The absence of appropriate instruments and observational techniques preclude the direct recording of photosynthetically active radiation, the spectral interval of 0.38 to 0.71 micron that is used by plants in the process of photosynthesis. However, observation data on direct-beam, diffuse, and total radiation can be converted into the photosynthetically active portion of radiation through the use of appropriate coefficients. According to theoretical and experimental findings, the photosynthetically active portion represents 43 percent of direct-beam radiation and 57 percent of diffuse radiation, or 50 percent of total radiation. The basic radiation data can be obtained either by generalizing the observations of actinometric stations or by computational methods. World maps of photosynthetically active radiation for the entire year and for the growing season have been compiled. Annual sums range from 30–40 to 100–110 Kcal/cm2, and growing-season sums from 0–10 to 100–110 Kcal/cm2, with the highest values in tropical desert and the lowest in the polar regions.