User Notes for the Milton Multiband Radiometer
Introduction
The Milton Multiband Radiometer (MMR) was first developed in 1978 as a low-cost
field portable radiometers sensing in the four Landsat MSS bands. An early version of
the instrument is described by Milton (1980), but the present instrument differs from
that early design in several ways, the most important of which are:




Silicon photodiode detectors for all four bands, giving improved linearity and
better signal-to-noise ratio;
Digital display giving better precision and less chance of transcription errors;
Internal data buffers, allowing near simultaneous measurements of target and
reference panel with a single sensor head (single beam mode);
The ability to use two sensor heads (dual beam mode) to make truly simultaneous
measurements of the target and the reference panel.
There have been many other improvements to the basic design, but the overriding aim
has been to produce an instrument which is simple to use, reliable and adaptable to the
varied needs of teachers and researchers in remote sensing. Inevitably, some
compromises have had to be made along the way, some of which may affect how you
use the instrument, so please read carefully through these notes before using the
radiometer.
Description of the Milton Multiband Radiometer
The MMR comprises a meter unit, one or two sensor heads, a connecting cable and,
optionally, a data logger. It is normally left to users to provide a suitable calibrated
reference panel and a means of supporting the instrument over the surfaces of interest.
The MMR can be operated hand-held, but it is better to ue it from a mast or a tripod
and a fixing is provided on the sensor head for this purpose. Power for the MMR
comes from a set of AA size batteries fitted within the meter unit. Rechargeable
NiMH batteries are recommended, although alkaline cells may be substituted in an
emergency (see important safety note below1). Many MMR supplied before 2001
were fitted with rechargeable nickel-cadmium cells but these are no longer
recommended due to their well-known ‘memory’ effects. The MMR circuit draws
very little current, so a fresh set of batteries will typically power the unit for over 48
hours continuously.
1
If non-rechargeable (i.e. alkaline) cells are fitted, a prominent label must be stuck on the outside of
the instrument, warning users not to attempt to recharge them.
1
The MMR in use
The MMR was designed to measure the spectral reflectance of spatially extensive
targets. The standard instrument is not calibrated in radiance units because it is
intended to be used with a calibrated reference panel to produce measurements of
reflectance factor. Care must be exercised when using the MMR to measure the
reflectance of small or spatially variable targets because the simple optics of the
instrument mean that the field-of-view is not sharply defined, nor is the point spread
function (psf) calibrated. With these limitations in mind, the MMR is well-suited to
making rapid measurements of the spectral reflectance of many different terrestrial
surfaces.
The MMR may be operated in either single beam or dual beam mode. The principle is
the same in each mode: the reflectance factor is defined by the amount of energy
reflected from the target in a particular waveband and in a particular direction, as a
proportion of that reflected from a calibrated reference panel. The panel must be
calibrated to determine its reflectance in each of the wavebands sensed by the MMR,
and it may also be desirable to calibrate the angular spectral response of the panel,
depending upon the aims of the experiment and the conditions of measurement. This
aspect is considered further below.
Dual beam mode is more precise than single beam mode as it avoids errors introduced
by temporal variations in the atmosphere. However, it involves the additional expense
of a second sensor head, and the two heads must be accurately inter-calibrated (see
below). The MMR is unique in that it allows measurements to be made either in dual
beam mode or in single beam mode with minimal time delay between target and
reference measurements. The normal delay inherent in the single beam method is
reduced to the minimum by using internal data buffers to store the data values from
the the target whilst those from the reference panel are acquired.
Operation in single beam mode
1. Connect the sensor head to the socket marked ‘S’ (see Figure 1).
2. Switch on the radiometer and select channel 1 on the rotary switch. This switch
controls which of the eight data buffers is displayed on the liquid crystal display
(LCD).
3. Set the toggle switch to ‘low’. This switch controls the sensitivity of the MMR.
The ‘low’ position is for low light levels, the ‘high’ position should be used if the
meter goes over-range on the ‘low’ scale.
4. Block off all light from entering the sensor unit by pressing it against an optically
dense object. It is not sufficient to press it into the palm of your hand as some near
infra-red energy could be transmitted through your hand. Press it into your body
or a thick piece of black cloth.
5. With the sensor head still blocked from receiving any light, press button ‘S’ for 3
seconds and then release. Then press button ‘R’ for a similar length of time and
release.
6. Turn the rotary switch to each of the eight data buffers in turn and make a note of
the reading shown on the LCD. These values are the ‘dark level offsets’ for each
2
band and they are a small error signal that must be removed from the data before
calculating the reflectance factor.
7. Position the sensor head vertically over the centre of the reference panel, checking
that it is not shading the panel and that the field-of-view is contained within the
panel.
8. Press the button marked ‘R’ and hold it down until the reading on the display
stabilises. If the illumination conditions are variable, the reading may never
become completely stable, in which case a judgement should be made as to when
to capture the data. When button ‘R’ is released, the data from all four spectral
bands are transferred to the first four data buffers.
9. Position the sensor head vertically over the target surface, checking that it is not
shaded by the operator or any of the equipment.
10. Press the button marked ‘S’ and, when the display stabilises, release the button.
When button’S’ is released, the data from all four spectral bands are transferred to
data buffers 5 to 8.
11. When the data from both reference panel and target have been sent to the data
buffers they can be displayed in turn using the rotary switch. Take care not to
press either of the buttons while reading out the eight data values. The data can be
transcribed manually or can be entered into a suitable data logger.
12. It is normally advisable to make more than one measurement of the reflectance
factor of a target, but it is not necessary to measure the dark level offsets every
time. These only change significantly if the temperature of the instrument alters,
so repeat the procedure from step (7).
Operation in dual-beam mode
1. First, intercalibrate the two sensor heads as described below.
2. Connect the sensor head to be used with the reference panel to the socket marked
‘R’, and that to be used over the target surface to the socket marked ‘S’.
3. Switch on the radiometer and select channel 1 on the rotary switch. This switch
controls which of the eight data buffers is displayed on the liquid crystal display
(LCD).
4. Set the toggle switch to ‘low’. This switch controls the sensitivity of the MMR.
The ‘low’ position is for low light levels, the ‘high’ position should be used if the
meter goes over-range on the ‘low’ scale.
5. Take the reference panel sensor head and block off all light from entering it by
pressing it against an optically dense object. It is not sufficient to press it into the
palm of your hand as some near infra-red energy could be transmitted through
your hand. Press it into your body or a thick piece of black cloth. With the sensor
head still blocked from receiving any light, press button ‘S’ for 3 seconds and then
release.
6. Turn the rotary switch to each of the first four data buffers in turn and make a note
of the reading shown on the LCD. These values are the ‘dark level offsets’ for
each band and they are a small error signal that must be removed from the data
before calculating the reflectance factor.
7. Repeat step (5) for the target surface sensor head.
8. Turn the rotary switch to each of data buffers 5 to 8 in turn and make a note of the
reading shown on the LCD.
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9. Position the sensor heads over the reference panel and the target. Take the same
precautions against shading and scattering from areas outside the field-of-view as
in the single beam mode described above.
10. Press one of the buttons (‘S’ or ‘R’, it does not matter which) when you are ready
to take a reading. Hold the button down until the reading on the LCD stabilises. If
the illumination conditions are variable, the reading may never become
completely stable, in which case a judgement should be made as to when to
capture the data. When the button is released, the data from all four spectral bands
from the reference panel and all four bands from the target are transferred to the
data buffers.
11. When the data have been sent to the data buffers each band can be displayed in
turn using the rotary switch. Take care not to press either of the buttons while
reading out the eight data values. The data can be transcribed manually or can be
entered into a suitable data logger.
13. It is normally advisable to make more than one measurement of the reflectance
factor of a target, but it is not necessary to measure the dark level offsets every
time. These only change significantly if the temperature of the instrument alters,
so repeat the procedure from step (9).
If the sensor heads are plugged into the sockets suggested above then the data
presented to the LCD in both modes of operation are as follows:
Rotary
switch
position
1
2
3
4
5
6
7
8
Data buffer
number
Data stored
1
2
3
4
5
6
7
8
Band 1 from target
Band 2 from target
Band 3 from target
Band 4 from target
Band 1 from reference panel
Band 2 from reference panel
Band 3 from reference panel
Band 4 from reference panel
Calculating the reflectance factor
Single beam mode
Reflectance factor =
S   VS   k 
R  VR  
Where:
S =
R =
VS =
digital number from surface in band 
digital number from the reference panel in band 
dark level offset from buffers 1-4 (surface, bands 1-4).
4
(1)
VR =
k
=
dark level offset from buffers 5-8 (reference panel, bands 1-4).
absolute reflectance of the reference panel in band 
Dual beam mode
Reflectance factor =
S   VS  k P 
R  VR   
(2)
Where:
S
R
VS
VR
k
P
=
=
=
=
=
=
digital number from surface in band 
digital number from the reference panel in band 
dark level offset from buffers 1-4 (surface, bands 1-4).
dark level offset from buffers 5-8 (reference panel, bands 1-4).
absolute reflectance of the reference panel in band 
panel calibration factor in band  (see below).
Figures 2 and 3 show worked examples of each of the modes of operation.
Intercalibration of two sensor heads
Before two sensor heads can be used together in dual beam mode it is necessary to
intercalibrate them. First, you should check that the two heads have the same filters
and detectors fitted2, and compare their spectral response curves if these are available.
Assuming that the two sensor heads have well-matched spectral responses, all that
remains is to relate their respective gains. The two sensor heads respond in a linear
manner to changes in illumination so a linear regression equation is sufficient to
achieve the intercalibration. The calibration equation can be most easily determined
from a set of data collected on a day with intermittent bright sunshine. Clear blue sky
interspersed with rapidly moving fair weather cumulus clouds is ideal. The two sensor
heads should be mounted together pointing at a uniform, highly reflective diffuse
target (a Spectralon reference panel is ideal). At least 50 measurements are then taken
from the paired sensor heads, covering as wide a range of illumination conditions as
possible. Don’t forget to measure the dark level offset. These data can then be plotted
on a graph and P determined from the slope of the regression equation fitted to the
data in each band. Figure 4 shows some typical results from this technique.
Correlation values (r) between sensor heads of 0.99 or better are to be expected.
Battery charging
If rechargeable batteries are fitted, the need to recharge them is shown by a message
appearing on the LCD. The mains adapter should then be connected to the socket on
the right-hand side of the meter unit marked ‘AUX’ and the unit left on charge for up
2
This is usually shown on a label fixed to the sensor head.
5
to 14 hours. Please note that it is necessary to have the sensor head(s) plugged in
while the unit is charging.
Routine maintenance
Checking the gelatin filters
The MMR has two sets of filters: glass filters fitted internally and gelatin filters taped
to the outside of the field stop. In time, the gelatin filters can become degraded, or
they may be damaged, in which case it is possible to replace them. Hot temperatures
can cause the gelatin filters to become brittle and crack, and hot, humid conditions can
cause them to become cloudy. The first problem is usually evident by looking down
the field stop and inspecting the surface of the filter. The second problem is only
evident by removing the filter. If in doubt, it is best to fit new filters.
The gelatin filters are available from the Kodak Wratten range (see Table) and are
available through Kodak dealers worldwide.
Band number
1
2
3
4
Wratten 45
Wratten 99
Wratten 29
Wratten 87
Kodak part number
75mm x 75mm
149-5761
149-6306
149-5621
149-6256
Colour
blue
green
red
near infra-red
Kodak gelatin filters are made by dissolving organic dye in liquid gelatin and coating
the proper amount of the solution onto prepared glass. After the coating is dry, the
gelatin film is removed from the substrate and coated with a very thin lacquer film for
protection. They should be handled with care and only held by the edges. They may
be cut by placing the filter in a sandwich of thin card. Spare filters should be stored
flat, interleaved with clean paper, in a cool, dark, dry place.
To replace the gelatin filters, first remove the field stop by undoing the four small
nuts. Note the orientation of the field stop relative to the sensor unit before you
remove it. The four gelatin filters will be revealed held between the field stop and the
front of the sensor unit, held by a piece of light trap material (black card or similar).
Take care that the new filters are placed over the correct detectors and that the light
trap is re-fitted and seated correctly, and then replace the field stop.
Checking the connecting lead
The multicore lead connecting the sensor unit to the meter unit can be damaged by
repeated flexing or by accidents. It is recommended that users carry a spare lead
during critical data collection. Take care not to damage the connection pins or sockets
when inserting and removing the lead. If a faulty lead is suspected, check to see if any
of the pins or sockets are bent or have become pushed back into the plastic housing. A
pin that has become unseated and pushed back can often be cured by opening up the
meter unit and applying gentle pressure from the inside. It should ‘click’ back into
place.
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Other adjustments
Changing the sensor head gain
The toggle switch on the front of the instrument marked ‘LOW’ and ‘HIGH’ switches
the display between two sensitivity scales. It does not alter the basic gain of the
MMR. Keep the MMR on the LOW setting, except if the display reads over-range
(‘1’ in left-hand digit, others blank).
As supplied, the MMR is suitable for field measurements over most surfaces with a
grey reference panel, but there are occasions when greater sensitivity (higher gain) is
required, such as if measurements are routinely made of dark soil surfaces under
cloudy skies. It is possible to vary the gain of each band independently over a wide
range to optimise the instrument for a range of applications. To do this you should
open the sensor unit and identify the various switches and variable resistors shown in
Figure ??. The following Table shows the relative gains possible by altering the
position of each of the four switches and rotating the variable resistors.
Variable resistor fully clockwise
Variable resistor fully anti-clockwise
Switch set at ON
Gain = 1 (default)
Gain = x 2.5
Switch set at OFF
Gain = x2
Gain = x5
Answers to Frequently Asked Questions
To be completed.
See:
http://www.soton.ac.uk/~ejm
and:
http://www.soton.ac.uk/~epfs
References cited
Milton, E.J., 1980. A portable multiband radiometer for ground data collection in
remote sensing. International Journal of Remote Sensing, 1, 153-165.
Nicodemus, F.F., Richmond, J.C., Hsia, J.J., Ginsberg, I.W. and Limperis, T.L.,
1977. Geometrical considerations and nomenclature for reflectance, 1977, National
Bureau of Standards Monograph 160, U.S. Govt. Printing Office, Washington D.C.
20402.
Glossary
anisotropy index:
A number that represents the degree to which the
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bidirectional reflectance
distribution function
(BRDF):
bidirectional reflectance
factor (BRF):
goniometer:
goniospectrometer:
hot-spot:
photometer:
radiometer:
solar principal plane:
spectral library:
Spectralon:
spectrometer:
directional reflectance of a surface departs from that
expected from a perfectly diffuse, or Lambertian
surface. Generally speaking, a Lambertian surface
has an anisotropy index of 1.0.
A mathematical function that describes the
reflectance of a surface in terms of the angles that
incident and reflected rays make with the normal to
the surface and with a reference direction in the
horizontal plane. The function is also wavelengthdependent. It has units of inverse steradians (sr-1).
The ratio of the radiant flux reflected by a sample
surface to that which would be reflected into the
same reflected-beam geometry by an ideal, 100%
reflective, perfectly diffuse (Lambertian) standard
surface, irradiated in exactly the same way as the
sample (Nicodemus et al., 1977, US National Bureau
of Standards Monograph 160).
A device to enable a sample to be positioned
precisely in relation to the geometry of illumination
and viewing.
The combination of a goniometer and a spectrometer.
The peak of reflectance observed in the solar
principal plane at which the source of illumination
(generally the Sun) is directly aligned with the
sensor, and therefore the point at which the sensor
views its own shadow and sees no shadows from
objects on the surface.
An radiometer with a spectral response deigned to
match that of the human eye-brain system.
Photometers are calibrated in photometric units (SI
unit = ???).
An instrument to measure the amount of radiant flux
contained within a finite solid angle. Radiometers are
calibrated in radiometric units (SI unit = mWsr-1nm-1)
The plane joining the source of illumination, the
sensor and the target.
A collection of calibrated reflectance spectra,
together with their associated documentation and
metadata. Spectra should be traceable to a recognised
national standard to be of value.
A synthetic white plastic material made from sintered
poly?? that is well-suited to use as a field reflectance
standard. It is highly reflective throughout the optical
region, chemically inert and resists surface
contamination. Spectralon is a commercial product of
Labsphere Inc.
An instrument to measure the energy reflected from a
surface into a solid angle in many different
wavelengths, such that, for practical purposes, the
measurement can be considered to be a continuous
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spectrum. Many spectrometers (but not all) are
calibrated in radiometric or photometric units.
Further reading
Clark, R.N., 1999, Spectroscopy of rocks and minerals, and principles of
spectroscopy. Chapter 1 in Remote Sensing for the Earth Sciences: Manual of
Remote Sensing, 3rd ed., Vol. 3, Rencz, A.N. (ed.), John Wiley & Sons Inc., New
York, 3-58. [Excellent in-depth review of the principles of reflectance
spectroscopy with many example spectra from rocks and minerals].
Deering, D.W., 1989, Field measurements of bidirectional reflectance. In: Theory
and Applications of Optical Remote Sensing, Asrar, G., ed. John Wiley & Sons
Inc., New York, 14-65. [Comprehensive, well-illustrated review of the subject
from the perspective of a NASA scientist].
Duggin, M.J. and Philipson, W.R., 1982. Field measurement of reflectance: some
major considerations. Applied Optics, 21, 2833-2840. [Describes some
intercalibration issues involved in using the dual-beam method and proposes a
methodology to achieve accurate BRF measurements using two
spectroradiometers].
Milton, E.J., 1987. Principles of field spectroscopy. International Journal of Remote
Sensing, 8, 1807-1827. [Review paper containing much practical advice and a list
of instruments widely used in the 1980s].
Milton, E.J., Rollin, E.M. and Emery, D.R., 1995. Advances in field spectroscopy.
In: Danson, F.M. and Plummer, S.E., ed. Advances in Environmental Remote
Sensing, John Wiley & Sons Ltd, Chichester, 9-32. [Reviews the growth of field
spectroscopy in the UK, especially thorough the activities of the UK Natural
Environment Research Council Equipment Pool for Field Spectroscopy].
Robinson, B.F. and Biehl, L.L., 1979. Calibration procedures for measurement of
reflectance factor in remote sensing field research. Society of Photo-Optical
Instrumentation Engineers, 196-Measurement of Optical Radiations, 16-26.
[Good introduction to the issues involved in making accurate field measurements].
Salisbury, J.W. 1998. Spectral measurements field guide.
Schaepman, M.E., 1998. Calibration of a field spectroradiometer, Remote Sensing
Series, Volume 31, Remote Sensing Laboratories, Department of Geography,
University of Zurich, 146p. [Thorough field and laboratory evaluation of the
GER3700 field spectrometer]
Silva, L.F., 1978. Radiation and instrumentation in remote sensing. In: Swain, P.H.
and Davis, S.M., ed. Remote Sensing: The Quantitative Approach, McGraw-Hill,
New York, 21-135. [Important chapter in a highly influential book. Thorough, nonmathematical discussion of the physical principles of field spectroscopy]
Steiner, D. and Guterman, T., 1966. Russian data on spectral reflectance of
vegetation, soil and rock types, Final Technical Report on US Army Contract DA91-591-EUC-3863/OI-652-0106, Department of Geography, University of Zurich,
232p. [Comprehensive survey of Russian literature on field spectroscopy,
including the extensive field data sets collected by Krinov and co-workers in the
1940s and 1950s].
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