denoted as "phase function". The angle is positive (or negative) when the
sensor and the zenith are on the same (or opposite) side with respect to the
sun, Fig. 7 depicts the phase functions at 8:48 AM of Date 137, 1982. The
curves from the top to the bottom correspond to TM4, TM5 and TM3,: respectively.
The phase function for TM3 shows little variation while that for TM4 shows
greatest variation. Generally speaking, the shapes of the curves looked like
a "V", The reflectance ratio R reaches minimum when the sensor is yertically
looking downward. The curves are somewhat asymmetric with tlie reflectance ratio
R at Omin being larger than that at Omax. The results can be explained in terms
of the sizes of reflective surface and the shadow due to the plant itself. The
size of the reflective surface reaches minimum and the size of the shadow rea-
ches maximum at 0 downward. While the size of the reflective surface are about
the same when the phase angle @ is at its extremes, the size of the shadow at
9 min is somewhat smaller than that at © max. The combination of these two
effects make the shape looking like a "V" and asymetric.
Fig. 8 is identical to Fig. 7 except for Date 141.at 8:30 AM. The general
variations are similar to those in Fig. 7. It appears that the difference
between max R and min R is bigger on Date 141 than that at Date 137. To see
the long-term variation, we plot in Fig. 9 the reflectance ratios at Q min,
© downward and © max for each band against Julian Date. With few exceptions,
R is max at © min and is min at 9 downward. Note that the difference between
max R and min R becomes much smaller after Date 170, when the plant height and
head length reach their maximums, and grains begin to form. The plant top will
look rougher after this date. Consequently, the anisótropy in. R will becomes
lesser. : :
The "phase function" near local noon behaves quite differently compared to that
at other local times. Fig. 10 illustrates the "phase function" at 11:38 AM of
Date 142. The reflectance ratio R shows few variations for all of the three
bands. Recall that Fig. 8 is for Date 141, which was one day earlier than that
in Fig. 10. The fact that Fig. 8 and Fig..10 differed so much is mainly due to
the difference in local time. At noon, when tlie sun is overhead; the ratio
between the sizes of the shadow and reflective.surface is, more or less,
independent of the viewing angle. Consequently, the reflectance ratios show
little variations with respect to different view angles, which is consistent
with what Fig. 10 indicates.
z
=
==
REFERENCES
1- Mcleans, Informations Sheet on Optical Properties of Pressed Halon Coatings,
NASA/GSFC, Greenbelt, Md., U.S.A., 1979.
2- Nicodemus, F.E., Reflectance Nomenclature and Directional Reflectance
and Emissivity, Appl. Optics 9(6) : 1474-1475, 1970.
3- Sung, Q.C., The Spectral Signatures of the Major Crop in Taiwan by Dual
Looking Ground Radiometer, preprint, National Central University, Chung-li,
Taiwan, R.0.C., 1981.
4- Tucker, C.J., W.H. Jones, W.A. Kley, and G.J. Sundstrom, The GSFC Mark-II
Three Band Hand-Held Radiometer, NASA Technical Memeorandum 80641, NASA/GSFC,
Greenbelt, Md., U.S.A., 1980.
