Laboratory Bi-directional Reflectance Distribution Function (BRDF) measurements of salt pan regolith samples are presented in this study in an effort to understand the role of spatial and spectral variability of the natural biome. The samples were obtained from Etosha Pan, Namibia (19.20 deg S, 15.93 deg E, alt. 1100 m). It is shown how the BRDF depends on the measurement geometry - incident and scatter angles and on the sample particle sizes. As a demonstration of the application of the results, airborne BRDF measurements acquires with NASA's Cloud Absorption Radiometer (CAR) over the same general site where the regolith samples were collected are compared with the laboratory results. Good agreement between laboratory measured and field measured BRDF is reported

Butler, James J.

Gatebe, Charles K.

Georgiev, Georgi T.

King, Michael D.

NASA Technical Reports Server

BRDF of Salt Pan Regolith Samples 
Georgi T. Georgieva, Charles K.  ateb be^, James J. Butlerc, and Michael D. Gngd 
'Science Systems and Applications, Inc., Lanham, MD 20706, e-mail: georai.t.aeor~iev(ii3nasa.aov 
b~oddard Earth Science and Technology Center, University of Maryland Baltimore County, Baltimore, MD 21228 
'Earth-Sun Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771 
d~aboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309 
Abstract-Laboratory BRDF measurements of salt pan regolith uncertainty was 1% (k = 1) or better, where k is the coverage 
samples are presented in this study in an effort to understand the factor, The laboratory results were compared to BRDF 
role of spatial and spectral variability of the natural biome. The measurements with an airborne radiometer, the Cloud 
samples were obtained from Etosha Pan, Namibia (19.20°S, Absorption Radiometer (CAR), which was developed at GSFC 
15.93'E, alt. 1100 m). It is shown how the.BRDF de~ends on the rn 
measurement geomktry - incident and scatter angles and on the 
sample particle sizes. As a demonstration of the application of the 
results, airborne BRDF measurements acquired with NASA's 
Cloud Absorption Radiometer (CAR) over the same general site 
where the regolith samples were collected are compared with the 
laboratory results. Good agreement between laboratory 
measured and field measured BRDF is reported. 
BRDF, Optical instramenfation and measurements, Metrology, 
Remote sensing 
The monitoring of land surface is a major science objective 
in Earth remote sensing. An important goal is to identify major 
biomes and to map and distinguish the changes in their 
composition introduced by anthropogenic and climatic factors. 
Currently, deforestation and desertification are the most 
important land cover area processes of scientific interest. A 
feasible way of characterizing surface anisotropy is by 
measuring the Bi-directional Reflectance Dislribution Function 
(BRDF) for different surfaces and ecosystems in many parts of 
the world. The BRDF describes the spatial and spectral 
interaction of light on a sample's surface as a hnction of the 
incident and scatter angles and wavelength. BRDF 
measurements of Earth scenes can be used as a sensitive tool 
for early detection of changes occurring in vegetation canopies, 
soils, and oceans [I]. In this study, we analyzed laboratory- 
based BRDF data of soil samples collected at Etosha Pan, 
Fig.1, which is situated in northern Namibia with an area of 
4590 square kilometers. The Etosha Pan is a core site for 
validation of the Earth Observing System (EOS) Terra and 
Aqua satellite instruments. The pan is desert-like, white in 
color, and a dry salt pan without any vegetation. However, 
during the rainy years, Etosha pan becomes a 10 cm deep lake 
and a breeding ground for thousands of flamingos. The Etosha 
Pan mineralogy is dominated by four compounds: feldspar and 
mica, feldspar and sepiolite, silicates, and calcite and dolomite; 
compounds that determine the pan's spectra. Etosha pan 
surroundings we dominated by mopane and acacia trees and 
grasslands. All the samples were measured using NASA 
Goddard Space Flight Center's (GSFC's) Diffuser Calibration 
Laboratory @CL) scatterometer [2]. The typical measurement 
Figure 1. Etosha Pan 
The BRDF is a function which defines the spectral and 
spatial reflection characteristic of a surface. The definition and 
derivation of BRDF are credited to Nicodemus [4] who 
presented a unified approach to the specification of reflectance 
in terms of both incident and reflected light beam geometries 
for characterizing both diffuse and specular reflecting surfaces 
of optical materials. He defined the BRDF as a distribution 
function relating the irradiance incident fiom one given 
direction to the reflected radiance in another direction. Thus, 
the BRDF is presented in radiometric terms as the ratio of the 
radiance, L, reflected by a surface into the direction (8, &) to 
the incident irradiance, Eb on a unit surface area fiom a 
specified direction (@, &) at a particular wavelength, A. This is 
expressed mathematically as: 
BRDF = d r ( o i , # i > ' r , # r ; ~ i )  
dEi (ei 9 #i 1 
where the subscripts i and r denote incident and reflected light 
respectively, 8 is the zenith angle, and $I is the azimuthal angle. 
https://ntrs.nasa.gov/search.jsp?R=20080030205 2019-08-30T05:00:42+00:00Z
Nicodemus further assumed that the incident beam has 
uniform cross section; the illumination on the sample is 
isotropic; and all scattering comes fiom the sample surface and 
none from the bulk. In practice, we are dealing with real 
samples' sudaces which ate not isotropic and the optical beams 
used to measure the reflectance are not perfectly uniform. 
Hence from practical considerations the BRDF can be defined 
as presented by Stover [5] as the scattered power per unit solid 
angle normalized by the incident power and the cosine of the 
detector zenith angle. It is expressed in t e r n  of incident 
power, scattered power, and the geometry of incident and 
reflected light: 
where Pi is the incident power, P, is the scatter power, 8, is the 
detector zenith angle, and is the solid angle determined by 
the area of detector aperture, A, and the distance from the 
sample surface to the limiting aperture at the detector assembly, 
R. The solid angle can be computed as Cl  =  AIR^. Pi is the 
incident radiant power, and 0, is the reflected zenith angle. The 
cos8, factor is a correction to account for the illuminated area, 
when viewed from the detector direction. BRDF has units of 
inverse steradians and can range fiom very small numbers (e.g. 
for off-specular black samples) to very large values (e.g. for 
highly reflective samples at specular reflectance). The BRDF 
angular convention is shown in Fig.2, where Oi is the incident 
zenith angle measured from the sample normal; 4 is incident 
azimuth angle; 8, and +s are scatter zenith and scatter azimuth 
angles, respectively. 
Figure 2. BRDF angular convention after Nicodemus 
In the case of CAR measurements, the spectral BRDF (Rd 
is expressed following the van de Hulst [6] formulation, Fig. 
9 .  
where IA is the measured reflected intensity (radiance); Fk is the 
solar flux density (itradiance) incident on the top of the 
atmosphere; 0 and are respectively the viewing and incident 
zenith angles; @ is the azimuthal angle between the viewing 
and incident light directions; and po = cosOo. The Rh is 
equivalent to the bidirectional reflectance factor (BRF) as 
defined by Nicodemus, which is dimensionless and 
numerically equivalent to BRDF times x. 
a sun A Z 
\ Observer 
Figure 3. BRF angular convention after van de Hulst 
The CAR instrument was flown aboard the University of 
Washington Convair CV-580 research aircraft during the 
Southern Africa Regional Science Initiative 2000 (SAFARI 
2000) dry season campaign [7]. The CAR scan mirror scans 
360' in a plane perpendicular to the direction of flight with a 
rate of 100 scans per minute and the data is collected through a 
190' apeme.  The analog signal is digitized internally to 16 
bits. The CAR has an instantaneous field of view of 1" and 14 
channels located between 335 and 2344 nm. 
A. Measurement setup and samples 
In this study, we present laboratory BRDF measurements of 
regolith samples. in an effort to show the role of spatial and 
spectral variability of the natural biome, a continuation h m  
our previous work [&I. The BRDF was measured using the 
scatterometer located in the Diffuser Calibration Labomtory at 
NASA's Goddard Space Flight Center (GSFC). The 
scatterometer, Fig.4 is located in a class 10000 laminar flow 
cleanroom. 
Figure 4. The Scatterometer: a. Goniometer with the "Rock" sample, b. 
Optical table 
The scatterometer is capable of measuring the BRDF and 
BTDF of a wide range of samples including white and gray- 
scale diffusers. black aainted or anodized diffusers. oolished or 
a .  
roughened metal sukaces, clean or contaminated mirrors, 
transmissive diffusers, liquids, and granular solids. The 
operational spectral range of the instrument is from 230 to 900 
nm. The scatterometer can perform principal plane and out-of 
principal plane BRDF measurements. It consists of a vertical 
optical source table, a sample stage, a detector goniometer, and 
a computer system for positioning control, data collection and 
analysis. The light source used was a short-arc Xenon lamp - 
monochromator assembly producing an incoherent, tunable 
light with a well-defined spectral bandpass width. The samples 
were mounted horizontally on the sample stage and aligned 
with the scatterometer axes of rotation. The sample stage can 
be moved in the X, Y and Z linear directions using three motor 
stages. There is also an additional degree of freedom allowing 
sample rotation in the horizontal plane. The detector field-of- 
view was centered on each sample target for all measurements 
and was underfilled by the incident beam, meaning the 
illuminated area on the sample was always smaller than the 
detector FOV. Scattered light was detected using a polarization 
insensitive ultraviolet enhanced silicon photodiode with output 
fed to a computer-controlled lock-in amplifier. The detector 
assembly can be rotated around the vertical and horizontal axes 
of the goniometer. The surface of the sample to be measured 
was positioned at the cross point of the two perpendicular 
goniometer rotation axes which defme the center of rotation of 
the goniometer system. All measurements were made for 
polarizations of the illumination beam both parallel, -p, and 
perpendicular, -s, to the plane of incidence. The BRDF for each 
polarization was calculated by dividing the net signal from the 
transmitted radiant flux by the incident flux and the projected 
solid angle from the calibration item to the limiting aperture of 
the detector. This setup facilitates the acquisition of 
computerized BRDF measurements at different incident and 
scattered geometries for a complete data acquisition at selected 
points and wavelengths. 
Figure 5. The regolith samples 
Four fractions of the same sample materia1 were studied, 
Fig.5. The first sample, "the rock", is a piece of rock sediment, 
hereafter referred to as EPR (Etosha Pan Rock), while the other 
three are regolith samples derived by grinding the sample 
material to different particle sizes. Particle sizes for the EPI 
sample (Etosha Pan I)  was sub 0.5 rnrn, for the EP2 sample 
was between 1 and 2 mm, and for the EP3 sample was sub 1 
mm. Etosha Pan samples were studied at different wavelengths 
and incident and scatter angles corresponding to the airborne 
measurements over the Etosha Pan. The laboratory-based 
BRDF of all samples was analyzed in the principal plane at 
412,555,667 and 869 nm, however; only 667 nm and 869 nm 
are similar to CAR'S wavelengths. The samples were 
characterized at an incident angle of 30' and at scatter zenith 
angles from 0" to 80' in steps of 5". The scatter azimuth angles 
of 0" and 180' correspond to the principal plane measurement 
geometry. The samples were placed in square 50 x 50 x 5 mm 
black plastic holders with the sample surface well flattened and 
with uniformly sized particles distributed through the entire 
surface area. 
B. Results and Discussion 
The laboratory-based BRDF at 30' incidence for the four 
Etosha Pan samples is shown in Fig. 6 at 667 nm. The rock 
sample's BRDF is higher as the particulate incident light 
shadowing and scatter light obscuration effects are the 
smallest. The finest structure sample, EP1, has distinctively 
higher BRDF than the two other larger fractions, samples EP2 
and EP3. It is worth noting that the shape of the BRDF curve 
for the rock sample is different than the shape of the 
particulate samples. It is also very important that all samples 
have apparent backscattering properties. Although the BRDF 
at Oi = 0, could not be measured due to the relative geometries 
of the scatterometer source optics and detector, the BRDF for 
all samples shows evidence of a significant opposition effect 
represented by increased light being retroscattered back in the 
direction of the incident beam. Sample No.2, with particle 
sizes between 1 and 2 mm, has the lowest BRRF. 
0.3 I 1 I I I 
+ Powder 1 
0.25 - +- Powder2 + Powder3 
-80 60 40 -20 0 20 40 60 80 
Viewing Angle ('1 
Figure 6. BRDF at 30' incident anglc and 667 nm 
In addition to BRDF measurements, the samples' spectral 
reflectance was measured with an Analytical Spectral Device 
(ASD) spectroradiometer in-plane at 30° incident angle and 
30' scatter angle from 350 to 2500 nm. This is shown in Fig. 
7. The ASD spectra present a full reflectance picture for the 
VIS-NIR spectral range providing additional information on 
the Etosha Pan sample's reflectance properties. 
200 400 600 8M) 1MX)1200140016M)18002000220024002600 
Wavefength, nm 
Figure 7. ASD spectra at 30' incidcnt angle and 30" scatter zenith angle 
In order to correctly compare the laboratory-based BRDF 
with the airborne measurements, we calculated the composite 
laboratory-based BRDF fi-om the BRDF of the rock and 3 
particulate Etosha Pan samples as measured in the laboratory. 
The contribution of each sampIe to the calculated composite 
laboratory-based BRDF was determined by the disbribution of 
the 3 components as seen by the CAR instrument during 
airborne measurements. The components' spectral reflectance 
weighting was calculated by the field campaign participants 
after a careful examination of photographs of the flight track 
over Etosha Pan. These weights were determined to be 25% 
EP1, 50% EP2, and 25% EP3. The simulated fi-actional 
laboratory-based data is compared to the CAR airbome data. 
I 
BD 60 -40 -20 0 20 40 W 80 
Viewing Angle ("1 
Figure 8. Etosha Pan simulated scene BRDF from the laboratory-based 
BRDF and CAR airborne data 
The same general shape of the laboratory-measured 
samples and airborne measurements can be seen in Fig. 8. The 
data matches well within the uncertainty for both wavelengths 
over the scatter angIe range with the exception of -80" where 
the CAR measured data are higher. However, the airborne data 
at the two wavelengths shown are very close. The laboratory 
based data show a larger difference between the data at 667 
nm and 869 nm. 
IV. CONCLUSIONS 
This work is intended to describe more completely the 
BRDF of regolith samples from Btosha Pan measured in a 
laboratory environment. In addition, the laboratory results are 
compared to airborne measurements of these areas by the 
CAR instrument. In the laboratory measurements, the BRDF 
depends on the incident and viewing angles, on the nature of 
the sample, on the sample status, and on the particle size 
fraction. It is highest for the rock sample and lowest for the 
larger size particles regolith sample. 
Laboratory-based and CAR airborne data sets from Etosha 
Pan were compared at 682 and 870 nm for the airborne data 
and 677 and 869 nrn for the laboratory data, respectively. The 
BRDF curves have the same general shape, and the data 
matches well into the uncertainty for both wavelengths over all 
viewing angular range. However, the airborne data show 
smaller BRDF differences between the two wavelengths than 
the laboratory-based data. The effects of atmospheric 
absorption aria scattering from CAR measurements could be a 
source of uncertainty. We believe the laboratory results are 
going to be of great use to the remote sensing community in 
their modeling and correction efforts of airbome data. 
The results presented here are traceable to the National 
Institute of Standards and Technology's (NISTYs) Spectral Tri- 
function Automated Reference Reflectometer (STARR). 
[ I ]  J.J. Butlcr, B.C. Johnson, R.A. Barnes, 'The calibration and 
characterization of Earth rcmote sensing and environmental monitoring 
insbuments", in Optical Radiometry, Eds. A.C. Parr, R.U. Datla, J.L. 
Gardner, Academic Press, New York, 2005 
[2] G.T. Georgiev, J.J. Butler, "Laboratory-based bidirectional distribution 
functions of radiometric tarps", Applicd Optics, vo1.47, pp. 3313-3323, 
2008 
[3] M.D. King. M.G. Strangc, P. Lcone, L.R. Blaine, "Multiwavelength 
scanning radiometer for airborne measurements of scattered radiation 
withinclouds", J. Arm. and Oceanic Tochn., vol. 3, pp. 513-522, 1986 
[4] F.E. Nicodemus, J.C. Richmond, J.J. Hsia, I.W. Ginsburg, T. Limperis, 
"Geometrical considerations and nomenclature for reflectance", National 
Bureau of Standards, NBS monograph 160, Oct. 1977 
[S] J.C. Stover, "Optical scattering: mcasurcment and analysis", SPIE Press, 
Bellingham, Washington, 1995 
[6]  H.C. van dc Hulst, Multiple light scattering, Tables, Formulas, and 
Applications, vol. 1. San Diego: Academic Prcss, 1980. 
[7] C.K. Gatebe, M.D. King, S. Platnick, G.T. Arnold, E.F. Vermote, B. 
Schmid, "Airborne spectral measurements of surface-atmosphcrc 
anisotropy for several surfaces and ecosystems over southern Africa", J. 
Geophysical Research, vol. 108, pp. 1-16, 2003 
r8l G.T. Gcor&v, C.K. Gatebe, J.J. Butler. M.D. King. "BRDF calibration - - 
of natural samples in support ofrernotc sensingn, IGARSS '07, pp.2877- 
2888,2007. 


BRDF of Salt Pan Regolith Samples

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server