=75°
Sy
-0°
Neither Walthall’s model nor our proposal to account for the
specular peak are based on theoretical assumptions. f. as
defined in equation 7 is just an empirical function to describe
the measured data.
4 RESULTS OF THE BRDF-MODEL-FITTING TO
THE SELECTED SURFACE SAMPLES
Fig. 5-10 show the BRDF of the samples as fitted by equation
1. There was no uniform angular dependence of the samples.
The BRDF of some of the samples increased with increasing
zenith angles (e.g. concrete), whereas the BRDF of other
samples decreased with increasing zenith angles (e.g. plastic).
The variation due to the diffuse component is usually less
than 15.96 (not taking into account zenith angles greater
than 75^). The intensity of the specular peak dominates the
visual impression of figures 5 to 10.
No simple rules could be deduced from the wavelength depen-
dence of the coefficients ao to as, which describe the diffuse
component. The wavelength dependence of the coefficients
as to ag, which describe the specular component, is almost
always linear or even constant.
The coefficients of the samples are given in tables 11 — 13 at
a wavelength of A = 650, 750,850 nm respectively. The er-
ror was calculated from the corresponding covariance matrix.
The relative error of the fitted BRDF varies between 2 and
7%.
All the selected samples pass the x? test. We conclude that
the function of [Walthall, 1985] with the modifications of
[Liang, 1994], extended by a specular peak, is best suited
for describing the BRDF of the samples studied by us.
5 OUTLOOK
We plan to apply the here discussed experimental results to
multispectral imagery recorded with the DAEDALUS AADS
1268 scanner. The imagery was acquired during several cam-
paigns in cooperation with the German Aerospace Research
Establishment (DLR) and is described in previous communi-
cations [Hepp, 1994].
ACKNOWLEDGEMENTS
Dr. Barbara Bartsch put the spectrometer OVID at our dis-
posal. She and Dr. Martin Kollewe enabled us to carry out
the measurements. We want to thank them for their contri-
butions.
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
