抄録
Most of studies on the utilization of solar radiation by crop stands have been concerned with the total short wave radiation. As visible radiation is selectively absorbed by green leaves, utilizations of photosynthetically active radiation by crop stands would differ from the utilizations of total short wave radiation by them. In order to clear the point, an equipment for measuring photosynthetically active radiation in crop stands was devised. 1. Fifteen photodiodes (SHARP, SBC-102) were used as photo energy detectors. In order to make sensitivities of each element uniform, their load resistances were regulated by trimming potentiometers (JAPAN RESISTOR CO., 1w). The values of load resistances of each elements adopted were between 8 and 10 ohmes. These silicon photodiodes were connected as series to sum out-put of each element (fig. 3). 2. Heat-absoring glass (OHARA KOGAKU CO., HG) was used to cut off infrared radiation. Its spectral characteristic is shown in fig. 1. 3. Having a linear responsibility between light intensity and out-put under the dim light, photodiodes adopted were shaded by the brass net (40 meshes) sprayed black enamel paint to 1% rate of light penetration. 4. The size of the photo-energy detector is 498×29×38 mm3, and its construction is shown in fig. 4. The load resistor was separated from photo energy detector, because the sensitivity must be adjusted by the load resistance in case of changing with time. 5. Calibration of the equipment were carried out with comparison to photosynthetically active radiometer (TOSHIBA, PSZ-1 type) under natural solar radiation. While the equipment had been set in the crop stands and run from July to November 1974, no sensitivity changes were found throughout the periods (fig. 5). 6. The cosine response of the equipment was improved by milky-white acryl plate(fig. 6).
