United States
Environmental Protection
Agency
Environmental Monitoring
and Support Laboratory
PO. Box 15027
Las Vegas NV 89114
EPA-600/4-78-045
August 1 978
Research and Development
&EPA
Environmental
Research Series
Tests of the Dual
Differential Radiometer
Under Field Conditions
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad categories
were established to facilitate further development and application of environmental
technology. Elimination of traditional grouping was consciously planned to foster
technology transfer and a maximum interface in related fields. The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL MONITORING series/This series
describes research conducted to develop new or improved methods and instrumentation
for the identification and quantification of environmental pollutants at the lowest
conceivably significant concentrations. It also includes studies to determine the ambient
concentrations of pollutants in the environment and/or the variance of pollutants as a
function of time or meteorological factors.
This document is available to the public through the National Technical Information
Service, Springfield, Virginia 22161
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EPA-600/4-78-045
August 1978
TESTS OF THE DUAL DIFFERENTIAL
RADIOMETER UNDER FIELD CONDITIONS
by
Robert W. Thomas
Monitoring Operations Division
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89114
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DISCLAIMER
This report has been reviewed by the Environmental Monitoring and Support
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Mention of trade names or commercial products does not constitute endorsement
or recommendation for use.
ii
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FOREWORD
Protection of the environment requires effective regulatory actions which
are based on sound technical and scientific information. This information
must include the quantitative description and linking of pollutant sources,
transport mechanisms, interactions, and resulting effects on man and his
environment. Because of the complexities involved, assessment of specific
pollutants in the environment requires a total systems approach which trans-
cends the media of air, water, and land. The Environmental Monitoring and
Support Laboratory-Las Vegas contributes to the formation and enhancement of a
sound monitoring data base for exposure assessment through programs designed to:
• develop and optimize systems and strategies for moni-
toring pollutants and their impact on the environment
• demonstrate new monitoring systems and technologies by
applying them to fulfill special monitoring needs of
the Agency's operating programs
This report assesses the use of the dual differential radiometer to re-
motely determine chlorophyll a_ values in United States lakes and reservoirs.
The results obtained should be of value to prospective users of this and
similar remote sensing instrumentation; many of the problems encountered are
common in the use of spectral data for remote sensing. Raw data and further
information can be obtained from the Water and Land Quality Branch, Monitoring
Operations Division.
George B. Morgan
Director
Environmental Monitoring and Support Laboratory
Las Vegas
111
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ABSTRACT
A dual differential radiometer was tested on numerous eastern United
States lakes and reservoirs. Remotely sensed data were compared with ground-
truth chlorophyll a^values. Results indicate that the instrument has only
limited application in the remote sensing of chlorophyll a^ in the nation's
lakes. At its present state of development, its use should be confined to
large, deep, relatively clear water bodies in conjunction with ground-truth
and surface survey efforts.
iv
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CONTENTS
Foreword iii
Abstract iv
List of Figures vi
List of Tables vi
1. Introduction 1
2. Conclusions 2
3. Recommendations 3
4. Theory of Operation 4
5. Description of the Dual Differential Radiometer .... 5
6. Procedures 6
7. Results 10
8. Discussion 14
References Cited 20
Appendix: Sensor Bundle Field-of-View Geometric Values
for Varying Height and Angle of View 22
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LIST OF FIGURES
Number
1
2
3
4
5
6
7
Radiometer calibration curve.
Radiometer strip chart record, Lake Mead
(December 1972).
Radiometer output vs. chlorophyll a_
(Spring 1973 data).
Radiometer output vs. integrated chlorophyll a_,
August 1973 data.
Data for selected large reservoirs.
Sensor bundle field-of-view geometry.
Absorption spectra for various algae.
Page
7
8
11
12
13
15
18
Number
LIST OF TABLES
Sensor field-of-view for varying height, and
off-nadir angle.
Page
16
vi
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INTRODUCTION
Remote sensing of water quality parameters would allow rapid, cost-effec-
tive surveys and monitoring of surface waters. Arveson et al. (1971) utilized
a dual differential radiometer to remotely measure chlorophyll a_ in Pacific
Coast waters with fair precision. Their success led to interest in the instru-
ment as a possible device to rapidly survey water bodies throughout the United
States and to estimate their trophic state on the basis of observed chloro-
phyll a_ values.
The National Eutrophication Survey (NES) was a U.S. Environmental Protec-
tion Agency (U.S. EPA) research effort investigating the threat of accelerated
eutrophication in freshwater lakes and reservoirs. Nationwide in scope, it
was designed to develop, in conjunction with State environmental agencies, in-
formation on nutrient sources, concentrations, and impacts on selected surface
water bodies. Consequently it was decided to test the differential radiometer
as a possible vehicle to measure chlorophyll a_ levels in NES lakes.
Two dual differential radiometers were obtained from the National Aero-
nautics and Space Administration's Ames Laboratory (NASA/Ames) and installed
aboard National Eutrophication Survey helicopters. These units were tested
during the 1972 and 1973 field operations. Tests were conducted on lakes and
reservoirs which varied in size, shape, depth, and water quality. Water
colors ranged from red/brown to blue, and Secchi disc transparency ranged from
a few centimeters to over seven meters. Although not quantitatively measured,
suspended sediments were noted in many of the water bodies at the time of the
sampling. These tests were conducted within the framework of the NES sampling
effort. Therefore, testing was limited largely to obtaining spot readings
when approaching or departing a lake sampling site. Few transects were
obtained due to time restrictions.
The purpose of this report is to present the results of our experience in
using the dual differential radiometer to measure chlorophyll a_.
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CONCLUSIONS
The concept of the dual differential radiometer is basically straightfor-
ward. The instrument is small, relatively inexpensive, and electronically
simple. It may be readily installed in light aircraft without difficulty.
Because of these considerations and the capabilities demonstrated by both
Arveson et al. (1971) and NES tests on Lake Mead, it is felt that the instru-
ment can be successfully utilized in some situations.
The radiometer could be of great value in contouring chlorophyll content
in a lake of relatively low turbidity. However, it cannot be effectively em-
ployed without collection of simultaneous ground-truth data until the presence
or absence of interferences is established. For successful utilization it
should be operated by a well-trained individual and receive a high operational
priority. Possible interferences should be known, considered, and compensated
for if necessary.
Although the National Eutrophication Survey tests of the dual differential
radiometer were often compromised by time, space, weather and other consider-
ations, sufficient data were collected to permit the following specific conclu-
sions:
1) The dual differential radiometer has limited application in the re-
mote sensing of surface chlorophyll a_.
2) A.large proportion of U.S. surface waters has chlorophyll a^ levels
above its effective range and/or has morphologic characteristics that preclude
its use.
3) The dual differential radiometer could be a useful tool on large,
relatively clear water bodies. It could provide adequate data for non-quan-
titative "survey" efforts, help define areas for most effective ground-truth
sampling, and assist in extrapolating quantitative ground-truth data outside
the immediate sampling area and interpolating between sample sites.
4) In spite of its basic simplicity, the instrument should be operated
by a trained observer and close attention should be paid to the many possible
interferences. Simultaneous ground-truth data should be collected.
5) Its use as a general field instrument on a broad range of lakes is
not supported by the National Eutrophication experience.
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RECOMMENDATIONS
The dual differential radiometer should not be considered a proven field
instrument and used blindly without regard to the various possible interfer-
ences. As presently configured, it cannot be successfully utilized to measure
chlorophyll a_ in areas of high turbidity. It is weather limited, requires a
water body of substantial size and depth, cannot tolerate excessive boat
wakes, and is subject to other interferences.
Based on the results of a literature review and the NES results, it is
felt that calibration curves should be constructed for each different water
body, and perhaps for the various seasons. (Typically, lacustrine phytoplank-
ton populations shift from domination by diatoms in the spring to predom-
inately blue-green or green algae in the summer. Suspended sediments would
also typically be greater in the spring months than later in the year.) It
also cannot be successfully used without considerable attention as to its
operation. Frequent checks on solar zero, angle of view with respect to solar
azimuth, elevation, and atmospheric conditions must be made.
Some ground tests of the dual differential radiometer should be conducted
utilizing both channels simultaneously to attempt to separate out chlorophyll
and non-chlorophyll effects. Immersion of the sensor bundle into the water
would eliminate the reflection and skylight influences. Further field testing
may allow for correction of many of the present interferences.
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THEORY OF OPERATION
The dual differential radiometer is capable of measuring upwelling sun-
light from bodies of water in four spectral bands. The system is designed for
airborne operation and real time detection of small changes in spectral
radiance. Its theory is based on a simple form of the correlation spectro-
meter. The instrument is configured to correlate the specific spectral
characteristics unique to chlorophyll and reject or cancel the background.
The absorption spectra for a variety of phytoplankton have been deter-
mined (Yentsch 1960, Friedman and Hickman 1972, Grew 1973). These spectra
indicate that for many species there is a maximum absorption in the blue re-
gion at about 440 nanometers (nm) due largely to chlorophyll, a relatively
transparent region between 530 and 650 nm, and a secondary absorption maximum
near the red at about 680 nm. These specific absorption bands modify up-
welling sunlight from water at characteristic wavelengths corresponding to
absorption maxima and minima.
To determine chlorophyll a^, a sample filter with a maximum transmission
at 443 nm, close to one of the absorption maxima of phytoplankton, is paral-
leled to a reference filter with a maximum transmission at 525 nm. This latter
filter lies outside the major absorption region of the phytoplankton. The two
selected wavelengths lie near the absorption minimum for water, thus minimiz-
ing its effect upon the returning radiation. Variations in upwelling light
from a water body because of water surface roughness, scattering, or haze
should have a similar effect on both wavelengths. Variations in the concen-
tration of algae will primarily affect the intensity at the sample wavelength
(443 nm). The resultant differential signal output can be calibrated by
comparison with ground-truth chlorophyll ^concentration (NASA/Ames n.d.,
Arveson et al. 1971). The rate of change of the differential signal obtained
is such that the effective range of the radiometer is between 0.01 yg/1 and
10 yg/1 chlorophyll a. Above 10 yg/1 the change in output voltage for a unit
change in chlorophylT a_ is too small to be effective.
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DESCRIPTION OF THE DUAL DIFFERENTIAL RADIOMETER
Two dual differential radiometers were obtained and labeled units A and
B. Each radiometer consists of two main components, a sensor assembly and an
electronics unit. The sensor assembly receives radiation through a fiber op-
tics bundle. The bundle consists of randomly mixed fibers and is split into
four sections. Behind each section is a bandpass filter to isolate the spec-
tral region of interest. The passed radiation is detected by a silicon
photodiode which produces an output voltage signal proportional to the inci-
dent radiation. The four output signals provide sample and reference signals
to two channels, A and B.
The four output signals from the sensor assembly are further amplified
within the electronics unit. The reference signals to channels A and B are
amplified by fixed factors while the amplification factors to the sample sig-
nals are independently variable. This allows normalization of the signal
pairs during solar standardization. The electronics unit also provides pro-
cessing electronics to compute the algorithm:
10(Ir - I.)
SIG = - J- - — volts
where I and I respectively represent the reference and sample signal levels
in either channel A or B. In addition, the electronics unit contains preci-
sion voltage sources and null meters to provide signal offset over the range
HO volts to 10 volts and to determine the output voltage for each channel.
During the National Eutrophication Survey, only one channel was ever em-
ployed at any one time. Occasional changes from channel A to channel B were
made, however this required internal switching of the filters and was not a
frequent occurrence.
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PROCEDURES
The dual differential radiometers were installed and tested aboard float-
equipped Bell UH-1H "Huey" helicopters which were utilized in the National
Eutrophication Survey. Voltage readings were obtained from the instruments
while either approaching or departing an NES sampling station. At each site
the helicopter would land on the water, perform in situ measurements, and
collect water samples for later analysis (U.S. EPA 1974 and 1975). Samples
for chlorophyll a_ analyses were collected at each station from water integrated
from the surface to 4.6 meters (m) or to the lower limit of the photic zone,
whichever was greater. In waters less than 4.6 m deep, the lower limit of the
integration was a point just off the bottom. Samples were collected in unused
polyethylene bottles and stored in an icebox aboard the helicopter. At day's
end, they were removed and analyzed using a modification of the fluorometer
procedure described by Yentsch and Menzel (1963). In addition, in 1973
surface samples were collected at specific sites for the specific purpose of
comparing surface chlorophyll a^ values with the radiometer data.
During the 1972 field year, use of the dual differential radiometer was
attempted utilizing an aligning yoke (located in the helicopter rotor well)
which directed the sensor bundle's field of view 20 from the vertical in any
selected quadrant. This configuration made calibration of the instrument a
tedious, torturous process and exposed the sensor assembly to damage. Radio-
meter data showed little correlation with ground-truth chlorophyll a_ levels.
This, coupled with a high amount of downtime, inadequate instructions in its
use, and.a work schedule demanding 10- to 12-hour days, 7 days a week, re-
sulted in field researchers viewing the radiometer largely as a hindrance to
more important work. Consequently, it received minimal attention and effort.
John Arveson of NASA/Ames Research Center visited the Environmental Moni-
toring and Support Laboratory-Las Vegas during the 1972-73 winter. He as-
sisted in repairs and calibration, advised as to methods of installation, and
instructed National Eutrophication Survey personnel in the use and peculiarities
of the instruments. He also provided a new calibration curve (Figure 1) which
incorporated additional data collected during 1972. Subsequently, the instru-
ment was tested over Lake Mead, Nevada. The remotely obtained data compared
very favorably with simultaneous ground-truth chlorophyll a^ data (Figure 2).
It was therefore decided to continue testing the instrument during the 1973
National Eutrophication Survey field season in the Eastern and Southeastern
United States.
The instrument was relocated such that the limnologist could aim the
sensor bundle through an open window of the helicopter. The dual differential
radiometer was calibrated daily (and often before each use) by aiming it at
-------
o
b
i
O
i
'00
I
0)
3v
c
<°
&
p
o
01
CHLOROPHYLL (pg/liter)
2 e -
Ul
o
Ul
o
o
o
m
0)
+
00
1
1
1
1
I
I
I
1/13/72 - Original Curve 1/13/73 - Modified Curve
Figure 1. Radiometer calibration curve (after Arveson personal communication, 1973).
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-3.6
-19
oo
-1.4
Ground -Truth Chlorophyll a (ug/liter)
0.90
TIME - DISTANCE
Figure 2. Radiometer strip chart record, Lake Mead (December 1972). Horizontal distance is
approximately 10 kilometers and the chlorophyll scale is from the 1972 calibration curve.
-------
the sun with a Teflon diffuser over the optic bundle. Since this was often
done while airborne, concern as to the effect of the rotor shadow was ex-
pressed. Several simple experiments using the radiometer and several photo-
meters were conducted. Measurements were made before and after starting the
helicopter (i.e., with the rotors motionless and up to speed) and simulta-
neously inside and outside the rotor shadow. No appreciable effect on the
calibration was observed, although a 5% light loss was encountered. Whenever
possible, calibration was performed prior to takeoff with the rotor still or
by banking the helicopter while in flight to allow a direct line to the sun.
Voltage readings were taken by pointing the sensor bundle at the water
surface in a direction away from the sun. Measurements were taken at eleva-
tions from 60 to 150 m above the lake level during acceptable weather condi-
tions.* Care was exercised to avoid including the helicopter's shadow, the
float bag, or portions of the shoreline within the 30 field of view. Through-
out these tests only one channel and filter pair per instrument were employed
at one time. Changes from channel A to channel B were made periodically with
no noticeable effect on the voltages obtained.
*Acceptable weather conditions are defined as enough available light
existing to throw a readily visible shadow, when neither the water body nor
the helicopter is in a cloud shadow, and water surface roughness does not
exceed 10% whitecaps.
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RESULTS
In the early spring of 1973, test flights dedicated to testing the dual
differential radiometer were made on transects down Las Vegas Bay into Lake
Mead proper. The area was selected for its proximity and because chloro-
phyll a_ levels were known to decrease from the upper end of the bay toward the
main lake body. Figure 2 presents a reproduction of the strip chart record
obtained from the radiometer on a transect flown about 150 m above lake level.
Ground-truth data collected at sites 1 and 2 compare quite favorably. Because
of rapidly deteriorating weather conditions, ground-truth data at sites 3 and
4 were not obtained.
During the 1973 NES field sampling in the Eastern and Southeastern United
States, use of the radiometer was attempted when weather permitted. Voltage
readings were obtained by directing the sensor bundle toward the sampling site
upon approach or departure from an elevation which varied from 60 to 150 m
above the lake. The lakes visited varied in size, morphology, trophic state,
phytoplankton assemblage, and chlorophyll a^ level. Data collected are pre-
sented in Figure 3. It is readily apparent that the correlation between the
output voltage and chlorophyll a^ levels is poor.
Mr. Arveson visited the NES team in the field and checked the calibration
of the internal filters. He also performed some minor maintenance and again
instructed personnel in the use of the instruments. Following this visit,
several more tests of the differential radiometer were conducted. The re-
sults, utilizing the integrated chlorophyll a^ value, are presented in Figure 4.
Data obtained were also reviewed on an individual lake basis (Figure 5).
Again, no consistent relationship was apparent in either case.
10
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CHLOROPHYLL a (jjg/liter)
01 6
01
o
o
o
O Output voltage vs. chlorophyll a_ content of surface
dipped water samples.
D Output voltage from Unit A vs. chlorophyll a_ content of
integrated water sample.
A Output voltage from Unit B vs. chlorophyll a. content of
integrated water sample.
(calibration curves superimposed)
Figure 3, Radiometer output vs. chlorophyll a_ (Spring 1973 data)
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CHLOROPHYLL a (jjg/liter)
ui
O
01
o
o
o
111
c
H
-0
C
H
O
o
m
en
1 I ' '"I
o
o o
'o
Figure 4. Radiometer output vs. integrated chlorophyll ,a, Aucnist i<»73
(calibration curves superimposed).
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50
O)
:10
Q_
O
CL
O
O
1
JT
. ____ . ____ j ________ _____ ____ ; ____
01 2345
OUTPUT VOLTAGE
O Old Hickory Reservoir, TN; 8/16/73
D Cumberland Reservoir, KY; 8/21/73
A Chickamauga Reservoir, TN; 8/23/73
-|- Rend Lake, IL; 8/8/73
Berlin Reservoir, OH; 7/30/73
X\Lake Carlyle, IL; 8/10/73
(.lines between points for clarity only)
Figure 5. Data for selected large reservoirs,
13
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DISCUSSION
The results of testing the dual differential radiometer by NES personnel
were generally poor. Little correlation between radiometer output voltages
and chlorophyll a was evidenced. There are several probable reasons why this
is so. Not the Teast of these is the fact that the instrument had a low
priority among NES objectives and personnel commitment to its use was cor-
respondingly low. The extremely hectic schedule of the NES field team left
little opportunity for involved personnel to review data in detail, perform
side experiments, and fully investigate the instrument and all its nuances.
As an example of this last consideration, the geometry of the system was
not thoroughly investigated at the time of use. Figure 6 presents the geome-
try of the radiometer as it was in 1973. Table 1 presents the major and minor
axes and area of the field-of-view of the sensor bundle at several elevations
and angles from the vertical.
As can be seen, the sensor bundle views an ellipse on the ground and in-
tegrates over this area. The area of the ellipse defining the field-of-view
is much larger than most of the NES team members realized. Consequently the
inclusion of shorelines, visible lake bottom, or other objects that would af-
fect the reflected light differentially probably occurred occasionally.
Light reflected from the water surface could mask the backscatter sig-
nals. Since the sensor bundle was always directed away from the sun, this
effect was minimized. However, experiments by both the National Aeronautics
and Space Administration's Langley Laboratory (NASA/Lang!ey) (NASA 1973) and
NES personnel indicate that a slight increase in voltage with increasing angle
is experienced utilizing the 443- and 525-nm filters.
These considerations, however, do not explain all the scatter observed.
Results obtained by researchers at NASA/Lang!ey utilizing the dual differen-
tial radiometer were also disappointing (NASA 1973, Witte 1975). They noticed
that, among other problems, atmospheric haze affected the solar standard-
ization zero; boat wakes could cause up to a 15 percent of full-scale deflec-
tion in the radiometer output; and the geometry of the system with respect to
viewing angle, solar orientation, and stream flow affected the results.
Many of the difficulties encountered apparently stem from one basic as-
sumption, that "changes in light intensity, variations in water surface rough-
ness, or scattering within the water body have similar effects on the intensity
of both wavelengths and are automatically corrected" (Arveson et al. 1971), Al-
though not stated, the implication is that the only significant variation in sig-
nal between the two wavelengths is due to chlorophyll absorption. This is def-
initely not the case. Clarke et al. (1970) showed that as the altitude of the
14
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D FiC
x'
X
/ a .
/ U
\
B /
N\ :
1
1
\
The distance DA = h tan (a + 15°)
The distance BA_ = h tan (a - 15°)
The minor radius a = (tan 15°H
c
The major radius b = (DA - BAJ/2
_ h
£ cos (a + 15°)
2 2
where: a = \| FA + h
FA = BA + b
and the area of the ellipse
cos (a + 15°)
Figure 6. Sensor bundle field-of-view geometry.
15
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TABLE 1. SENSOR FIELD-OF-VIEW FOR VARYING HEIGHT, ANGLE*
h
meters
60
80
100
150
300
a
20°
30°
20°
30°
20°
30°
20°
30°
20°
30°
a
meters
17.3
19.0
23.0
25.4
28.8
31.7
43.2
47.6
86.4
95.2
b
meters
18.4
22.0
24.5
29.3
30.6
36.6
45.0
54.9
91.9
109.8
Area 2
meters
997.8
1,313.6
1,774.1
2,334.6
2,772.2
3,648.4
6,236.2
8,208.0
24,944.6
32,835.0
*
See Appendix for English units.
16
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the sensor above the water body increased, there was a differential increase
in the percent of incident light upwelled with the larger increase occurring
at shorter wavelengths. Considering the elevations encountered during the NES
study this effect would probably be negligible. However, since it is largely
attributed to "air light" (light backscattered by the atmosphere) the presence
or absence of haze or smog, differences in humidity, and the increasing
influence of "air light" as the bundle is increasingly directed off-nadir,
would differentially affect the two measured spectra.
Prewett et al. (1973) found that the relative reflectances from four
ponds of differing suspended solids concentrations changed differentially.
Ritchie et al. (1974) were able to correlate these changes with suspended
sediments. In both studies reflectances were greater at 525 nm than at 440 nm.
This would result in an increased difference in the signals received and give
higher than true chlorophyll a^ values. This was indeed evidenced in NES data.
Although NES did not measure suspended sediments, their presence was observed
in many of the reservoirs, particularly in the Southeast.
Another consideration is the composition of the phytoplankton population.
Spectral absorption curves for different algae can be expected to vary as the
various dominant pigments change. Figure 7 presents absorption curves redrawn
from data presented in Freidman and Hickman (1972), Grew (1973), and NASA/Ames
(n.d.) for chlorophyll ^ and various algae species. The absorption maximum
for each curve was arbitrarily assigned a unit value and the remainder of the
curve adjusted proportionally. The curves are therefore not quantitative.
The sample filter central wavelength, interestingly, is generally near
the maximum phytoplankton absorption, but is well off the peak for chlorophyll a_.
The sample wavelength does occur well up the absorption maximum curves (not
shown) for other chlorophyll species, however. In addition, the width of the
filter transmission curve (NASA/Ames, n.d.) would permit significant transmis-
sion at the chlorophyll ^maximum wavelength absorption. The shapes of curves
for the marine genera Cyclotella, Amphidium, and Chlamydomonas are remarkably
similar while a fourth, Isochrysis. is reasonably close. Freshwater species
show more variation; in two cases (Chlorella and Agmenellus) the larger maxi-
mum occurs at 630-680 nm. The most distinct form, Porphyridium, is a terres-
trial red algae. It should also be noted that curves obtained by different
researchers for Chlamydomonas, although similar, differ distinctly. Although
quantitative data are not illustrated, the magnitude of the absorption varies
by a factor of 2 to 4 between different genera.
The reference wavelength is seen to be subject to more variation than the
sample wavelength. In several cases the reference wavelength intersects the
algae-absorption curves significantly up the slope toward the maximum absorp-
tion. On a spectral signature obtained by Grew (1973) over Clear Lake,
California, (not shown) the signal strength at 525 nm is nearly one-half that
at 443 nm. Clearly, one should not expect chlorophyll admeasurements based on
spectral characteristics centered at 443 nm and 525 nm to be the same in each
of these cases.
17
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I I I I I I I I I I I I I I I I I I I I I
Agmenel 1 um
quadruplication
Wavelength (ran)
Wavelength (nm)
Vertical lines denote center of sample (443nm) and reference (525nm) filters.
Vertical scale is arbitrary—curves adjusted on basis of maximum absorption.
Figure 7. Absorption spectra for various algae.
(Adapted from cited references)
18
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Grew (1973) also discusses the results of muHispectral analyses along a
flight line over the New York Bight. Changes were observed in spectral bands
centered at 468 nm and at 543 nm, indicating that the 525-nm reference band is
affected by other parameters. He suggests that one possibility for these
changes is due to particle size. In the atmosphere and in clear water, back-
scattered light is predominantly due to Rayleigh scattering which occurs when
the size of the scatterers is much smaller than the light wavelengths. The
amount of scatter is inversely proportional to the fourth power of the wave-
length. A second type of scattering, Mie scattering, becomes important as
particle size approaches 1 micrometer. Mie scattering is predominantly in the
forward direction and nonselective as to wavelength. The result is a dif-
ferential change in the radiometer output signal due to particle size alone.
Other factors could also affect the values differentially. The presence
of large amounts of pollens, dyes, or other colored substances could be ex-
pected to result in a differential output. Surface films of oils would great-
ly reduce the upwelled signal from the water and probably increase the re-
flected light. Since none of these factors which may differentially affect
the sensor channels were allowed for during the testing, the poor results are
not surprising.
A final consideration in the poor success of the dual differential radio-
meter was that many of the NES lakes sampled in 1973 had chlorophyll a_ concen-
trations greater than lO.yg/liter. These are above the effective range of the
radiometer. However, as can be seen in Figures 3-5, output signals from water
bodies with high chlorophyll a_ concentrations were usually significantly offset
from the calibration curves. These offsets are indicative of the presence of
interfering factors other than excessive concentration alone.
19
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REFERENCES CITED
Arveson, J. C., E. C. Weaver, and J. P- Millard. 1971. Rapid Assessment of
Water Pollution by Airborne Measurement of Chlorophyll Content. Paper
presented at the Joint Conference on Sensing of Environmental Pollutants,
Palo Alto, California, November 1971. AIAA paper N. 71-1097- 7 pp.
Clarke, 6. L., C. C. Ewing, and C. J. Lorenzen. 1970. Spectra of Backscat-
tered Light from the Sea Obtained from Aircraft as a Measure of Chloro-
phyll Concentration. Science 167:118-1121.
Friedman, E. J. and G. D. Hickman. 1972. Laser Induced Flourescence in
Algae: A New Technique for Remote Detection. NASA Contract INAS6-2081
Final Report, October 1972. 103 pp.
Grew, G. W. 1973. Remote Detection of Water Pollution with MOCS: An Imaging
Multispectral Scanner. Proceedings, Second Conference on Environmental
Quality Sensors, Las Vegas, Nevada. 11-17 - 11-40.
NASA, Langley. 1973. EPA-NASA Water Pollution Detection Sensor Evaluation
(Status Review, September 1973). Langley Research Center.
NASA/Ames Research Center. No date. Operating Manual for Dual Differential
Radiometer. 19 pp.
Prewett, 0. E., D. R. Lyzenga, F. C. Polcyn, and W. L. Brown. 1973. Tech-
niques for Measuring Light Absorption, Scattering, and Particle Concen-
trations in Water. Environmental Research Institute of Michigan. 32 pp.
Ritchie, J. C., J. R. McHenry, F. R. Schiebe, and Wilson. 1974. The Relation-
ship of Reflected Solar Radiation and the Concentration of Sediment in
the Surface Waters of Sediments. Proceedings of the Third Remote Sensing
of the Earth Resources Conference.
U.S. Environmental Protection Agency. 1974. National Eutrophication Survey
Methods for Lakes Sampled in 1972. (National Eutrophication Survey
Working Paper No. 1) Environmental Monitoring and Support Laboratory,
Las Vegas, Nevada and Environmental Research Laboratory, Corvallis,
Oregon. 40 pp.
U.S. Environmental Protection Agency. 1975. National Eutrophication Survey
Methods 1973-1976. (National Eutrophication Survey Working Paper
No. 175) Environmental Monitoring and Support Laboratory, Las Vegas,
Nevada, and Environmental Research Laboratory, Corvallis, Oregon. 91 pp.
20
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Witte, W. G. 1975. Evaluation of the Dual Differential Radiometer for Remote
Sensing of Sediment and Chlorophyll in Turbid Waters. Paper presented
at the Fourth Annual Remote Sensing of Earth Resources Conference,
Tullahoma, Tennessee, March 1975.
Yentsch, C. S. 1960. The Influence of Phytoplankton Pigments on the Color
of Sea Water. Deep-Sea Research 7:1-9.
Yentsch, C. S. and D. W. Menzel. 1963. A Method for the Determination of
Phytoplankton Chlorophyll and Phaeophytin by Fluorescence. Deep-Sea
Research 10:221-231.
21
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Appendix:
Sensor Bundle Field-of-View Geometric Values for Varying Height (h) +
Off-Nadir Angle (a)
h
ft
200
300
400
500
1,000
a
0
20°
30°
20°
30°
20°
30°
20°
30°
20°
30°
a
ft
58
64
86
95
115
126
144
159
288
317
b
ft
62
73
92
110
122
146
153
183
306
366
£
S
ft
201
207
301
311
402
414
502
518
1,004
1,035
A_
9
ft
244
283
366
424
488
566
610
707
1,220
1,414
I.
c
ft
215
237
322
355
430
473
537
592
1,075
1,184
Area
?
ft^
11,206
14,478
24,856
32,830
44,257
57,991
69,215
91,410
276,862
364,494
Symbols are those defined in Figure 6. Values of a , a , and a are included
for comparison with h. 9
22
iM.S. GOVERNMENT PRINTING OFFICE: 1978 - 786-151/1266 Region No. 9-1
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/4-78-045
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
TESTS OF THE DUAL DIFFERENTIAL RADIOMETER UNDER
FIELD CONDITIONS
5. REPORT DATE
August 1978
6. PERFORMING ORGANIZATION CODE
N/A
7. AUTHOR(S)
R. W. Thomas
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFpRMING ORGANIZATION N.AME AND ADDRESS . ,
nvironmentaT Monitoring & Support Laboratory
En
Office of Research & Development
U.S. Environmental Protection Agency
Las Vegas, NV 89114
10. PROGRAM ELEMENT NO.
1BD613
11. CONTRACT/GRANT NO.
N/A
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency—Las Vegas, NV
Office of Research & Development
Environmental Monitoring & Support Laboratory
Las Vegas. NV 89114
13. TYPE OF REPORT AND PERIOD COVERED
Interim 03/72 to 09/73
14. SPONSORING AGENCY CODE
EPA/600/07
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A dual differential radiometer was tested on numerous eastern United States
lakes and reservoirs. Remotely sensed data were compared with ground-truth
chlorophyll a^ values. Results indicate that the instrument has only limited
application in the remote sensing of chlorophyll a_ in the nation's lakes. At its
present state of development, its use should be confined to large, deep, relatively
clear water bodies in conjunction with ground-truth and surface survey efforts.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
* Radiometer
* Chlorophylls
Field tests
Remote sensing
7C
20F
18. DISTRIBUTION STATEMEN1
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
32
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
A03
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
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