Description is given of apparatus for and principle of measuring the color of fluorescent materials illuminated by daylight covering the' ultraviolet and visible spectrum regions. The light from the source is dispersed by a grating monochromator and the monochromatic light illuminates a sample. The fluorescent or reflected light from the sample enters the second grating monochromator and is detected by a photomultiplier tube. (Method I). As a simple method, the first monochromator is removed and the sample is illuminated directly by the source, and both the fluorescent and reflected lights from the sample are dispersed by the monochromator and detected by the photomultiplier tube. (Method II). In both methods, a xenon short arc lamp is used as the light source.
By using cloths whitened by a fluorescent whitener, the color rendering characteristics of the natural daylight in several phases and artificial white light are examined. It is found that the Middleton's data of the overcast sky over hypothetical urban area are correct and that the xenon lamp is the most suitable light source for the Method II although the ultraviolet intensity of the lamp is considerably high.
Useful tables of reduction of spectrofluorometric data to tristimulus values are shown. Results of the measurements on colors of some daylight fluorescent materials are given.