The spectral energy distribution of daylight

A series of measurements, extending over a year, was made at Enfield, Middlesex, using a grating monochromator to determine the spectral energy distribution of daylight between 3000 and 8000 Å, particularly of total sky radiation without direct sunlight. The results are considered with respect to the structure of the curves, their equivalent chromaticities and the related full radiator distributions. Average distributions are given for several daylight sources including those at the limits of the usual range of chromaticities used in colour matching. From these, specifications for real or artificial daylight at any intermediate colour temperature can be set up. The values so obtained are more realistic than the theoretical data of the Abbot-Gibson formulation.