A sensitometer is described which affords a continuous variation of illumination across the sensitive surface. Two intensity-ranges are available, using the same source of light and the same exposure-time; with both ranges, the instrument can give an over-all variation of intensity in the ratio 2000: 1. No lenses or moving parts are needed, and the sensitometer is strictly neutral with regard to wave-length.
W. M. CADY, "A New Photographic Sensitometer," J. Opt. Soc. Am. 29, 470-470 (1939)