with a
waist external to the LED. The M2 value is very large but the beam still fits the hyperbolic
17
envelope well. The existence of an external beam waist allows the effective measurement of the
direct propagation envelope.
Table 1 Calculated beam parameters for Osram IRED LED
Parameter
Value
Waist position from LED vertex zo 5.61
Waist diameter Wo
3.38
Rayleigh distance Zr
9.76
Divergence 4
347
M2
969
5.2.2
Units
mm
mm
mm
mrad
Two Lens transform of IR LED
Achromats
LED
Rotating
diffuser
ND Filter and
Filter holder
CCD camera
Planofocal
Zoom Lens
Secondary
optical bench
Primary optical
bench
Figure 16 Optical set-up for apparent source size measurement
using two achromat lenses
The second step in the validation of the measurement method is to predict t e beam waist
diameter. This requires the use of a transfo m lens in the optical arrang ment (see Figure 16).
Two achromats were used to produce a beam waist that would not overfill the field of view of
the CCD. The lenses also ensured that the Rayleigh Length was sufficiently long to provide an
appropriate number of measurement planes. Figure 17 shows the results of these measurements
with a hyperbolic fit to the data points.
18
IRED 2 Lens Transform X-axis
7.5
7.0
CSM width (mm)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
220
230
240
250
260
270
Distance past vertex (mm)
Figure 17 Hyperbolic fit to data from IR LED through two achromat lenses
A summary of the beam characteristics for the Osram LED is presented in Table 2. The
complete summary of the Transform Validation Experiment is located in Tables 4 and 5 of
Section 5.3 as it was thought more appropriate to put them in the context of the 12-bit system
and hence allow comparison.
Table 2 Summary of beam characteristics for IRED LED using 8-bit camera system
SFH 400 IRED using 8-bit Camera
Transformed Beam
Beam property
Waist position from LED vertex Zo
254.1
Waist diameter Wo
3.12
Rayleigh distance Zr
8.6
361
Divergence 4
M2
950
mm
mm
mm
mrad
Goodness of fit
0.01
0.10
0.27
16
67
Original Beam from Inverse Transform
Beam property
Uncertainty
Waist position from LED vertex Zo
4.9 mm
3.0
3.47 mm
Waist diameter Wo
0.22
Rayleigh distance Zr
0.5
10.7 mm
26
324 mrad
Divergence 4
M2
129
950
Uncertainties Used
Focal length etc.
Datum positions
Width measurements
Measurement locations
0.5
0.1
0.025
0.15
19
%
mm
mm
mm
5.3
12-BIT TRANSFORM VALIDATION EXPERIMENT RESULTS
The 12-bit PCO Sensicam camera with the Leica Z16 zoom lens was used for further
measurements of apparent source size on the Osram IRED LED and a range of visible LEDs.
The details of all LEDs measured can be seen in Appendix 3.
5.3.1
Low Level Noise
It was found that the discriminator level used to eliminate low level noise caused a much greater
effect upon measured beam width than might be expected. The intensity of the imaged LED was
always set as close as possible to the saturation point of the CCD to produce the greatest
dynamic range possible. With the 12-bit system, the maximal number of bits of dynamic range
would be 4096. Figure 18 shows the effect upon the second moment beam size due to change of
discrimination level. Hereafter the discrimination setting was kept at 5 bits.
IRED raw beam width vs Discriminator Level
Pos 1
Second moment (X-axis)
48
Pos 3
Pos 6
43
Pos 6
Pos 10
Pos 11
38
Pos 12
Pos 13
33
Pos 14
Pos 15
28
0
5
10
15
20
Discriminator Level
Figure 18 Variation in discriminator level with beam width
5.3.2
Transform Validation Experiment results from IRED LED
The measurements performed using the 8-bit system (described in section 5.2) were repeated to
demonstrate the differences between the two systems and to validate the procedure for
measurement of angular subtense. The beam width of the LED was measured directly using the
measurement arrangement shown in Figure 14.The resulting data is plotted in Figure 19.
The next step in the Transform Validation Experiment was to measure the beam of the LED via
two achromat lenses. Several measurements were made and a hyperbolic fit was made to the
resulting data. The fit is shown in Figure 20.
20
IRED Raw - X-axis (SEP 30)
Converged 2M beam width (mm) - 2% uncertainty
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
5
10
15
20
25
30
Distance past vertex (mm)
Figure 19 Plot of converged beam width for IR LED
IRED 2 Lens Transform X-axis
7.5
7.0
CSM width (mm)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
220
230
240
250
260
270
Distance past vertex (mm)
Figure 20 Hyperbolic fit to data through two achromats from IR LED
21
Table 3 Summary of beam characteristics for IRED LED using 12-bit camera system
SFH 400 IRED
Transformed Beam
Beam property
Goodness of fit
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
252.8
2.73
10.7
255
588
mm
mm
mm
mrad
0.02
0.09
0.35
12
43
Original Beam from Inverse Transform
Beam property
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
4.7
3.00
12.9
232
588
mm
mm
mm
mrad
Uncertainties Used
Focal length etc.
Datum positions
Width measurements
0.5
0.1
0.025
%
mm
mm
Measurement locations.
0.15
mm
Uncertainty
2.7
0.19
0.6
18
78
Beam propagation parameters calculated from the hyperbola equation. These parameters
were then back propagated by calculation through the lens using the known lens
parameters allowing calculation of the LED emitted beam properties. The beam
parameters are presented in Table 3.
These can then be compared with the previous measurements of the LED direct beam.
Tables 4 and 5 give a complete summary of the Transform Validation Experiment
results for both the 8-bit and 12-bit camera systems.
22
Table 4 Validation results for IRED LED - X axis
LED
SFH 400 IRED (950 nm)
12-bit camera + Z16 lens
Trans. UC
Property
Direct UC
beam
%
beam %
(DB)
Waist
location
4.28
0.04
252.8
0.03
(mm from
vertex)
Waist
diamter
3.06
0.01
2.73
0.14
(mm)
Rayleigh
length
12.13 0.03
10.7
0.56
(mm)
Divergence
253
1
255
19
(mrad)
X – Axis
Inverse
transformed
beam (ITB)
8-bit camera system + monozoom 7 lens
Waist
location
5.6
254.1
(mm from
vertex)
Waist
3.38
3.12
diamter
(mm)
Rayleigh
9.76
8.6
length
(mm)
Divergence
346
361
(mrad)
Note:
UC = Uncertainty (%)
23
UC
%
Difference
(ITB-RB)
Agreement %
(Difference/RB
x 100)
4.7
9.7
-0.42
-9.8
3
0.58
0.06
2.0
13
1.8
-0.87
7.2
232
55
21
8.3
4.9
0.7
12.5
3.47
-0.09
2.7
10.7
-0.94
9.6
324
22
6.4
Table 5 Validation results for IRED LED - Y axis
LED
SFH 400 IRED (950 nm)
12-bit camera + Z16 lens
Trans. UC
Property
Direct UC
beam
%
beam %
(DB)
Waist
location
4
0.03
252.8
0.03
(mm from
vertex)
Waist
3.19
0.007
2.77
0.16
diamter
(mm)
Rayleigh
11.93
0.03
10.3
0.58
length
(mm)
Divergence
267
1
270
21
(mrad)
Y - AXIS
Inverse
transformed
beam (ITB)
8-bit camera system + monozoom 7 lens
Waist
location
5.66
253.9
(mm from
vertex)
Waist
3.34
3.09
diameter
(mm)
Rayleigh
10.01
8.36
length
(mm)
Divergence
333
370
(mrad)
Difference
(ITB-RB)
Agreement %
(Difference/RB
x 100)
4.5
9.9
-0.5
-12.5
3.05
0.61
0.14
4.4
12.5
1.8
-0.57
-4.8
245
60
22
8.2
4.45
1.21
21.4
3.45
-0.11
3.3
10.43
-0.42
4.2
2
0.6
331
24
UC
%
5.4
YELLOW LED - LIGITEK LUY 3833/A29
The measurements of the yellow LED were performed using an arrangement of two
achromats. Images of the beam were taken and the beam width calculated. The
measurement points and the resultant hyperbolic fit are plotted in Figure 21. The
hyperbolic fits the measured data well. It is always difficult to make an initial estimate
of the position of the waist from the LED vertex. Ideally iterative measurements would
allow the spread of data to be symmetric around the beam waist position.
Yellow LED 2 Lens Transform X-axis
7.5
7.0
CSM width (mm)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
220
230
240
250
260
270
Distance past vertex (mm)
Figure 21 Hyperbolic it to data from a yellow LED through two achromats
25
Table 6 Summary of beam characteristics for Yellow LED using 12-bit camera system
Yellow LED - Ligitek LUY 3833/A29
Transformed Beam
Beam property
Goodness of fit
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
242.4
3.81
16.7
228
735
mm
mm
mm
mrad
0.02
0.05
0.20
4
20
Original Beam from Inverse Transform
Beam property
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
Uncertainties Used
Focal length etc.
Datum positions
Width measurements
Measurement locations
-3.9
4.67
25.0
186
735
0.50%
0.1
0.025
0.15
mm
mm
mm
mrad
Uncertainty
2.8
0.19
0.7
9
64
mm
mm
mm
Table 6 presents a summary of the beam characteristics for the yellow LED. From the results
the waist position of the inverse transformed beam (the direct beam) can be seen to be inside the
LED chip. This provides an interesting contrast to the IR LED. It should also be noted that the
divergence of this LED is significantly less than the other "display" LEDs examined in this
study.
26
5.5
BLUE LED - NICHIA NSPB500 RANK WS
Measurements of the 2-lens transformed beam from the blue LED were made at positions either
side of the beam waist and the results are shown in Figure 22.
The departure of data from the smooth fitted curve, shown in Figure 22, was thought to be due
to filter changes creating intereflections and problems for the CSM beam width measurement.
Blue LED 2 Lens Transform X-axis
5.4
CSM width (mm)
5.2
5.0
4.8
4.6
4.4
4.2
4.0
235
240
245
250
255
260
Distance past vertex (mm)
Figure 22 Hyperbolic fit to data from Blue LED through two achromats
27
Table 7 Summary of beam characteristics for Blue LED using 12-bit camera system
Blue LED - Nichia NSPB500 Rank WS
Transformed Beam
Goodness of fit
Beam property
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
249.3
4.05
11.8
343
1171
mm
mm
mm
mrad
0.03
0.17
0.51
21
112
Original Beam from Inverse Transform
Beam property
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
1.4
4.47
14.4
310
1171
Uncertainties Used
Focal length etc.
Datum positions
Width measurements
0.50%
0.1
0.025
mm
mm
0.15
mm
Measurement locations
mm
mm
mm
mrad
Uncertainty
2.7
0.30
0.8
27
170
With these results an externally located beam waist location outside the Blue LED package can
be seen from the positive waist position in the inverse transform section of Table 7. This is
similar to the Osram SFH 400 IRED, but the waist is not as conveniently far away from the
LED vertex which facilitated the direct beam measurement in section 5.2.
28
5.6
GREEN LED - NICHIA NSPG500 RANK GS
Measurements of the beam width were made at positions either side of the beam waist and the
results are shown in Figure 23.
Green LED 2 Lens Transform X-axis
5.4
CSM width (mm)
5.2
5.0
4.8
4.6
4.4
4.2
4.0
235
240
245
250
255
260
Distance past vertex (mm)
Figure 23 Hyperbolic fit to data from Green LED through two achromats
Figure 23 shows a smaller data divergence from the fitted curve. These deviations can be
disregarded because the rest of the data fits so well.
29
Table 8 Summary of beam characteristics for Green LED using 12-bit camera system
Green LED - Nichia NSPG500 Rank GS
Transformed Beam
Beam property
Goodness of fit
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
248.6
4.12
12.7
325
1129
mm
mm
mm
mrad
0.03
0.15
0.47
17
93
Original Beam from Inverse Transform
Beam property
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
0.5
4.82
17.4
278
1129
mm
mm
mm
mrad
Uncertainties Used
Focal length etc.
Datum positions
Width measurements
0.5
0.1
0.025
%
mm
mm
Measurement locations
0.15
mm
Uncertainty
3.0
0.29
0.8
22
148
The results in Table 8 summarise the beam characteristics for the green LED. It shows an
external waist from the LED. The measurement procedure requires at least 10 measurements of
the beam width either side of the waist position, therefore the waist would not be positioned far
enough away from the LED vertex to be easily measured as a direct beam.
30
5.7
RED LED - KINGBRIGHT L-53SRC/E
Measurements of the beam width were made at positions either side of the beam waist and the
results are shown in Figure 24.
Red LED 2 Lens Transform X-axis
5.4
CSM width (mm)
5.2
5.0
4.8
4.6
4.4
4.2
4.0
235
240
245
250
255
260
Distance past vertex (mm)
Figure 24 Hyperbolic fit to data from Red LED through two achromats
31
Table 9 Summary of beam characteristics for Red LED using 12-bit camera system
Red LED - Kingbright L-53SRC/E
Transformed Beam
Beam property
Goodness of fit
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
244.0
4.14
13.8
301
1052
mm
mm
mm
mrad
0.03
0.13
0.44
13
75
Original Beam from Inverse Transform
Beam property
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
-3.8
4.94
19.6
252
1052
mm
mm
mm
mrad
Uncertainties Used
Focal length etc.
Datum positions
Width measurements
0.5
0.1
0.025
%
mm
mm
Measurement locations
0.15
mm
Uncertainty
3.0
0.28
0.9
18
126
A summary of the beam characteristics for the red LED is presented in Table 9. As seen with
the yellow and blue LEDs in sections 5.4 and 5.5, a beam waist location inside the red LED
package can be seen from the negative waist position in the inverse transform section of Table
9.
32
5.8
WHITE LED - NICHIA NSPW500 RANK BS
Measurements of the beam width were made at positions either side of the beam waist and the
results are shown in Figure 25.
White LED 2 Lens Transform X-axis
5.0
4.8
CSM width (mm)
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
235
240
245
250
255
260
Distance past vertex (mm)
Figure 25 Hyperbolic fit to data from White LED through two achromats
33
Table 10 Summary of beam characteristics for White LED using 12-bit camera system
White - Nichia NSPW500 Rank BS
Transformed Beam
Beam property
Goodness of fit
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
245.9
3.54
11.6
305
911
mm
mm
mm
mrad
0.03
0.16
0.52
19
90
Original Beam from Inverse Transform
Beam property
Waist position from LED vertex Zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
-3.6
4.35
17.5
248
911
mm
mm
mm
mrad
Uncertainties Used
Focal length etc.
0.5
%
Datum positions
Width measurements
0.1
0.025
mm
mm
Measurement locations.
0.15
mm
Uncertainty
3.5
0.29
0.9
21
131
A summary of the beam characteristics for the white LED is presented in Table 10.
34
5.9
ORANGE LED - TOSHIBA TLOH190P
Orange LED One Lens Transform. X-axis
13.
CSM width (mm)
12.
11.
10.
9.0
8.0
7.0
6.0
180
200
220
240
260
280
Distance past vertex (mm)
Figure 26 Orange LED one achromat transform
The orange LED only used one achromat to perform the beam transformation to give an
appropriate image to analyse on the 12-bit camera.
The orange LED data did not fit well to a hyperbola. This is clearly demonstrated in Figure 32.
Further study of the measurement data indicated that the propagating beam was astigmatic and
hence would not fit to the beam propagation model. Astigmatic beams could be treated using the
methodology described in ISO 11146-2 7 but this is beyond the scope of this study.
The astigmatic nature of the beam can be discovered from the steady change of azimuth angle as
the beam propagates, see Figure 33. The apparent sudden jump of the angle is due to the beam
widths in the X and Y direction reaching the same value at that point in the Z direction. This
indicates a nearly circular beam and makes the azimuth angle indeterminate. As described in the
ISO standard 11146-2 the insertion at this point of a cylindrical lens at the right azimuth angle
may remove the astigmatism.
35
Astigmatism in beam from Orange LED
20.00
15.00
10.00
WoX
WoY
Elipticity x 10
Azimuth (degrees)
5.00
0.00
170
190
210
230
250
270
290
310
-5.00
-10.00
Distance past Vertex (mm)
Figure 27 Plot demonstrating the astigmatism of the orange LED and hence the lack
of fit to a hyperbola.
36
5.10 HIGH POWER BLUE LED - LUXEON STAR
Figure 28 Photo of Luxeon Star LED with Fraen 10° lens
The Luxeon Star LED, including the Fraen 10° lens associated with the LED, is shown in
Figure 29. Details for the Luxeon Star are given in Appendix 3.
Luxeon V-Star Batwing LED (Royal blue) + Fraen 10° Lens. (x-axis)
13.0
CSM width (mm)
12.0
11.0
10.0
9.0
8.0
7.0
340
350
360
370
380
390
400
410
Distance past vertex (mm)
Figure 29 Hyperbolic fit to data from high power royal blue LED through one achromat
37
The values for the beam width of the Luxeon Star LED are plotted in Figure 30. The angular
subtense of this device far exceeds Įmax (100 mrad) and hence falls outside the region where the
coefficient C6 (IEC 60825-1) value depends upon angular subtense. It should be noted that this
LED carries a Class 2 warning label.
38
6 UNCERTAINTY ANALYSIS
With reference to the ISO Guide to Uncertainty in Measurement
separated into:
Type A, those uncertainties evaluated by statistical methods
and Type B, those evaluated by other methods.
18
the uncertainties are
The equations used to derive the beam propagation parameter are partially differentiated with
respect to all the measured quantities to produce contributions to the uncertainty budget.
A simplified summary of the Type B uncertainty budget is shown in the Table 11. The
uncertainties quoted are the reduced values (coverage factor k=1).
Table 11 List of Type B uncertainty values
Source of Uncertainty Value
Focal lengths
0.5 %
Datum positions
Width measurements
0.1 mm
0.025 mm
Measurement locations 0.15 mm
The second moment width measurements were fitted to a hyperbolic curve and the curve
coefficients were then used to derive the beam propagation parameters. The uncertainty of this
measurement was therefore derived by the partial differentiation of the equations defining the
propagation parameters. This was checked using a step-wise uncertainty analysis, which
produced close agreement with the original method (partial differentiation is a more rigorous
method).
Correlation has not been considered in this analysis but the uncertainties were combined using
sum of squares to give the most conservative estimate of uncertainty.
The uncertainty derivations had to consider three measurement configurations
a) No Transform lenses used (LED has an external waist)
b) One Transform lens used
c) Two Transform lenses used
The tables below give examples of each configuration. The Type B uncertainties listed in Table
11 are added in quadrature to provide the uncertainty values for component positions.
39
6.1.1
No Transform Lens Used
This example is for the 12-bit direct beam measurement of Osram IR LED. Table 12 details the
uncertainty components. The uncertainty references are at the end of the Section 6.
Table 12 Uncertainties for measurement of Osram IR LED
Waist position zo
Waist diameter Wo
Rayleigh distance Zr
Divergence 4
M2
4.28
3.06
12.13
252
639
mm +/mm +/mm +/mrad
+/-
0.04
0.0067
0.0287
0.81
3.17
The uncertainties for the raw beam were calculated by knowledge of the Type B uncertainty in
the measurement of distance modified by the local gradient of the hyperbola. The resulting
covariances were then added in quadrature to obtain estimates for the 1 standard deviation level.
6.1.2
One Lens datasheet
This example is for the 12-bit direct beam measurement of high power Luxeon Star LED Osram
IR LED (optical arrangement and dimensions are shown in Figure 31). Table 13 details the
uncertainty components for this measurement.
Figure 30 Diagrams illustrating the required dimensions for the Luxeon star LED
40
Table 13 Uncertainties for measurement of Luxeon Star LED
Symbol
Formulae
Distance
Uncertainty
1V
(mm)
Uncertainty calculation code
LED Vertex to lens datum
L4
253.00
0.21
RMS
Datum of Lens 1 to 1st principle
plane
L8
19.7
0.16
M/F
Separation of principle planes
L9
5.3
0.11
M/F
Effective focal length of lens
fe
76.2
0.38
M/F
Location of measured beam
waist
Zo1
Z0
370.08
0.04
Ref 1
Distance of measured waist from
focal plane
X1
X 1 Zo1 L4 L8 L9 f e
15.88
0.48
RMS
Rayleigh Length of measured
beam
Zr1
Zr
24.51
0.50
Ref 2
Waist width of measured beam
Wo1
Wo
A
B2
4C
7.56
0.16
Ref 3
Far-field divergence of measured
beam
41
4
Wo
u10 3
Zr
308.3
8.97
Ref 4
Transform Parameter
G1
G
6.81
0.24
Ref 5
Distance of output waist from
focal plane
Y1
Y1 G ˜ X 1
108.1
5.0
Ref 6
Location of output beam waist
wrt vertex of LED
Zo2
Zo2
L4 L8 Y1 f e
88.4
5.0
Ref 7
Waist width of LED output beam Wo2
Wo 2
Wo1 ˜ G1
19.7
0.5
Ref 8
Rayleigh Length of LED output
beam
Zr2
Zr2
G ˜ Z r1
166.9
6.8
Ref 9
Far-field divergence of LED
output beam
42
4
Wo
u10 3
Zr
118.2
18.3
Ref 10
B
2˜C
1 §
1 2·
¨ A˜C B ¸
4 ¹
C ©
f2
( X 12
Z r2 )
41
6.1.3
Uncertainty Budget For Beam Waist (two lens transform)
Figure 32 illustrates some of the critical measurements made for the Ligitek LUY 3833/A29
Yellow LED evaluation.
Figure 31 Critical measurements for two lens transformation
Table 14 Uncertainties for measurement of Yellow LED
Symbol
LED Vertex to lens 2
datum
Datum of Lens 2 to
1st principle plane
Separation of
principle planes of
Lens 2
Effective focal length
of Lens 2
LED Vertex to lens 1
datum
Datum of Lens 1 to
1st principle plane
Separation of
principle planes
Effective focal length
of lens
Formulae
Uncertainty
Distance
1ı
calculation
(mm) Uncertainty
code
L5
161.8
0.14
RMS
L10
13.23
0.15
M/F
L11
4.9
0.10
M/F
fe2
100
0.50
M/F
L4
81.60
0.14
RMS
L8
19.7
0.16
M/F
L9
5.3
0.11
M/F
fe1
76.2
0.38
M/F
242.43
0.02
Ref 1
-37.50
0.55
RMS
1 §
1 2·
¨ A ˜ C B ¸ 16.70
C ©
4 ¹
0.20
Ref 2
B
2˜C
Location of measured
beam waist
Zo1
Distance of measured
waist from input
focal plane of Lens 2
X1
X1 Zo1 L4 L8 L9 fe
Rayleigh Length of
measured beam
Zr1
Zr
Z0
42
B2
4C
Waist width of
measured beam
Wo1
Wo
A
Far-field divergence
of measured beam
41
4
Wo
u10 3
Zr
Transform Parameter
G1
G1
Y1
Y1
3.81
0.05
Ref 3
228.2
3.85
Ref 4
5.93
0.16
Ref 5
-222.5
6.8
Ref 6
2
f2
2
( X 1 Z r21 )
Distance of waist of
intermediate beam
from focal plane of
Lens 2
Location of
intermediate beam
waist wrt vertex of
LED
Waist width of
intermediate beam
Zo2
Z o2
L5 L10 Y1 f e2 297.5
6.8
Ref 7
Wo2
Wo 2
Wo1 ˜ G1
9.3
0.2
Ref 8
Rayleigh Length of
intermediate beam
Zr2
Zr2
G ˜ Z r1
99.1
2.9
Ref 9
Far-field divergence
of intermediate beam
42
4
Wo
u103
Zr
93.7
0.6
Ref 10
Distance between
intermediate waist
and input focal plane
of Lens 1
X2
X 2 Zo2 L4 L8 L9 f e1 114.7
6.8
RMS
Transform Parameter
G2
G2
0.2526
0.018
Ref 5
Distance of waist of
input beam from
focal plane of Lens 1
Y2
Y2 G2 ˜ X2
28.98
2.7
Ref 6
Location of input
beam waist wrt
vertex of LED
Zo3
Z 03
L4 L8 Y2 f1
-3.88
2.8
Ref 7
Waist width of LED
output beam
Wo3
Wo 3
Wo 2 ˜ G2
4.67
0.2
Ref 8
Rayleigh Length of
LED output beam
Zr3
Z r3
G2 ˜ Z r 3
25.03
0.73
Ref 9
Far-field divergence
of LED output beam
43
43
Wo3
u103
Zr 3
186.41
9.36
Ref 10
G ˜ X1
2
f1
2
( X 2 Z r22 )
43
6.1.4
Uncertainty References
The fitting used for the measurement data was :
(W r V W ) 2
A r a (B r b) ˜ z (C r c) ˜ z 2
Example of one lens analysis as fitted to the Luxeon blue LED propagation data.
Where A =
13074
B=
-70.35
and a =
0.972
b=
0.00518
C=
0.0950
c = 6.911E-06
Example of two lens analysis as fitted to the Yellow LED transformed output beam
Where A =
3075
B=
-25.25
C=
0.0521
and a =
0.143
b=
0.00117
c=
2.395E-06
sl =
0.1
These are the partially differentiated equations for the derived parameters and therefore provide
the measurement uncertainty of the named parameters.
Reference 1:
1
§c˜B·
˜ b2 ¨
¸
2 ˜C
© C ¹
V Zo
2
2
Reference 2:
2
§ c § B2
··
1
2 § bB·
a ¨ ¸ ¨¨ ¨¨ A¸¸¸¸
2CZr
© 2C ¹ © C © 2C ¹¹
VZ
r
2
Reference 3:
2
1
§ bB · § cB ·
2
a ¨ ¸ ¨¨ 2 ¸¸
2Wo
© 2C ¹ © 4C ¹
VW
o
Reference 4:
V4
Reference 5:
2
103
V W2o
Zr
§
W ·
¨¨V Zr ˜ o ¸¸
Zr ¹
©
2
2
ı
Gi
wGi
where
wX i
Reference 6:
VY
Reference 7:
VZ
2
§ wG ·
§ wG ·
§ wG ·
ı ¨¨ i ¸¸ ı 2f ¨¨ i ¸¸ ı 2 ¨¨ i ¸¸
X i wX
Z ri wZ
© wf ¹
© i¹
© ri ¹
i
o(i 1)
V
wGi
Gi2 X i
2
,
2
wf
f
2
Gi
˜ X i V X i ˜ Gi
2
2˜Vl 2 VYi2 V 2f
2or1
2
Reference 8:
2
2
VW
o(i1)
§ V Gi ˜ Woi ·
2
¸ VW ˜ Gi
¨
oi
¨ 2 G ¸
i ¹
©
44
G
2 i
f
wGi
and
wZ ri
Gi2 Z ri
2 2
f
Reference 9:
Reference 10:
VZ
r(i1)
V 4i
V Gi ˜ Zri 2 V z
ri
103
VW2oi
Zri
˜Gi
§
W ·
¨¨V Zri ˜ oi ¸¸
Zri ¹
©
2
2
45
7 CONCLUSIONS
The results from this investigation show that this method can be applied successfully to the
analysis of beam propagation parameters 17 and hence the apparent source size determination for
stigmatic and simple astigmatic beams from LEDs. It is also capable of identifying beams that
suffer from general astigmatism and which are not currently eligible for relaxation of
classification limits using this simplified form of analysis. The good agreement obtained for the
ISO Transform Validation Experiment for both the 8-bit and 12-bit systems shows that the
technique is highly robust. This level of agreement was unexpected, due to the increased noise
levels inherent in the 8-bit system as a result of the smaller dynamic range and the un-cooled
analogue camera.
Assuming a maximum permissible inaccuracy is 10% at a measuring distance of 100 mm, the
geometric approximation can only be used with beams whose Rayleigh Length is less than
approximately 50 mm. This limit effectively marks the boundary above which diffraction
effects become noticeable and classical ray-tracing optics cannot be used. The measured LED’s
Rayleigh Lengths were all less than 50 mm which allows us to consider the apparent source size
and its location, to be the same as the direct beam waist.
The detection of general astigmatism in the beam from the orange LED shows that the in built
checks of the techniques applicability work well.
Figure 33 shows the high level of agreement between the propagation parameters derived
through the 8-bit and 12-bit methods using the IR LED. It also includes the parameters derived
through the Transform Validation Experiment. Figure 34 shows the measurement results from
each of the LED’s plotted on the contour plot for angular subtense as a function of the measured
beam characteristics of LEDs. The size of the ellipse indicates the level of measurement
uncertainty for each LED.
The montage, presented in Figure 32, clearly depicts how the beam images, profiles and results
correspond. The beam image mapped onto the beam propagation envelope with the appropriate
2D spatial intensity profiles is a helpful visualization of the evolution of the beam as it travels
through space. The important aspects to note are that the point in the beam propagation where
the LED chip structure is in focus does not correspond to the position of the beam waist. This is
an important result because it has been the practice of some safety assessors to use the position
of sharp focus to estimate the apparent source size. In this situation this methodology would
result in an estimate of the apparent source size that was greater that the real value. This would
produce a lower value of the potential hazard of the LED than actuality.
46
Figure 32 Montage of the spatial beam profiles which make up the
propagation envelope of the LED
47
Com parison of Beam Param eter Measurem ents - Y-axis
14
12
8
12 -b it raw beam
12 -b it t ransf ormed
8-bit raw beam
8 -bit t ransf ormed
6
4
2
0
Zo
Wo
Zr
Thet a
Comparison of Beam Parameter Measurements - X-axis
16
14
12
millimeters or d/rad
millimeters or d/rad
10
10
12-bit raw beam
12-bit transformed
8
8-bit raw beam
8-bit transformed
6
4
2
0
Zo
Wo
Zr
Theta
Figure 33 Comparison of 8-bit and 12-bit camera results
48
Figure 34 Plot showing contours of angular subtense, including results for the
measured LEDs
49
7.1
FUTURE DIRECTIONS
This project has allowed the identification of many areas where the technique can be improved
with further work. An improvement of the length measuring system would result in a reduction
of the uncertainty of the validation process and would produce greater accuracy in the
determination of beam waist, divergence and thus retinal hazard. An optically encoded servo
motor slide would reduce the distance measurement uncertainty by an order of magnitude. The
replacement of a manual vernier slide with an electrically driven version would remove the need
to manually read measurement position. This would allow better exclusion of background light
by the use of a local light tight enclosure coated with diffusing black paint. The lack of
extraneous light would improve the measurement dynamic range and reduce the probability of
problems caused by optical artefacts on the CCD images.
A custom produced achromat with a larger diameter and shorter focal length would serve to
reduce the cumulative uncertainty. The reduction in the number of optical surfaces through
which the light propagates would serve to reduce aberration of the beam wavefront and the
production of scattered light. A custom manufactured graticule would allow calibration of the
whole field of view at higher magnification zoom settings.
An ideal development of this project would be to determine the real spot size produced by a
given source by producing an “artificial” eye or eye analogue. Apparent source size was created
as an artifice to allow the comparative measurement of the effect of viewing sources larger than
a “point” source yet smaller than the 100 mrad subtense advocated in the IEC safety standard.
This would then inform the debate about the effect of problematic beam profiles on the retina
and thus would allow a thermal diffusion model to be produced with finite element analysis.
Figure 35: Diagram showing the lens of an eye transforming a LED beam. The size of
the spot on the retina is not measurable with current techniques.
The correct treatment of generally astigmatic beams from LEDs and other intermediate sources
would require the use of the mathematical method outlined in “ISO 11146-2 7 Lasers and laserrelated equipment. Test methods for laser beam widths, divergence angle and beam propagation
ratio. Part 2: General astigmatic beams”. The existing software could be adapted and the
measurement technique amended to provide the required technique 8.
50
APPENDIX 1: SECOND MOMENT, AZIMUTH AND PRINCIPLE
WIDTH DERIVATION
The reduced second order moments can be determined by a measurement of the energy density
distribution over a limited area or window:
2
V
x ( z)
2
V
y ( z)
y2
x2
y1
x1
y2
x2
y1
x1
¦ ¦ (
x x ) ˜
I ( x, y, z) { ¢ x ²
¦ ¦ I ( x, y, z)
2
2
y2
x2
y1
x1
y2
x2
y1
x1
¦ ¦ ( y y) ˜
I ( x, y, z) { ¢ y ²
¦ ¦ I ( x, y, z)
2
2
where the summations are carried out over a rectangle parallel to the x- and y-axes and:
x1
3
x dVx
2
x2
x
3
dVx
2
3
y dVy
2
y2
y
3
dVy
2
and
y1
The concept of second moment measurements is extended to include the “mixed moments” of
the spatial and divergence properties of the beam. For example, the spatial mixed moment is:
2
V xy ( z )
y2
x2
1
1
¦ y ¦x ( x x )( y y ) ˜ I ( x, y, z )
{ ¢ xy ²
y
x
¦ y ¦x I ( x, y, z )
2
2
1
1
The three spatial moments describe the lateral extent of the power density distribution of the
beam in the reference plane. The directions of minimum and maximum extent are called
principal axes which are always orthogonal to each other. Any power density distribution is
characterized by the extents along its principal axes and the orientation of the principal axes.
The beam width along the direction of that principal axis, which is closer to the x-axis of the
laboratory system, is given by:
dVx z ­
2
ª
°
2 2 ®§¨ x 2 y 2 ·¸ J «§¨ x 2 y 2 ·¸ 4 xy
¹
¹
©
°¯©
2º
½½
½
°
» ¾
¼ °¿
and the beam width along the direction of that principal axis, which is closer to the y-axis by:
dVy z ­
2
ª
°
2 2 ®§¨ x 2 y 2 ·¸ J «§¨ x 2 y 2 ¸· 4 xy
©
¹
¹
©
¯°
51
2º
½
½½
°
¾
¼ °¿
»
where J
sgn§¨ x 2 y 2 ·¸
¹
©
x2 y2
x2 y2
Finally, the azimuthal angle between the principal axis that is closer to the X-axis and the Xaxis is :
M
ª
2 xy
½ arctan «
« x2 y2
«¬
52
º
»
»
»¼
APPENDIX 2: DESIGN AND TECHNICAL SPECIFICATION FOR A
FACILITY TO DETERMINE THE APPARENT SOURCE SIZE OF
LIGHT EMITTING DIODES
Introduction
The angular subtense of an apparent source of radiation in the 400 nm to 1400 nm wavelength
range is required by current laser safety standards to permit calculation of the relaxation factor
C6, for extended sources. It is the ratio of the angular subtense of the source in question to that
of a source that would form the realistic minimum spot size on the retina (1.5 mrad).
Classification or assessment of the thermal hazard from a source requires that both the angular
subtense and location of an extended source be known before there can be a relaxation of the
maximum permissible exposure (MPE) of the eye. The location of a source is required so that
the angular subtense can be calculated for viewing from the minimum conceivable eye
accommodation distance of 100 mm.
It is a simple matter to measure the physical size of the chip of a LED that has a Lambertian
radiation pattern but it is more difficult to know or measure the location or size of the apparent
source of collimated beams from an LED. Such beams can have a nearly plane wavefront which
would imply that the apparent source is located at infinity with an unknown angular extent.
However, recent advances in the characterization of optical beams, both coherent and
incoherent, enable prediction of their propagation envelopes. It is now possible to assess the
intrabeam viewing hazard by using known beam characteristics to estimate the angular subtense
of an extended source that would present the greatest hazard to a retina.
Measurement of the optical constants of the propagation envelope of a beam have been the
subject of considerable research over the last ten years. A consequence of this work is the
evolution of ISO standards1 for the measurement of the diameter and divergence of a beam. The
procedures and techniques that are proposed here for the determination of the diameter and
location of the apparent source of a beam2 are based on the principles underlying the ISO
standards for simple astigmatic beams. Should a beam display general astigmatism (twist) no
relaxation of the laser safety criteria should be given.
Beam measurements
There are a number of methods available for measurement of the diameter of a beam as well as
its far-field divergence. The basic principles for those methods have been established by the ISO
standard. They are applicable to laser beams with a relatively small beam propagation ratio, M2.
Recent research has demonstrated that adequate steps have to be taken to counter the effects of
noise and offset errors when measuring the transverse irradiance distribution of a beam. When
these steps are taken, the propagation behaviour of incoherent broadband beams as well as high­
1
ISO 11 146:1999. “Test methods for laser beam parameters: beam widths, divergence angle and
beam propagation factor”.
2
The measurements proposed in this document are applicable to beams whose full divergence angle
is less that 30°.
53
quality laser beams can be predicted reproducibly with considerable precision. The methods
leading to estimates of the diameter of a beam use a procedure known as the Converging Second
Moment diameter or width measurement (CSM). Those methods are being defined in the
revision of ISO 11146 that is currently in preparation.
The preferred method for measuring all the propagation characteristics of a beam is to perform
CSM diameter measurements at a number of locations either side of the beam waist.
The Optical System
The beam measurement process consists of using a CCD sensor to image the irradiance profile
at about ten locations, ideally either side of the beam waist. The proposed optical system
contains variable magnifying optics that are designed to facilitate imaging the transverse
irradiance profiles so that they occupy approximately one quarter of the sensor screen height.
Other components are included in the system to attenuate the beam power and avoid sensor
saturation and to provide spatial calibration of the pixel array of the sensor.
A schematic diagram of the proposed system is displayed later in this section.
The main base bench contains two sub-systems, the LED Bench and the Imaging Bench.
The LED Bench
The LED bench is capable of axial movement so as to transport the emerging beam relative to
the imaging bench. The movement can be achieved with manual or automatic means. The
movement distance relative to a datum should be determined with an uncertainty less than 0.1
mm. The bench is to be 400 mm long and fitted with two or three component holders that are
capable of smooth and fine adjustment about a number of specified axes.
Unit 1 - The first carriage is a holder for a range of LEDs. The LED should be firmly held in a
cantilevered stand and restrained from any movement that might arise from movement of its
power supply leads. An arrangement consisting of a split clamping plate with a suitable range of
collets that will accommodate the diameters of the selected LEDs. This unit should be capable
of adjustment along the transverse x and y axes, the axial z axis and rotation about the Ox and
Oy axes. The LEDs will probably all have a cylindrical form with discrete diameters of 3 mm, 5
mm, 8 mm or 10 mm.
Unit 2 - The next carriage has a cantilevered lens holder that is capable of placing a lens with
its vertex touching the output face or vertex of the lens of the LED. The mount should hold the
lens normal to the optical axis of the system and be capable of smooth adjustment along the
transverse x and y axes and the axial z axis.
i)
The lens should not need to have an aperture greater than f/2. This lens is
designed to transform beams whose waist is virtual and behind the LED into one with
an accessible waist in the measurement region of the system. The design of the lens
should be based on a multi-element composition aimed at minimising aberrations. It
should also be broadband A/R coated to minimise reflections at the wavelength range
that is to be studied.
ii)
An examination of the capability of a +50 mm focal length transform lens on
the range of beam parameters under consideration is given in the Annex. The results
suggest that this lens will produce transformed beams whose parameters are all within
the measurement capability of the proposed system.
54
A second carriage, similar to Unit 2 should be provided so that a second transform lens can be
mounted on the LED bench for the “validation” trials.
The Imaging Bench
A static imaging bench is to be provided with three main carriages. These are for an attenuator
wheel, a calibration graticule and the CCD camera fitted with a “zoom” microscope lens system.
Unit 3 - This is the main beam attenuation device. It consists of an indexable wheel containing,
say, eight neutral density filters. The apertures in the filter holder should be sufficient to
accommodate at least 95% of the beam power. The ND filters should exhibit a high degree of
uniformity across 95% of their full aperture and, ideally, should be antireflection coated for the
wavelength range that is to be studied.
The filter wheel holder should be capable of adjustment along both transverse x and y axes. It
should also be capable of rotation about the Oy axis so that reflections can be diverted from the
beam path.
Unit 4 - This carriage contains a holder for a calibration graticule. It should be capable of fine
adjustment along all the x, y and z axes. The location of the graticule along the z axis should be
capable of placement with a separation D1 from the objective of the microscope between 30
mm and 300 mm. The graticule should be capable of being adjusted by rotation about an Oz axis
through the centre of its aperture. The design of the graticule should enable viewing with the
CCD array to present contrast and sharpness sufficient to enable use of the provided calibration
software with an adequately low level of uncertainty. Trials will be performed with a number of
graticules to permit analysis and select a suitable design.
The function of the graticule is to enable calibration of the transverse dimensions and linearity
of the CCD sensor array. The graticule is not required during actual beam measurements
although it could be advantageous to use the beam to illuminate the graticule at its own
wavelength.
The graticule has to be removed from the beam path during measurements but it should be
capable of being relocated in its original position in the object plane of the microscope without
requiring readjustment.
Units 5 and 6 - A sturdy carriage is required to hold firmly a zoom microscope and CCD
camera with a high degree of stability. The function of the zoom microscope is to enlarge the
image of the transverse irradiance distribution in the object plane and focus it on the CCD array
of Unit 6. The zoom function enables adjustment of the magnification so that, ideally, the image
fills approximately one quarter of the array.
i)
The Microscope - The zoom microscope is required to have a field of view that
will be adjustable between 0.25 mm and 50 mm. A microscope that can satisfy this
requirement is the Leica MonoZoom77 Video Microscope System. When fitted with
either x0.25 or x2 objectives in combination no amplifier lens or a x3 amplifier, it may
just cover the required field size range when used in conjunction with an appropriately
sized CCD array sensor. In addition, an aperture diaphragm (P/N 007/023) can be
incorporated into the system to provide continuously variable attenuation of the
transmitted power. This is a very attractive facility since it will enable power
adjustment between the steps available with the ND filters.
55
A MonoZoom 7 instrument has been hired to enable assessment of its optical
performance and the magnitude of residual aberrations. Should the performance be
inadequate an alternative set of components could be assembled from the Leica Z16
APO system.
ii)
The Camera - It is thought that a CCD sensor system with a 12-bit dynamic
sensitivity is required to provide sufficient resolution and noise control for beam
profile analysis. Furthermore, a pixel number in excess of 1 million is estimated to be
required to give sufficient resolution for spot diameter measurements. However, these
aspects are to be studied in one of the principle topics of this programme.
One further aspect influencing the selection of a camera is the physical size of the
CCD array. The height dimension of a 4:3 video array is the dimension that will
control the field of view seen through the microscope. The dimensions of an array are
not always given in a camera specification. It could be referred to as a b” screen where
it will have a height of 6.6 mm and width of 8.8 mm, giving an 11 mm diagonal.
Alternative array sizes can be quoted as ½” and a”. These dimensions must be
obtained before all the lens accessories for the microscope system can be selected.
Additional Components
Three more optical aspects of the system need to be considered and provided. These are a
“beam stop” and background illumination shields. The need for these components arises from
the requirement to eliminate background optical noise from the CSM width estimates. When the
beam passes through optical components, residual multiple internal reflections and other sources
of stray light will combine with the main beam and tend to enlarge estimates of beam widths.
The first precaution is to place the whole system in a light-tight enclosure so that light from the
environment does not reach the CCD array. The next stage is to attenuate any light scattered or
reflected out of the main beam by placing an absorbing shield with an aperture at some distance
down the beam path. The aperture should limit beam divergences above 30°.
The final precaution is to make a record of any residual light or optical imperfections by
recording a frame of the average background field and subtract it from the beam profile. This
can be done by inserting a physical beam stop into the path of the beam so that the only light
that passes is from the sources of extraneous noise and recording this as the background. The
exact nature and location of this beam stop must be the subject of further discussion.
56
Figure 36 Proposed optical apparatus for measurement of apparent source size
57
APPENDIX 3: LED TECHINICAL DATA SHEETS
VISIBLE LEDs
Red:
Kingbright L-53SRC/E
http://www.us.kingbright.com/data/spec/W1503SRC-D.pdf
Yellow:
Ligitek LUY 3833/A29
http://www.ligitek.com/2-2.htm
Green:
Nichia NSPG500 Rank GS
http://www.nichia.co.jp/specification/led_lamp/NSPG500S.pdf
Blue:
Nichia NSPB500 Rank WS
http://www.nichia.co.jp/specification/led_lamp/NSPB500S.pdf
White:
Nichia NSPW500 Rank BS
http://www.nichia.co.jp/specification/led_lamp/NSPL500S.pdf
IR LED
IR:
Osram SFH 400
http://www.osram.convergy.de/scripts/product_family.asp?CLSOID=10024&FAMILYOID=20412
HIGH BRIGHTNESS LEDS
Orange:
Toshiba TLOH190P
http://www.semicon.toshiba.co.jp/td/ja/Opto/Visible_LED/20030620_TLOH190P(F)_datasheet.p
df
Blue:
Luxeon Star
http://www.lumileds.com/pdfs/DS23.pdf
Table 15 Summary of optical and electrical characteristics of LEDs
LED
model
Size
(mm)
Colour
Peak
Oҏ(nm)
Kingbright
L-53SRC/E
Ligitek
LUY
3833/A29
Nichia
NSPG500
Rank GS
Nichia
NSPB500
Rank WS
Nichia
NSPW500
Rank BS
Osram
SFH 400
Toshiba
TLOH190P
Luxeon
Star
5
Red
5
Typ
current
(mA)
660
Typical
Lumious
Intensity
(mcd)
1500
Yellow
595
5
Green
5
Bandwidth
(nm)
Divergence,
½ 4q
20
Max
Fwd
voltage
(V)
2.5
20
15
2700
20
2.8
-
12
520
11600
30
4.0
40
-
Blue
470
3460
30
4.0
30
-
5
White
595
6400
30
4.0
N/a
-
5
Ired
950
-
300
5
55
6
10
Orange
612
20000
50
4
10
6
-
Blue
470
100000
700
5
25
10
58
REFERENCES
1
“ICNIRP statement on light-emitting diodes (LED’s) and laser diodes:
implications for Hazard assessment”, Health Physics June 2000, Volume 78,
Number 6 (http://www.icnirp.de/documents/led.pdf)
1.
2
Ward B.A. “Measurement of Laser and LED Beams for prediction of
Angular Subtense ILSC 2003 conference Jacksonville, FL, USA
3
BS EN 60825-1:1994, Incorporating Amendment 1,2 and 3, Safety of laser
products. Equipment classification, requirements and user’s guide
4
ISO 11146:1999 Lasers and laser-related equipment -- Test methods for laser
beam parameters -- Beam widths, divergence angle and beam propagation factor
5
BS EN ISO 11554:2003 Optics and optical instruments. Lasers and laser-related
equipment. Test methods for laser beam power, energy and temporal
characteristics
6
01/714513 DC ISO/CD 11146-1. Lasers and laser-related equipment. Test
methods for laser beam widths, divergence angle and beam propagation factor.
Part 1: Stigmatic and simple astigmatic beams
7
01/714514 DC ISO/CD 11146-2. Lasers and laser-related equipment. Test
methods for laser beam widths, divergence angle and beam propagation ratio.
Part 2: General astigmatic beams
8
ISO/PDTR 11146-3. Lasers and laser-related equipment. Test methods for laser
beam widths, divergence angle and beam propagation ratio. Part 3: Alternative
test methods and geometrical laser beam classification and propagation (BSI
draft 01/714515 DC)
9
ISO 13694:2000 Optics and optical instruments – Lasers and laser-related
equipment – Test methods for laser beam power (energy) density distribution
10
IEC 60825-13. Ed.1. Safety of laser products. Part 13: Measurements for
classification of laser products (BSI draft 03/307798 DC)
11
IEC TR 60825-14 ed. 1. Safety of laser products. Part 14. A user's guide (BSI
Draft 02/206661 DC)
12
Henderson R and Schulmeister K “Laser Safety” Bristol, IOP,2004
13
ISO 11145:2001, Optics and optical instruments . Lasers and laser-related
equipment . Vocabulary and symbols.
59
14
ISO 13694, Optics and optical instruments . Lasers and laser-related equipment .
Test methods for laser beam power (energy) density distributions
15
IEC 61040:1990, Power and energy measuring detectors. Instruments and
equipment for laser radiation.
16
Amarande S, Giesen A Hügel H “Propagation analysis of self-convergent beam
width and characterization of hard-edge diffracted beams” APPLIED OPTICS
Vol. 39, No. 22, 1 August 2000
17
Siegman A.E “ Defining the Effective radius of Curvature for a Nonideal
Optical Beam” IEE Journal of Quantum Electronics Vol 27 No 5 May 1991
18
ISO Guide to the expression of uncertainty in measurement 1995, ISBN 92-6710188-9
19
Wood. RM “Laser-Induced Damage of Optical Materials” Bristol IOP 2003
60
GLOSSARY
Gamma
A numerical value, or the degree of contrast in a television picture, which is the
exponent of that power law which is used to approximate the value of the magnitude of
the output signal as a function of the input signal over the region of interest.
Interline Transfer
A technology of CCD design, where rows of pixels are output from the camera. The
sensor's active pixel area and storage register are both contained within the active image
area. This differs from "frame transfer" cameras that move all active pixels to a storage
register outside of the active area.
Vignetting
In an optical system, the gradual reduction of image illuminance as the off-axis angle
increases, resulting from limitations of the clear apertures of elements within the
system. This is called vignetting and is shown in Figure 37.
Figure 37 Illustration of vignetting effect
Stigmatism
Property of a beam having circular power density distributions in any plane under free
propagation and showing power density distributions after propagation through a
cylindrical lens all having the same or orthogonal orientation as that lens
Simple astigmatism
Property of a non-stigmatic beam whose azimuth shows a constant orientation under
free propagation, and retains its original orientation after passing through a cylindrical
optical element whose axis is parallel to one of the principal axes
NB: The principal axes of a power density distribution corresponding to a beam with
simple astigmatism are called the principal axes of that beam.
61
Generalised Rayleigh length (ZR,g)
Distance along the beam axis from the generalized beam waist where the generalized
beam diameter is a factor of ¥2 larger than the generalized beam waist diameter.
EFL (Effective Focal length)
The effective focal length (EFL) or equivalent focal length (denoted f in Figure 38) is
the distance from the focal points of the lens (F and F" in the Figure) to the respective
principal points (H or H"). The EFL determines magnification and hence the image size.
The term f appears frequently in the lens formulas and tables of standard lenses.
Unfortunately, the principal points are usually inside the lens, so that it is an
inconvenient measurement for precisely positioning a lens or determining mechanical
clearances. Consequently, most lenses specifications include measurements made from
the focal planes to the surfaces (verticies) of the optic (e.g., the front focal length ff, and
the back focal length fb).
Figure 38 Illustration of optical path through a lens
Back Focal Length
The Back focal length fb is the distance from the secondary vertex (A2) to the rear focal
point (F"), as illustrated in Figure 38.
Front Focal Length
The front focal length ff is the distance from the front focal point (F) to the primary
vertex (A1), as illustrated in Figure 38.
62
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