Photoelectric measurements of the luminous efficiency of daylight

Abstract

In the course of numerous measurements of daylight which we have made from time to time with certain stable photoelectric cells (Poole and Atkins, 1935), we have compared the values thus obtained with simultaneous measurements of the total radiation, as recorded by a thermopile. Separate determinations were made for sun and sky by screening off the former by means of a small object held at some distance from the cell, and from the combined results the luminous efficiencies of the radiation from sun and sky on various occasions were found on the photoelectric scale. Some of these results have already been published (Atkins and Poole, 1936). It might be objected that the photoelectric scale used differs so much from the visual that these figures are misleading. Criticism might, in particular, be directed against the result obtained that the radiation from a clear blue sky generally has a higher luminous efficiency than sunlight, though the weakness of sky light in infra-red explains this. We have accordingly gone into the matter rather more fully, paying especial attention to the question as to the possible overrating of sky light by the selenium cell. It will be seen that, as a result of this further consideration, the general correctness of the previous results has been confirmed.