The problem of obtaining accurate photometric data at brightness levels below 0.5 foot-lambert is reviewed briefly. A method of calibrating a photoelectric photometer for heterochromatic measurements in this socalled Purkinje range is described with some details given of the design of the instrument used. By arbitrarily dividing the luminous emission of a tungsten lamp into twelve overlapping spectral bands it has been possible to calibrate a photometer, using a single correction filter for the phototube. The practical advantages of such a calibration include much time saved in brightness measurements on phosphor samples of different colors.
W. S. PLYMALE, JR. and G. T. HICKS, "Physical Photometry in the Purkinje Range," J. Opt. Soc. Am. 42, 344-345 (1952)