Colorimetric specifications are given for Wratten light filters, including the tristimulus values, trichromatic coordinates, dominant wave-lengths, purity, and luminous transmittance for ICI illuminants A and C and for an artificial daylight produced by the use of a Wratten filter. These data are based on the spectral transmittance values published in the seventeenth edition (1944) of the booklet entitled “Wratten Light Filters.”Wave-lengths for thirty selected ordinates and for supplementary ordinates are given for colorimetric computations involving the artificial daylight source, which consists of a tungsten filament lamp operated at a color temperature of 2360°K, the light from which is passed through a Wratten filter No. 78.