radiometer and the Exotech 4-band radiometer. Both instruments have a 15° FOV
capability. The distance between tubes for the three-band instrument is 3.8 cm,
and 6.35 for the four-band instrument. The relation between the radius of the
target area and the height of the radiometer (h) is
For a 15° FOV, r = 0.132 h. The diameter of a target circle is 26.4 cm when
the radiometer is held at 1 m, and 52.7 cm when held at 2 m. In other words, the
diameter is roughly one-fourth of the height that the radiometer is held. This is
a useful approximation when estimating target areas over row crops.
Figure 9A shows the ratio of the coincidence area to the target area as a
function of the radiometer height above the target for the Mark II 3-band and the
Exotech 4-band instruments. At 2 m in height, any two bands of the three-band
instrument will view about 91 percent of the same area. At 1 m, about 82 percent
of the same area is viewed. The coincident area for all three bands is about 87
percent at 2 m, dropping to 74 percent at 1 m.
The greater tube separation of the Exotech 4-band causes a smaller
coincident area than for the three-band. Figure 9B shows the ratio for two
adjacent bands, two diagonal bands, and for all four bands for this instrument.
At 2 m, the ratio is 85 percent, dropping to 70 percent at 1 m. The ratio for the
four-band coincident area is 71 percent at 2 m and 47 percent at 1 m.
We have considered only the height perpendicular to a flat target. In a
field, the soil surface is considered the flat target, and the radiometer is held
vertically a distance h above the soil. Plants, protruding above the surface,
alter the picture somewhat. Consider a situation in a field where the radiometer
is held 2 m above the soil surface. If plants are in the scene, the coincident
are will be less for the tops of the plants than at the soil sur face. Figure 10
shows a side view and a top view of what a single band (15° FOV) radiometer
“sees” when held 2 m above the soil surface. The centers of the plant rows
(designated by the horizontal lines) are 0.3 m apart (approximately the row
spacing of wheat in the northern Great Plains), the row width is 0.1 m and the
plant height is 0.2 m.
At 2 m, a 15° FOV radiometer will see portions of 1-1/2 rows of plants,
depending upon where the radiometer was located above the row. Depending upon
location, it is possible that the radiometer could view most of two plant rows in
one instance and only slightly over one row in another. Since it is difficult to
hand-hold a radiometer much higher than 2 m, it is necessary to take a series of
measurements at various horizontal locations (maximum height) across the rows in
order to get an adequate sample of the reflectance properties of the entire plot.
This problem can be reduced by increasing the field of view of the instrument;
however, the danger exists of getting portions of the operator’s body in the
radiometer scene. Figure 11 depicts this possibility in the form of a person
standing on a plank (to increase radiometer height) holding a radiometer. Two
fields of view, 15° and 24°, are shown. The edge of the 24° scene is about 20 cm
from the plank. The radiometer is shown level. In practice, it is very difficult
to hold a radiometer sufficiently level to guarantee no “foreign” bodies in the
scene. Furthermore, the total field of view is usually somewhat larger than that
specified by the manufacturer. Peripheral