The colour temperature of daylight

A series of measurements of the colour temperature of daylight, extending over two years, was made at Teddington, Middlesex, with a visual colour temperature meter. The mean value of colour temperature, when calculated from measurements taken in four quadrants of the sky was 150.3 mireds (6650°K); and the mean value when the sky was measured as a whole, including direct sunlight, when the sun was not obscured by cloud, was 167.0 mireds (5990°K). Histograms are given to show the distribution of the colour temperature at different times of day and year. The present work, combined with the work of Henderson and Hodgkiss in 1963 on the spectral distribution of daylight, is proposed as a basis for a new artificial daylight standard. The relevance of this proposal to the provisional recommendations made in 1964 by the Commission Internationale de l'Eclairage is mentioned.