Illumination items: Practical color photometry

By the usual methods of photometry, two lights are compared by balancing the brightness of adjacent surfaces which are illuminated by the two sources. In this way, lights from sources of slightly different color can be compared with a probable error of photometric measurement of about 1 or 2 per cent. When the color difference is considerable, however, as when red, blue or green light is compared with white light in determining the transmission of colored glass or accessories for colored lighting and the luminous output of colored lamps, the measurements become extremely difficult and the observational errors may be quite large.