Not all the radiation that reaches the downlooking scanner 
originates at the surface element being viewed. There is a quantity 
called path radiance, which is extraneous radiation that is scattered 
into the field of view by the atmosphere. 
Figure 5 shows that path radiance depends strongly on visual range 
and wavelength. The wavelength dependence is much different than that 
for the irradiance shown in Figure 4. The reason for this difference 
is that path radiance depends on the reflectance spectrum of the objects 
surrounding the material being viewed. In this instance, the spectrum 
is that of green vegetation. 
Path radiance also depends on the scan angle from nadir and the 
location of the sun. Figure 6 shows the scan-angle dependence of path 
radiance for three different sun positions on a very hazy day. The 
scan plane includes the sun, and one readily can see the characteristic 
peak in backscattered radiance at the antisolar angle. When the scan 
plane does not include the sun, the path radiance is a much more symmetric 
function of scan angle. Figure 7 shows how path radiance would vary 
throughout one day if the aircraft were flying East or West at an 
altitude of 1 km on a medium hazy day. The scan is in the North-South 
plane. Note the increase and decrease of the radiance at any given scan 
angle as the time changes throughout the day. Also note the increase 
in path radiance at noon when the sun moves into the scan plane. This 
type of information is useful for planning flight lines and times. 
A scanner measures the total radiance from the view direction; thus, 
the atmospheric effects of path radiance and transmittance are both 
present in its signals. Figure 8 simulates the total radiance that 
would be received by the scanner from a diffusely reflecting surface 
with the reflectance spectrum of green vegetation. A comparison of 
Figures 7 and 8 shows that the path radiance is a substantial component 
of the total received radiance. Both figures show the large and systematic 
changes in signal level that are associated with time of day, that is, 
with sun position. 
Now let us examine the scan angle dependence more closely. Figure 
9 presents a comparison between experimental measurements of sky radiance 
and calculated values for three different surface albedos. Note the 
excellent agreement between the shapes of the experimental and the 
calculated curves of sky radiance. Total radiance calculated with the 
model was compared to total radiance measured by a multispectral scanner 
for several fields of soybeans. The data are presented in Figure 10. 
One immediately notes the difference in angular response. We believe 
that much of this difference is due to the surface reflectance character- 
istics. The simulation model assumed a perfectly diffuse, or Lambertian, 
surface, whereas results presented by Suits [13] and results of other 
investigators show that agricultural crops, like soybeans, do have 
definite bidirectional reflectance characteristics. The model is being 
modified to include such reflectarices. 
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