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Spectral reflectance and photometric properties of selected rocks
REMOTE SENSING OF ENVIRONMENT 2, 95-100 (1972)
95
Spectral Reflectance and Photometric Properties of Selected Rocks 1 R O B E R T D. W A T S O N U.S. Geological Survey, Denver, Colorado 80225
Studies of the spectral reflectanceand photometric properties of selected rocks at the USGS Mill Creek, Oklahoma, remote sensing test site demonstrate that discrimination of rock types is possible through reflection measurements, but that the discrimination is complicated by surface conditions, such as weathering and lichen growth. Comparisons between fresh-broken, weathered, and lichen-coveredgranite show that whereas both degree of weathering and amount of lichen cover change the reflectancequality of the granite, lichen cover also considerably changes the photometric properties of the granite. Measurements of the spectral reflectance normal to the surface of both limestone and dolomite show limestone to be more reflectivethan dolomite in the wavelength range from 380 to 1550 nanometers. The reflectancedifferencedecreases at view angles greater than 40° owing to the differencein the photometric properties of dolomite and limestone.
Introduction The feasibility of using airborne multispectral remote sensing reflection measurements for discrimination of rock types has recently been investigated (Watson and Rowan, 1971), and the results have demonstrated that this technique may be significant in the automatic generation of geologic maps. There are, however, several factors that determine the significance of the measurements and influence the interpretation of the data. (1) The spectral bandwidth and the sensitivity of the measuring device determine the degree to which small differences in the reflection of rocks can be resolved. (2) Reflected radiation from both rock surface and background (soil, grass, etc.) will be integrated by the measuring device. Also, the viewed area of the rock surface can include various stages of weathering and surface conditions (coatings, lichen or moss cover, soil contaimination, etc.). Both factors complicate interpretation of reflectance, as has already been established at infrared wavelengths (Watson, 1970). (3) Not all rock surfaces are Lambert reflectors, and the photometric (bidirectional reflectance) properties can be important in the interpretation of reflectance data and in discriminating between rock types. In order to properly evaluate and predict the influence of these factors, laboratory and field measurements were made of the visible and nearinfrared photometric and spectral reflectance of 1 Publication authorized by the Director, U.S. Geological Survey.
some selected rocks 'in an area where airborne multispectral reflection experiments were being conducted. The four rock types, quartz sandstone, dolomite, limestone, and granite (fresh-broken, weathered, and lichen- and (or) moss-covered), are well exposed in different structural and topographic settings (Rowan et al., 1970) near Mill Creek, Oklahoma, one of several areas being studied by the U.S. Geological Survey.
Measurement Techniques The laboratory apparatus used in the measurement of the total reflected radiation (reflectance) of the rock types is shown in Fig. 1. The basic unit is a Bausch and Lomb colorimeter with an integrating sphere reflectance attachment coated with ceramic fibers (fiberfrax). The effective spectral bandwidth is 20 nm (nanometers) over the wavelength region from 375 to 950 nm. Light emanating from the lamp is focused on the entrance slit, reflected and dispersed into monochromatic light by the diffraction grating, and refocused on the exit slit. The sample is then illuminated by the monochromatic light of beam size 2 by 6 mm and the reflected light is measured by the photomultiplier. Fiberfrax is used as the reflectance standard, and all results are normalized to this standard. The measurements of the reflectance of fiberfrax made by the author (Watson, 1971) are shown in Fig. 4 and are in general agreement with those made by Trytten and Flowers (1966). The photometric properties of the rock surfaces were measured by use of the instrument shown in
Copyright © 1972 by American Elsevier Publishing Company, Inc.
96
ROBERT D. WATSON
SHUTTER~ M P I . . E SHUTTER~ CONTROL"
/
I
PHOTO- I MULTIPLIER
~, ~ INTEGRATING I SPHERE LAMP
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L
/= 1
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LENSA ~KNTRANCE SLIT
=7 1
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L .S.
~"~'~'~"~-~1------ DIFFRACTIONGRATING
SPECTRONIC20 CAM FIG. 1.
Diagram of the colorimeter with integrating sphere used for laboratory measurements of spherical reflectance.
Fig. 2. The source light is a Westinghouse C D D 100-watt lamp with appropriate optics for collimation to a 25-mm beam at the sample surface. The receiver consists of a 6199 photomultiplier detector coupled to a tektronix model 565 memory-oscilloscope. All measurements were made in the vertical plane defined by the projection of the source and detector, and fiberfrax was used as the standard. The bidirectional reflection properties of a surface (Nicodemus, 1965) were measured with the source light at an incident angle of 45 ° and with the angle of observation (0) varied between +70 ° and - 7 0 °. (Sign convention: observation angle positive on the side of normal opposite to source.) Filters with half-transmittance values given in parenthesis were used to obtain the spectral photometric properties at 430 (50) nm, 525 (65) nm, and 630 (65) nm. The spectral bidirectional reflectance of the four rock types was measured in the field by means of an ISCO (Instrumentation Specialties Company) model SR spectroradiometer modified to include a variable aperture fiber optics remote probe sensing head (Fig. 3). All field measurements were made with the remote probe head perpendicular to, and at a constant height above, the rock surface. The field of view was limited to approximately 30 °, and a circular area with a
I
FIG. 2. Schematic diagram of apparatus used for laboratory measurements of the photometric function.
FIG. 3. ISCO spectroradiometer with modified viewing head recording the spectral radiance of fiberfrax. Note the viewing head perpendicular to the surface. 30-cm diameter was viewed. Consistency with the laboratory measurements was obtained by use of fiberfrax surfaces as standards. The ratio of the bidirectional spectral radiance of the rock to that of the standard fiberfrax surface gives the bidirectional reflectance at a known wavelength.
Results The average spectral reflectance and standard deviations for the four rock types as measured in the laboratory are shown in Fig. 4. The three curves for granite are for fresh-broken, weathered, and lichen-covered surfaces. None of the reflectance curves show band structure, but both fresh and weathered granite show slight enhancement in the reflectance at 525-650 nm; this is explained as caused by coloration due to the presence of hematite in various degrees of oxidation in the feldspar. This effect is masked in the curve of "granite-
SPECTRAL REFLECTANCEAND PHOTOMETRICPROPERTIESOF SELECTEDROCKS
97
09
Fiherfrax J
0.8 ±
l
O~xJrtz sondstone - fres hly broken
0.7
0.6
Z
0.5 G ronite--freshly broken
,?, .J
==
04 Gronite--green- ilchencovered Li m e t , one --wecther'ed
0.3
Granitl--weathered Dolomite--weathered
0.2
I 400
q 500
I 600 WAVELENGTH
I 700
t 800
(NANOMETERS)
Fie. 4. Average spectral reflectance and standard deviation of quartz sandstone, limestone, dolomite, and granite (freshbroken, weathered, and lichen-covered) as measured in the laboratory. Fiberfrax represents the standard surface (see text). green lichen covered by the lichen cover (Parmelia Genus). 2 For all the rock types, reflectance increases as the wavelength increases. This is characteristic of many rocks, including laboratory-prepared samples and, in particular, those rocks containing quartz (Watts, 1966). However, an example of the contrast that can exist between a prepared sample and a natural surface can be seen by comparing the reflectance of freshly broken granite with the weathered and lichen-covered granite. Fresh granite is seen to be more reflective throughout most of the visible spectrum than the weathered and lichen-covered granite. The effects of weathering and lichen cover on reflectance are readily demonstrated by the granite curves. Weathering reduces the granite reflectance at all wavelengths, whereas green lichen coverage of the weathered surface enhances the visible reflectance and also noticeably increases the percentage difference in reflectance in the red.
Limestone (Fig. 4) is found to be more reflective than dolomite throughout the visible spectrum, and the percentage difference in reflectance between the limestone and dolomite is higher in the blue than in the red. Quartz sandstone has the highest reflectance of the four rock types examined because of its clean and uniform surface. The bidirectional spectral reflectances of the four rock types measured in situ are shown in Figs. 5, 6, and 7. Although many of the features observed in the laboratory are present in the field, there are some striking differences. The
2 Appreciation is expressed to the University of Oklahoma Biological Research Station for their help in species identification.
FIG. 5. Bidirectional reflectance and standard deviation of limestone and dolomite as measured normal to the surface in the field.
98
ROBERT D. WATSON
o~
(NANO~TICM)
~L[N~
FIG. 6. Bidirectional reflectance of granite (weathered, lichen-covered, and moss-covered) measured normal to the surface in the field.
Y m t cE ~ n 4 t N A ~ r t B I
FIG. 7. Bidirectional reflectance of quartz sandstone measured normal to the surface in the field.
measured bidirectional reflectance of all rock types increases with wavelength as in the laboratory measurements. However, the field reflectances of granite (weathered and lichen-covered), dolomite, and quartz sandstone are somewhat lower throughout the visible spectrum than the corresponding laboratory measurements, and there is a reversal in reflectance differences between weathered granite and lichen-covered granite at 600 nm, with weathered granite having a higher reflectance at longer wavelengths. The increased bidirectional reflectance of weathered granite in the 525- to 650-nm wavelength region as observed in laboratory measurements of reflectance also occurs in the field measurements. Results of the field measurements of the bidirectional reflectance of lichen- and black moss-covered granite are similar to those obtained
>-
by Gates, Keegan, Schleter, and Weidner (1965) of dry lichen and moss attached to sandstone and demonstrate the perturbing effect of mosses and lichens on the rock reflectance. The limestone and dolomite bidirectional reflectances shown in Fig. 5 are average values and standard deviations obtained from observations on six different samples of each rock type. The largest differences in bidirectional reflectance between these two rock types are observed at the shorter wavelengths. The field measurements also demonstrate the rapid increase in bidirectional reflectance of both rock types at wavelengths longer than 750 nm. A word of caution is in order with regard to comparison of field and laboratory results. In these experiments there is a significant difference in the field of view of the laboratory and field instruments so that different surface conditions are viewed. This can cause basic differences in the reflectance spectra and required carefully controlled sampling of field specimens for laboratory measurements in order to draw comparisons between results. The spectral photometric properties of the four rock types are shown in Figs. 8 through 13.
Oil
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,
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OBSERVATION A N G L E (DEGREES)
FIG. 9. Photometric properties of dolomite in the red, green, and blue regions of the spectrum.
as
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.
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OBSERVATION ANGLE (DEGREES)
FIG. 8. Photometric properties of quartz sandstone in the red, green, and blue regions of the spectrum.
-%o
-~o
I
I
OBSERVATION ANGLE
(DEGREES)
FIG. 10. Photometric properties of limestone in the red, green, and blue regions of the spectrum.
SPECTRALREFLECTANCEAND PHOTOMETRICPROPERTIESOF SELECTEDROCKS
De
~r*on N
*d
Blu
O4
O2
~o
-OBSERVATION ANGLE (DEGREES)
FIG. 11. Photometric properties of weathered granite in the red, green, and blue regions of the spectrum.
1963), the behavior of lichen-covered granite is readily understood. It also appears that the non-Lambertian behavior of limestone and dolomite can be partially explained by the influence of lichen cover, inasmuch as each of these surfaces has a uniformly fine scale lichen growth that partially covers the surface. However, further analysis is necessary to explain why non-Lambertian behavior occurs in both the forward and the backward directions and why the departures are more pronounced for the dolomite surface.
Summary
~o~ 8~lue"-G~n ~ed
N
O2
_%
.
.
.
.
:
,
,
,
OBSERVATION ANGLE (DEGREES)
FIG. 12. P h o t o m e t r i c
properties
o f fresh b r o k e n g r a n i t e
in the red, green, and blue regions of the spectrum.
~a \ \\\ me, 77 /
N
\ \
~Bn
\ \
\',
~o~ _o
-;o
. .' . .
'o
'
+g
+L
÷~ '
.
OBSERVATION ANGLE (DEGREES)
FIG. 13. Photometric properties of granite covered with green lichen as measured in the red, green, and blue regions of the spectrum. Dashed curve represents results obtained with a He-Ne laser operating at 632.8 rim. The ordinate in figures 8 through 13 is "normalized intensity" which is the intensity normalized to the maximum intensity measured throughout the range of angles of observation and is therefore a dimensionless quantity. Although quartz sandstone, fresh granite, and weathered granite all approximate Lambertian behavior, lichencovered granite, limestone, and dolomite show departures, particularly at angles of observation greater than 20 °. Because lichens are known to strongly backscatter light (Hapke and Van Horn, 9
99
and Conclusions
The photometric, spectral reflectance (total), and spectral bidirectional reflectance of four rock types have been measured. Differences between the results of the field and laboratory measurements are due to surface conditions. Lichencovered granite, and limestone and dolomite covered with fine-scale lichen depart from Lambertian behavior. All rock types show an increase in reflectance at the longer wavelengths, but the percentage difference in reflectance between each rock type is greatest at the shortest wavelengths. Limestone should be readily discriminated from dolomite inasmuch as it is more reflective at all wavelengths from 380 to 1550 nm. However, the reflectance difference decreases at view angles greater than 40 °, owing to the difference in the photometric properties of limestone and dolomite. These results confirm the need for caution when defining rock-discrimination experiments and interpreting remote sensing reflectance data.
References
H. E. Bennett, and J. O. Porteus, 1961, Relation betweer~ surface roughness and specular reflectance at normal incidence: J. Opt. Soc. Am. 51, 123 (1961). K. L. Coulson, G. M. Bouricius, and E. L. Gray, General Electric Space Sci. Lab. Tech. Rept. R64SD74 (1964). K. L. Coulson, G. M. Bouricius, and E. L. Gray, General Electric Space Sci. Lab. Tech. Rept. R65SD64 (1965). K. L. Coulson, E. L. Gray, and G. M. Bouricius, General Electric Space Sci. Lab. Tech. Rept. R65SD4 (1965). D. M. Gates, H. I. Keegan, J. C. Schleter, and V. R. Weidner, Appl. Opt. 4, 11 (1965). B. W. Hapke, and Hugh Van Horn, 1963, J. Geophys, Res. 68, 4545 (1963).
100 J. A. Howard, R. D. Watson, and T. D. Hessin, in 7th Internat. Symposium on Remote Sensing of Environment, Ann Arbor, Mich., May 17-21, 1971, proceedings (in press). E. L. Krinov, Spectral Reflectance Properties of Natural Formations (in Russian, 1947), Natl. Research Council Canada Tech. Translation TT-439, Ottawa (1953. F. E. Nicodemus, Appl. Opt. 4, 767 (1965). Rudolf Penndorf, Luminous and Spectral Reflectance as Well as Colors of Natural Objects, Geophys. Research Paper 44 (1956). L. C. Rowan, T. W. Offield, Kenneth Watson, P. J.
ROBERT D. WATSON Cannon, and R. D. Watson, Bull. Geol. Soc. America 81, 3549 (1970). Grover Trytten, and Wayne Flowers, Appl. Opt. 5, 1895 (1966). R. D. Watson, Geophys, Res. 75, 480 (1970). R. D. Watson, Appl. Opt. 10, 1685 (1971). R. D. Watson, and L. C. Rowan, in 7th Internat. Symposium on Remote Sensing of Environment, Ann Arbor, Mich., May 17-21, 1971, proceedings (in press). H. V. Watts, U.S. Geol. Survey, Natl. Tech. Inf. Service, Springfield, Va., Report N70-38884 (1966). W. W. Wendlandt, and H. G. Hecht, Reflectance Spectroscopy, Interscience Publishers, New York (1966).
95
Spectral Reflectance and Photometric Properties of Selected Rocks 1 R O B E R T D. W A T S O N U.S. Geological Survey, Denver, Colorado 80225
Studies of the spectral reflectanceand photometric properties of selected rocks at the USGS Mill Creek, Oklahoma, remote sensing test site demonstrate that discrimination of rock types is possible through reflection measurements, but that the discrimination is complicated by surface conditions, such as weathering and lichen growth. Comparisons between fresh-broken, weathered, and lichen-coveredgranite show that whereas both degree of weathering and amount of lichen cover change the reflectancequality of the granite, lichen cover also considerably changes the photometric properties of the granite. Measurements of the spectral reflectance normal to the surface of both limestone and dolomite show limestone to be more reflectivethan dolomite in the wavelength range from 380 to 1550 nanometers. The reflectancedifferencedecreases at view angles greater than 40° owing to the differencein the photometric properties of dolomite and limestone.
Introduction The feasibility of using airborne multispectral remote sensing reflection measurements for discrimination of rock types has recently been investigated (Watson and Rowan, 1971), and the results have demonstrated that this technique may be significant in the automatic generation of geologic maps. There are, however, several factors that determine the significance of the measurements and influence the interpretation of the data. (1) The spectral bandwidth and the sensitivity of the measuring device determine the degree to which small differences in the reflection of rocks can be resolved. (2) Reflected radiation from both rock surface and background (soil, grass, etc.) will be integrated by the measuring device. Also, the viewed area of the rock surface can include various stages of weathering and surface conditions (coatings, lichen or moss cover, soil contaimination, etc.). Both factors complicate interpretation of reflectance, as has already been established at infrared wavelengths (Watson, 1970). (3) Not all rock surfaces are Lambert reflectors, and the photometric (bidirectional reflectance) properties can be important in the interpretation of reflectance data and in discriminating between rock types. In order to properly evaluate and predict the influence of these factors, laboratory and field measurements were made of the visible and nearinfrared photometric and spectral reflectance of 1 Publication authorized by the Director, U.S. Geological Survey.
some selected rocks 'in an area where airborne multispectral reflection experiments were being conducted. The four rock types, quartz sandstone, dolomite, limestone, and granite (fresh-broken, weathered, and lichen- and (or) moss-covered), are well exposed in different structural and topographic settings (Rowan et al., 1970) near Mill Creek, Oklahoma, one of several areas being studied by the U.S. Geological Survey.
Measurement Techniques The laboratory apparatus used in the measurement of the total reflected radiation (reflectance) of the rock types is shown in Fig. 1. The basic unit is a Bausch and Lomb colorimeter with an integrating sphere reflectance attachment coated with ceramic fibers (fiberfrax). The effective spectral bandwidth is 20 nm (nanometers) over the wavelength region from 375 to 950 nm. Light emanating from the lamp is focused on the entrance slit, reflected and dispersed into monochromatic light by the diffraction grating, and refocused on the exit slit. The sample is then illuminated by the monochromatic light of beam size 2 by 6 mm and the reflected light is measured by the photomultiplier. Fiberfrax is used as the reflectance standard, and all results are normalized to this standard. The measurements of the reflectance of fiberfrax made by the author (Watson, 1971) are shown in Fig. 4 and are in general agreement with those made by Trytten and Flowers (1966). The photometric properties of the rock surfaces were measured by use of the instrument shown in
Copyright © 1972 by American Elsevier Publishing Company, Inc.
96
ROBERT D. WATSON
SHUTTER~ M P I . . E SHUTTER~ CONTROL"
/
I
PHOTO- I MULTIPLIER
~, ~ INTEGRATING I SPHERE LAMP
[~/ I II
L
/= 1
l i
I
)) It / / II
LENSA ~KNTRANCE SLIT
=7 1
L~CTT[~M20
OCCLUDER
I
L .S.
~"~'~'~"~-~1------ DIFFRACTIONGRATING
SPECTRONIC20 CAM FIG. 1.
Diagram of the colorimeter with integrating sphere used for laboratory measurements of spherical reflectance.
Fig. 2. The source light is a Westinghouse C D D 100-watt lamp with appropriate optics for collimation to a 25-mm beam at the sample surface. The receiver consists of a 6199 photomultiplier detector coupled to a tektronix model 565 memory-oscilloscope. All measurements were made in the vertical plane defined by the projection of the source and detector, and fiberfrax was used as the standard. The bidirectional reflection properties of a surface (Nicodemus, 1965) were measured with the source light at an incident angle of 45 ° and with the angle of observation (0) varied between +70 ° and - 7 0 °. (Sign convention: observation angle positive on the side of normal opposite to source.) Filters with half-transmittance values given in parenthesis were used to obtain the spectral photometric properties at 430 (50) nm, 525 (65) nm, and 630 (65) nm. The spectral bidirectional reflectance of the four rock types was measured in the field by means of an ISCO (Instrumentation Specialties Company) model SR spectroradiometer modified to include a variable aperture fiber optics remote probe sensing head (Fig. 3). All field measurements were made with the remote probe head perpendicular to, and at a constant height above, the rock surface. The field of view was limited to approximately 30 °, and a circular area with a
I
FIG. 2. Schematic diagram of apparatus used for laboratory measurements of the photometric function.
FIG. 3. ISCO spectroradiometer with modified viewing head recording the spectral radiance of fiberfrax. Note the viewing head perpendicular to the surface. 30-cm diameter was viewed. Consistency with the laboratory measurements was obtained by use of fiberfrax surfaces as standards. The ratio of the bidirectional spectral radiance of the rock to that of the standard fiberfrax surface gives the bidirectional reflectance at a known wavelength.
Results The average spectral reflectance and standard deviations for the four rock types as measured in the laboratory are shown in Fig. 4. The three curves for granite are for fresh-broken, weathered, and lichen-covered surfaces. None of the reflectance curves show band structure, but both fresh and weathered granite show slight enhancement in the reflectance at 525-650 nm; this is explained as caused by coloration due to the presence of hematite in various degrees of oxidation in the feldspar. This effect is masked in the curve of "granite-
SPECTRAL REFLECTANCEAND PHOTOMETRICPROPERTIESOF SELECTEDROCKS
97
09
Fiherfrax J
0.8 ±
l
O~xJrtz sondstone - fres hly broken
0.7
0.6
Z
0.5 G ronite--freshly broken
,?, .J
==
04 Gronite--green- ilchencovered Li m e t , one --wecther'ed
0.3
Granitl--weathered Dolomite--weathered
0.2
I 400
q 500
I 600 WAVELENGTH
I 700
t 800
(NANOMETERS)
Fie. 4. Average spectral reflectance and standard deviation of quartz sandstone, limestone, dolomite, and granite (freshbroken, weathered, and lichen-covered) as measured in the laboratory. Fiberfrax represents the standard surface (see text). green lichen covered by the lichen cover (Parmelia Genus). 2 For all the rock types, reflectance increases as the wavelength increases. This is characteristic of many rocks, including laboratory-prepared samples and, in particular, those rocks containing quartz (Watts, 1966). However, an example of the contrast that can exist between a prepared sample and a natural surface can be seen by comparing the reflectance of freshly broken granite with the weathered and lichen-covered granite. Fresh granite is seen to be more reflective throughout most of the visible spectrum than the weathered and lichen-covered granite. The effects of weathering and lichen cover on reflectance are readily demonstrated by the granite curves. Weathering reduces the granite reflectance at all wavelengths, whereas green lichen coverage of the weathered surface enhances the visible reflectance and also noticeably increases the percentage difference in reflectance in the red.
Limestone (Fig. 4) is found to be more reflective than dolomite throughout the visible spectrum, and the percentage difference in reflectance between the limestone and dolomite is higher in the blue than in the red. Quartz sandstone has the highest reflectance of the four rock types examined because of its clean and uniform surface. The bidirectional spectral reflectances of the four rock types measured in situ are shown in Figs. 5, 6, and 7. Although many of the features observed in the laboratory are present in the field, there are some striking differences. The
2 Appreciation is expressed to the University of Oklahoma Biological Research Station for their help in species identification.
FIG. 5. Bidirectional reflectance and standard deviation of limestone and dolomite as measured normal to the surface in the field.
98
ROBERT D. WATSON
o~
(NANO~TICM)
~L[N~
FIG. 6. Bidirectional reflectance of granite (weathered, lichen-covered, and moss-covered) measured normal to the surface in the field.
Y m t cE ~ n 4 t N A ~ r t B I
FIG. 7. Bidirectional reflectance of quartz sandstone measured normal to the surface in the field.
measured bidirectional reflectance of all rock types increases with wavelength as in the laboratory measurements. However, the field reflectances of granite (weathered and lichen-covered), dolomite, and quartz sandstone are somewhat lower throughout the visible spectrum than the corresponding laboratory measurements, and there is a reversal in reflectance differences between weathered granite and lichen-covered granite at 600 nm, with weathered granite having a higher reflectance at longer wavelengths. The increased bidirectional reflectance of weathered granite in the 525- to 650-nm wavelength region as observed in laboratory measurements of reflectance also occurs in the field measurements. Results of the field measurements of the bidirectional reflectance of lichen- and black moss-covered granite are similar to those obtained
>-
by Gates, Keegan, Schleter, and Weidner (1965) of dry lichen and moss attached to sandstone and demonstrate the perturbing effect of mosses and lichens on the rock reflectance. The limestone and dolomite bidirectional reflectances shown in Fig. 5 are average values and standard deviations obtained from observations on six different samples of each rock type. The largest differences in bidirectional reflectance between these two rock types are observed at the shorter wavelengths. The field measurements also demonstrate the rapid increase in bidirectional reflectance of both rock types at wavelengths longer than 750 nm. A word of caution is in order with regard to comparison of field and laboratory results. In these experiments there is a significant difference in the field of view of the laboratory and field instruments so that different surface conditions are viewed. This can cause basic differences in the reflectance spectra and required carefully controlled sampling of field specimens for laboratory measurements in order to draw comparisons between results. The spectral photometric properties of the four rock types are shown in Figs. 8 through 13.
Oil
R*d
~°+ ~. ~ z
Blu*
Rid
-
1~ °'t~
oz
o
BO
g
_+'o
- 0
,
2O
,
0
,
+~0
g
0
,
+SO
,
+SO
OBSERVATION A N G L E (DEGREES)
FIG. 9. Photometric properties of dolomite in the red, green, and blue regions of the spectrum.
as
o.a
alu,
!o+ N 5
O4
ok
z
o2
O2
0,'o
81ul
,n
' -60
•0
. -20
.
.0 .
+. 2 0
+40
~
÷ ;o
OBSERVATION ANGLE (DEGREES)
FIG. 8. Photometric properties of quartz sandstone in the red, green, and blue regions of the spectrum.
-%o
-~o
I
I
OBSERVATION ANGLE
(DEGREES)
FIG. 10. Photometric properties of limestone in the red, green, and blue regions of the spectrum.
SPECTRALREFLECTANCEAND PHOTOMETRICPROPERTIESOF SELECTEDROCKS
De
~r*on N
*d
Blu
O4
O2
~o
-OBSERVATION ANGLE (DEGREES)
FIG. 11. Photometric properties of weathered granite in the red, green, and blue regions of the spectrum.
1963), the behavior of lichen-covered granite is readily understood. It also appears that the non-Lambertian behavior of limestone and dolomite can be partially explained by the influence of lichen cover, inasmuch as each of these surfaces has a uniformly fine scale lichen growth that partially covers the surface. However, further analysis is necessary to explain why non-Lambertian behavior occurs in both the forward and the backward directions and why the departures are more pronounced for the dolomite surface.
Summary
~o~ 8~lue"-G~n ~ed
N
O2
_%
.
.
.
.
:
,
,
,
OBSERVATION ANGLE (DEGREES)
FIG. 12. P h o t o m e t r i c
properties
o f fresh b r o k e n g r a n i t e
in the red, green, and blue regions of the spectrum.
~a \ \\\ me, 77 /
N
\ \
~Bn
\ \
\',
~o~ _o
-;o
. .' . .
'o
'
+g
+L
÷~ '
.
OBSERVATION ANGLE (DEGREES)
FIG. 13. Photometric properties of granite covered with green lichen as measured in the red, green, and blue regions of the spectrum. Dashed curve represents results obtained with a He-Ne laser operating at 632.8 rim. The ordinate in figures 8 through 13 is "normalized intensity" which is the intensity normalized to the maximum intensity measured throughout the range of angles of observation and is therefore a dimensionless quantity. Although quartz sandstone, fresh granite, and weathered granite all approximate Lambertian behavior, lichencovered granite, limestone, and dolomite show departures, particularly at angles of observation greater than 20 °. Because lichens are known to strongly backscatter light (Hapke and Van Horn, 9
99
and Conclusions
The photometric, spectral reflectance (total), and spectral bidirectional reflectance of four rock types have been measured. Differences between the results of the field and laboratory measurements are due to surface conditions. Lichencovered granite, and limestone and dolomite covered with fine-scale lichen depart from Lambertian behavior. All rock types show an increase in reflectance at the longer wavelengths, but the percentage difference in reflectance between each rock type is greatest at the shortest wavelengths. Limestone should be readily discriminated from dolomite inasmuch as it is more reflective at all wavelengths from 380 to 1550 nm. However, the reflectance difference decreases at view angles greater than 40 °, owing to the difference in the photometric properties of limestone and dolomite. These results confirm the need for caution when defining rock-discrimination experiments and interpreting remote sensing reflectance data.
References
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