Derivation of the photometric flux of daylight from filtered measurements of global (sun and sky) radiant energy.

The study reported on here is a continuation of an earlier investigation by the same authors into the relationship between natural illumination and shortwave solar radiation. Whereas the initial approach treated the illumination of sunlight as derived from the corresponding filtered direct radiation component, the results now given extend this work to parallel determination of the (more generally applicable) illumination of integral daylight on the basis of similarly filtered global (sun and sky) radiation. Characteristics are outlined of the instrumentation employed in the investigation undertaken at different locations, viz., Newport, Jerusalem (Israel), and Mauna Loa (Hawaii), as compared with the basic work that utilized data assembled at Pretoria (South Africa). Included is an extension to recent solar radiation measurements obtained on high-altitude aircraft. In general, it is established that it is possible to derive such estimates of natural illumination from radiometric measurements with an accuracy comparable with that obtainable in the best direct photometric efforts.