Íslenskar landbúnaðarrannsóknir - 01.03.1980, Side 15

Íslenskar landbúnaðarrannsóknir - 01.03.1980, Side 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

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