Íslenskar landbúnaðarrannsóknir - 01.03.1980, Page 14

12 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
Figure 1. Hand-held digital radiometer used as a biomass estimation device. The instrument consist oi'a
digital radiometer that measures radiances from sample plots through two filtered probes.
found to be rather awkward to use in the
field and the pocket calculator and control
and interface circuit were removed (Fig-
ure 1). The radiometric technique ofin situ
biomass estimation had previously been
tested on U. S. Great Plains shortgrass
prairie vegetation (Tucker et al. 1973)
and on algal mats in Yellowstone National
Park, VVyoming, USA.
Instrumentation
The Tektronics radiometer was modified
to accomodate a second uncorrected
probe. Each probe contained a custom-
built interference filter that sharply li-
mited the wavelength band of the
radiance entering the probe (Figure 2).
The íilter bandwidths were set as narrow
as possible consistent with allowing
enough radiant energy through to the sili-
con photodiode in each probe to give a
sufíiciently noise-free output (Pearson et
al. 1976).
The photodiode mounts were re-
positioned so that the diode is~4 cm be-
hind the front viewing window in each
probe. At this distance the probe has an
~24° viewing angle or a solid angle of
~0.15 sr. When held 1.25 m above the
grassland or pasture surface, the probes
subtend a circular area with a diameter of
~50 to 60 cm. This translates to a circular
area of 25 m2.
The two filters were specifically chosen
to becentered around 0.675 and 0.800 jum,
respectively. The 0.650 to 0.700 gm filter
is centered over the in vivo red region of
maximum chlorophyll absorption.