Photometry and Colorimetry of Natural Objects

Abstract

Apparent luminances and colors of natural objects were measured by three types of telephotometers, which were designed originally as telephotometers to measure distant light sources and extinction of light in the atomsphere, but used as brightness meters in this study. Two of them are based on the photoelectric and photographic spectroradiometric methods and the third on the visual color matching with a comparison light consisting of an incandescent lamp and three Lovibond glasses.The range of the luminance observed in the daytime for the sky near the horizon is 6, 600-7, 400 cd/m2 on a clear day and 1, 200-5, 500 cd/m2 on an overcast day. The luminance of the snow surface in the vicinity of the instrument on a clear day. ranging 10, 000-14, 300 cd/m2, is largest among the natural objects. The luminance of other objects near the observer is generally much smaller than that of the sky. But with the increasing distance it approaches the luminance of the sky.The colors of natural objects are chiefly yellow green or yellow when the distances are small. With the increasing distance their chromaticity points on the chromaticity diagram move into blue green region along the blackbody locus. The domain, in which the chromaticity points of natural objects aredistributed, is fairly wide in the direction extending from blue to yellow green approximately parallel blackbody to the locus but much narrower in the direction vertical to it. Foliage at a distance less than 1 kilometer has yellow-greenish colors of dominant wavelength 542-582 mμ. Field crops and vegetables have a dominant wavelength of 561-570 mμ. Their colors are most saturated among the observed objects, having a maximum excitation purity of 52%. Earth colors are yellow of 561-585 mμ. With the increasing distance the colors of natural objects changes to blue. The chromaticity points for buildings, sea, snow, sky and distant objects are distributed over a small area around the achromatic point and its blue side.Spectral distribution curves for natural objects are characterized by their flat appearance, especially when the distances are large, in accord to the low saturation of their apparnet colors.These data were applied for discussing the visual range of colored lights, for instance signal lights, and objects through the atmosphere in the daytime.