Abstract
Average solar radiation must be considered to evaluate the amounts of photosynthesis and growth of plants in a greenhouse. Nevertheless, finding the average solar radiation in a greenhouse is not easy because the light incidence angle and greenhouse frame shadows strongly affect the internal solar radiation the solar radiation amount varies spatially. For this study, three simple methods were used to ascertain the average irradiance in a greenhouse: pyranometer improvement, computer simulation using greenhouse models, and comparison of direct measurements on clear and cloudy days. Results clarified that when the solar transmittance in the greenhouse was measured during cloudy weather, when the greenhouse had no frame shadows, the solar radiation transmittance remained constant even when the solar radiation varied over time. Comparison between clear and cloudy days revealed no difference in transmittance. These findings suggest a method to measure the average solar radiation in a greenhouse. First, using two upward-facing pyranometers, one must be installed outside the greenhouse. The other must be installed inside the greenhouse. Then measurements are taken on cloudy days to obtain the transmittance. Next, the outside pyranometer output must be multiplied by the solar radiation transmittance to obtain average solar radiation in the greenhouse for any time. If necessary, a downwardfacing pyranometer should be installed in the greenhouse. Then its measured value must be subtracted from the inside upward-facing pyranometer output to calculate the average amount of solar radiation in the greenhouse.
