Íslenskar landbúnaðarrannsóknir - 01.03.1980, Page 15

TEST OF A HAND-HELD RADIOMETER 13
Radiances in thes wavelength region show
an inverse nonlinear trend as the (green)
biomass (and hence the chlorophyll) in-
creases. The spectral response asymptotes
over and beyond that level of total dry
biomass corresponding to~450 g/m2.
The 0.775 to 0.825 jím filter was chosen
to straddle the 0.800 /tm midpoint and is
situated in a region of the spectrum where
green vegetation has characteristically
high levels of reflectance. Reflectance or
radiance in the 0.775 to 0.825 jitm region
shows a direct and nonlinear trend as the
biomass increases. The spectral response
for the 0.775 to 0.825 /im region
asymptotes at a much higher level ofgreen
biomass than the 0.650 to 0.700 jum re-
gion (Tucker 1977).
The ratio of the (0.775 to 0.825 ,um) /
(0.850 to 0.700 /um) bands was used for
spectral biomass estimation purposes.
The specific wavelength configurations
may change slightly for difíerent resear-
chers, but in most cases a photographic
infrared (IR) radiance or refiectance
measured somewhere in the 0.75 to 1.00
Atm region is divided by a red radiance or
reflectance measured somewhere in the
0.60 to 0.70 /um region (Tucker 1979).
The ratio of IR/red radiances is unitless
and allows for not having to calibrate the
readings by the solar intensity undir
somewhat constant conditions. The 0.65
to 0.70 and 0.775 to 0.825 /xm bands are
quite close in a spectral sense and any
changes in the solar intensity are
minimized. If, for example, the bands
were much more widely seperated, a
greater degree of variability would be in-
troduced by changes in the solar flux.
Because the response of the 0.650 to
0.700 /um and 0.775 to 0.825 /u'11 bands
are both nonlinear, the ratio of the (0.775
to 0.825 /um) / (0.650 to 0.700 /um)
bands would be expected to be nonlinear
also. As such, it exhibits a combination of
the asymptotic properties of its compo-
nents with respect to biomass.
Operation procedure
The operating procedure for the instru-
ment is quite simple. The operator holds
the probes pointing downward at the pre-
scribed height above the posture surface
and reads from the digital display the
radiance measured in each probe in
/uWatts/cm2. By minipulating a small
switch mounted on the radiometer, the
operator is able to read from first one
probe and then the other. The operator
then records the numbers or calls them
out to an assistant who records them. The
ratios are clculated later once the data is
being reduced, as can be other voriables
such as the vegetation index (VI). The VI
is calculated as the (IR—RED) /
(IR+RED).
The operator is able to scale the read-
out-display unit by simply pressing one of
several decade range switches depending
upon conditions. If the light intensity be-
ing measured is too great for the meas-
urement range that has been selected, the
display blinks at a 6-Hz rate to call this to
the operator‘s attention.
Each probe contains a READOUT
HOLD switch that allows the user to store
the last displayed reading as long as de-
sired. This is advantageous in a field
situation because a specific number can be
recorded for a given reading and not „vis-
ually integrated or averaged“ by the
operator.
The time involved in positioning the