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 so-called 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.
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