Measurements of global radiation using pairs of thermopile pyranometers, one of which was filtered with an RG8 hemisphere, were made under high insolation conditions in Israel and USA. For 1/2 h periods, when solar elevation was above 10.degree., the relative flux density of photosynthetically active radiation .eta., was nearly constant at 0.49 .+-. 0.05. Daily total values measured throughout 1 summer gave .eta. = 0.47 .+-. 0.07 and monthly values of .eta. measured at 3 stations gave .eta. = 0.49 .+-. 0.02. Within the accuracy with which it can be defined and measured, .eta. can be taken as 0.5 for periods of 30 min or longer when the solar elevation is greater than 10.degree.