Solar energy and reflected radiation flux measurements on mountainous test sit

Brennan, P. A.

English

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https://ntrs.nasa.gov/search.jsp?R=19690021679 2020-03-12T03:38:43+00:00Z
YEARTH. RESOURCES PROGRAM
TECHNICAL LETTER
NATIONAL AERONAMICS AND SPACE ADAMAMAWN
(ACCESSION NUMBER)	
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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
Technical Letter 16
ALBEDO MEASUREMENTS AT THE MT. LASSEN TEST SITE
Peter A. Brennan
NASA Contract NA -7779
University of Nevada
Mackay School of Mines
Reno, Nevada 89507
December 1968
CONTENTS
Page
Introduction
	
1
Instrument and instrument performance
	
2
Data Collection and Sampling
	
4
Data Reduction
	
6
Evaluation of Albedo Measurements
	
8
Application of Albedo Measurements
	
12
References
	
14
Tables
1. Albedo of White Card
	
7
2. Visual Albedo Parameters of cinder
	
8
3. Nomenclature for wavelengths
	
12
Figures
1. Photo of ISCO Spectroradiometer
	
3
2. Close up photo of ISCO
	
3
3. ISCO data form
	
6
4. Albedo plot comparing cinder pit and Cinder Cone
samples
	
9
5. Albedo plot of grey cinder
	
10
6. Albedo plot of red cinder
	
11
7. Albedo plot of varigated cinder
	
11
8. Albedo plot of basalt rock
	
12
NASA Technical Letter No. 16
	
i
9--
ALBEDO MEASUREMENTS AT THE MT. LASSEN TEST SITE
Ir-i --- J..-1I--
Albedo is a measure of the reflectivity of a material. The term
is used for the decimal fraction of the incident light in the visible
and near-infrared portion of the spectrum that is reflected by a surface.
The albedo of a material directly or indirectly influences the response
of instruments measuring energy fluxes in the visible infrared and micro-
wave, and is thus a very important "ground truth" parameter.
A large portion of the incoming solar energy is concentrated in the
visible and near-infrared portion of the spectrum. The incoming radiation
is either reflected or absorbed. The reflected energy is directly corre-
lative with the photographic quality of the terrain. The absorbed energy
influences the temperature of the material, but the correlation is affected
by thermal parameters of the material and is, therefore, an indirect
relationship.
NASA Technical Letter No. 16 	 1
Instrument and Instrument Performance
The ISCO model SR spectroradiometer measures the energy intensity
(flux density) of incident light. Energy is expressed in units of energy
flux per unit time per cross-sectional area per wave length. The values
in uw/mu/cm2
 obtained from direct readout of spectral intensity have a
maximum error of ± 40% of the actual values. Precise values may be
obtained by using correlation charts provided with the instrument.
Basically, the instrument consists of a diffusing screen, light
chopper, monochrometer, measuring photocell, amplifier, coherent detector
and indicating meter. The spectroradiometer incorporates two wave length
ranges, the visible 380-750 mu, and the near infrared, 750-1550 mu.
The monochrometer is a wedge interference filter, i.e. the space between
the reflecting layers of the filter is wedge shaped. It operates in the
second interference order on the visible range and the first order on the
near infrared range. The visible range detector is a silicon junction
photo cell. The near infrared detector is a germanium junction photo cell.
The ISCO instrument is capable of recording the gross aspects of
spectra in the wave length range of .38 to 1.55u. It is possible to
estimate the position of features in the spectrum to .005u • in the range
.38 to .75u and .Olu in the range .75 to 1.55u.
NASA Technical Letter No. 16	 2
Figure 1. ISCO spectroradiometer with remote fiber optics probe.
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Figure 2. Close up of ISCO spectroradiometer.
NASA Technical Letter No. 16	 3
Data Collection and Sampling Site Selection
Since albedo is a ratio of reflected radiation to incoming radiation,
two separate measurements, solar energy flux and reflected radiation flux,
are involved. The reflected radiation is best measured using the ISCO with
a remote fiber optics head (90 0
 field of view) placed 3-5 inches above the
target. The incoming radiation, for conformity, is also most easily meas-
ured using the fiber optics head. Two methods have been used to obtain
incident radiation measurements. Each method has its drawbacks; each is
partially successful. 	 The methods yield slightly different results in
different situations.
Method 1 ir ,iolves reversing the optics head 180° and directly measur-
ing the incoming radiation.
Advantages of Method 1
1. ease of calculation
2. absence of intermediate reflectors that may introduce errors
Disadvantages of Method 1
1. reflections and radiation from the horizon (angles 45° to
135°) are not included
2. shadows projected by the instrument are not included
Method 2 incorporates a white card which is placed over the sample
surface and the incoming radiation reflected off of the card,whose albedo
is well known, is recorded.
Advantages of Method 2
1. incorporation of minor reflections from along the horizon
and the shadow cast by the instrument
NASA Technical Letter No. 16 	 4
Disadvantages of Method 2
1. the albedo of the white card varies with age of the card,
dirtiness of the card and sun angle.
No matter what method is used, good curves may be generated rather
rapidly from points at .025u intervals in the range .40 to .75u and .051,
intervals in the range .75 to 1.55u. These readings are not closely enough
spaced to delineate any but major absorption bands, however, the curves are
not designed to show anything but the broad characteristics of the spectra.
Additional points do not seem to add enough detail to warrant the time
spent taking them and reducing the data.
Site selection is a matter of choosing the correct location and the
correct time. The ideal conditions may be enumerated as follows:
1. local noon on a day near the summer solstice
2. cloudless sky with stable atmospheric conditions and low
atmospheric moisture
3. the target should be representative in color and texture of the
desired rock or soil type
4. the measurements should be taken on a broad flat expanse of the
target material with a clear unobstructed horizon.
Even when these conditions are meet, small fluxuations occur in the
intensity of the incoming solar radiation. It is, therefore, advisable
to make a series of data runs on the same material, each being bracketed
in time by measurements of the intensity of the incoming radiation. Data
forms have been so designed (see Figure 3).
NASA Technical Letter No. 16	 5
0118
	TWL--	 LOCATO
	
fTMfA14r,^;..,,f^ IfAp
	
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A2S
A90 1
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Figure 3.	 ISCO data form (normal size 8-1/2 x 11).
DATA REDUCTION
The albedo at any given wavelength is the ratio of the intensity
of the reflected to the incoming light.
reflected intensity (Ra)
albedo (aa) s
incident intensity (Ia)
Because albedo is the ratio of the instrument readings, none of the
measurements need be corrected for imperfect instrument transmission.
If a white card is used for a standard, a factor for the albedo of the
card must be included.
reflected intensity (Ra)*albedo of card (asp,)
albedo (aa)	 Mident intensity a
NASA Technical Letter No. 16	 6
w
The white card used as a standard for a portion of the Mt. Lassen
data was white poster board. Several measurements on various cards
were averaged. The albedo for each normally measured wave length is
listed in Table 1; the column on the table labeled a is the average
measured albedo; the column rr is readings from a smoothed albedo curve.
If each of the datum points are calculated, albedos for 31 separate
wavelengths are obtained. The most meaningful way which this data can be
presented is as a graph plotting albedo (reflectivity) against wavelength.
All of t^, : albedo data in the following section is presented in this
;fanner.
TABLE 1
4LUDO OF STANDARD WHITE CARD
(observed vertically)
a Q 7
.400 .610 (.61;
.425 .610 (.61
.450 .616 (.62;
.475 .629 63
.500 .639 ^.64
.525 .659 (.65
.550 .638 (.66
.575 .703 (.68;
.600 .680 (.69;
.625 .681 (.68
.650 .688 (.69
.675 .687 (.69
.700 .695 (.69,
.725 .695 (.70
.750 .702 (.71
Q
.80 .724 (.72)
.85 .724 (.72;
.90 .700 (.71)
.95 .724 (.71)
1.00 .706 (.71)
1.05 .702 (.70
1.10 .705 (.70)
1.15 .706 (.69)
1.20 .662 (.67)
1.25 .654 (.66)
1.30 .654
1.35
1.40
.589 (64)
.713 ^-.67)
1.45 .650 (.66)
1.50 .649 (.62)
1.55 .543 (.58)
NASA Technical Letter No. 16 	 7
EVALUATION OF ALBEDO MEASUREMENTS
The following data from the Mt. Lassen test site indicate that
the variously rough basalt surfaces and the various colors of cinder
each exhibit distinct albedo curves. The reflectivities of the
samples are much as one might expect; the rough black basalts have
the lowest reflectivities. They are followed by the smooth dark grey
basalts, the grey cinder, and, finally, the brightly varigated cinder,
with the highest reflectivity. Table 2 shows the estimated visual order
of albedos of six cinder samples of varying color, their reflectivity at
.50u, .60u and .70u, near the peak of the solar energy output, and the
average of the albedos from these three points. The order of the aver-
ages is identical to the order of the reflectivities estimated by eye,
though no single wavelength seems to be a good index of albedo.
TABLE 2
Summary of Visual Albedo Parameters of Six Cinder Samples
Estimated Albedo	 Albedo Albedo Average
Sample No. Relative Albedo .50u(green)	 .60u(orange) .70u(red)	 .50,.60,.70u
N971(AL-6) 1 1 5.75	 (2)	 6.81	 (1) 7.12 (1) 6.56 (1)
N966(AL-1) 2 2 5.75 (2)	 6.9C	 (2) 7.85 (2 6.83 (2)
N967(AL-2) 3 3 4.86	 (1)	 7.53	 (3) 11.97 (4) 8.12 (3)
N968(AL-3) 5 4 5.94 (4)	 8.70 (4) 9.99 (3) 8.21 (4)
N969(AL-4) 5 7.83	 (6)	 11.38 (6) , 12.12 (5) 10.44 (5)
N970(AL-5) 6 6 6.74	 (5)	 11.20	 (5) 13.45 (6) 10.46 (6)
1.	 N971 Dark grey and dark olive brown cinder.
2.	 N966 Slightly varigated greyish cinder.
3.	 N967 Dark red and reddish-brown cinder.
4.	 N968 Slightly varigated brown and olive cinder,.
5.	 N969 Varigated olive, orange, brown and grey cinder.
6.	 N970 Bright varigated orange, red, olive, brown and grey cinder.
NASA Technical Letter No. 16	 8
The dark grey cinder, which covers much of the test site and a
large portion of the surrounding area, seems to be rather consistent.
Figure 4 shows three plots, the average of two grey cinder samples
from the test site, grey cinder from a cinderpit 10 miles to the north-
east, and grey cinder from the cinderpit recorded by a Beckman spectro-
meter operated by Jet Propulsion Laboratories. Figure 5 shows the indi-
vidual plots of the two samples, N971 and N966, from the test site, the
average of which appear in figure 4. The first of these samples is
entirely dark grey and olive brown cinder, the latter includes a small
amount of dark red, red-brown and light brown material as well.
WAVELENGTH in MICRONS
Figure 4. Albedo plot comparing grey cinder from the cinderpit and
from the Cinder Cone area.
NASA Technical Letter No. 16	 9
N97t Grey CWWW
-• "M uyn Grq Gft
1-3	 iA	 ID
VAVELENGiH m wwonf
Figure 5.
	 Two albedo plots of grey cinder from the Mt. Lassen test site.
The red and reddish-brown cinder is pecularily different from any
of the other cinder samples. This dull colored cinder has a very low
reflectivity in the region below .55u and then rises sharply in the red
portion of the spectrum, with a broad highly reflective region around
1.25u. The red sample, as shown in figure 6 has a very low reflective
h_	
component in the green or yellow where most of the other samples are
relatively high reflective in this region of the spectrum.
The varigated cinders, because they are mixtures of a number of
distinctly different colors of cinder, have irregularly shaped albedo
curves. The varigated cinders all have low reflectivities in the
violet and blue, with the albedo rising on the yellow and red and
continuing high and irregular throughout the infrared portion of the
spectrum. Figure 7 shows curves for three varigated cinder samples.
NASA Technical Letter No. 16 	 10
8d
r,
8
J
Figure 6. Albedo plot of dark red cinder.
NWV.VWN 0 IIrIPDM
Figure 7. Albedo curves for three varigated cinder samples.
NASA Technical Letter No. 16
	
11
M .
The basalt, whether rough or smooth, is remarkably uniform in re-
flectivity. The rock has a low reflectivity throughout the visible
and near-infrared spectrum. The only prominent feature is a broad,
slightly more reflective region, between .55u and .70u. As might be
expected, the rough basalt has a slightly lower albedo than the smooth
sample, as shown in figure 8.
Figure 8. Albedo curves for two basalt samples from the flows near the
Cinder Cone.
Applications of Albedo Measurements
Albedo measurements may be used in calculations of total energy
absorption and multispectral sensor response. Table 3 shows the wave-
length ranges of the various colors as the eye perceives them.
TABLE 3
Nomenclature for Wavelen gths Sensed by
 the ISCO Instrument
.33u to .40u	 near ultraviolet
.40u to .43u	 violet
.43u to .49u	 blue
.49u to .51u green
.57u to .59u	 yellow
.59u to .62u orange
.62u to .75u	 red
.75u to 1.55u near infrared (Photographic IR)
1_1	 NASA Technical Letter No. 16	 12
When the true solar radiation flux and the albedo are known, the re-
lative detector or photographic response for each wavelength may be
computed by using the following formula:
RX = aA *IA*CA
where
RA = true reflected intensity
ax = albedo
I a = measured incident radiation
c, = correction factor for instrument response
(graph included with ISCO instruction manual)
Similarly, the total absorbed energy in calories/sec./cm 2 over the range
.40 to 1.55u for any substance may be computed by the following related
formula:
Ea =	 an [(1-(%j *IA *ca*2.39*l0-7]
NASA Technical Letter No. 16	 13
REFERENCES
1. Blinn, J. C. III, P. E. Chapman, and J. G. Quade: Airborne Multi-
frequency Microwave Radiometric Sensing of an Exposed Volcanic Pro-
vince, Technical Memorandum 33-405, Jet Propulsion Laboratory, Calif-
ornia Institute of Technology, Pasadena, California.
2. Brennan, P. A., P. E. Chapman, J. G. Quade, J. P. L. Microwave
Experiment Support. Technical Letter #15, University of Nevada,
Mackay School of Mines, Reno, Nevada
3. Chapman, P. E., P. A. B rennan, and J. G. Quade: Preliminary Ground
Truth Report of the Mt. Lassen Test Site #56. Technical Letter #8
University of Nevada, Mackay School of Mines, Reno, Nevada.
4. Quade, J. G., P. E. Chapman, P. A. Brennan and P. Robertson: Ground
Truth Flight Data, Mission #55 and #56. Technical Letter #10,
University of Nevada, Mackay School of Mines. Reno, Nevada
5. Quade, J. G., P. E. Chapman, P. A. Brennan and Staff: Ground Data
Investigations Mt. Lassen, Site #56 - Mission #76. Technical Letter
#13, University of Nevada, Mackay School of Mines, Reno, Nevada.
6. Quade, J. G., P. E. Chapman, J. Blinn III and P. A. Brennan: Ninety
Day Report - Mission #76, Mt. Lassen Test Site #56. Special Report
#1, University of Nevada, Mackay School of Mines. Reno, Nevada
NASA Technical Letter No. 16	 14


Albedo measurements at the Mt. Lassen test site

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