Photoelectric measurements of the luminous efficiency of daylight

Determinations of the ratio of the illumination to the total radiant energy have been made, using L. A. Jones's special green filter to reduce the wave-length sensitivity curve of the selenium rectifier cell to a close agreement with that of the eye. The cells used were standardized for mean noon sunlight as previously described. The results for mixed sunlight and sky light are in good agreement with Kimball's measurements on the visual scale both with the green filter, and for the selenium cell without it, and for the Burt sodium cell. The selenium cell without filter may overrate blue sky light by a few per cent, but the sodium cell overrates it considerably. In general, daylight gives rather over 100 lumens/watt, sky light up to 158 for light blue, and down to 48 for heavy grey; sunlight may vary from 35, for low sun, to 120 lumens/watt for high altitude light in summer. These are mostly far higher than artificial sources of light, but data for animal light lead to a very high value, which is probably unreliable.