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A working model of a monochromatic sensitometer covering the wave-length region from 3500 to 11,000A has been built. Monochromatic radiation used for exposure is obtained from a double monochromator employing transmission replica gratings as the dispersing elements. The intensity of this radiation is measured with a thermopile. Intensity-scale exposures upon the photographic material result from the use of a specially designed intermittent intensity-scale sector wheel. The same time of exposure is employed at all wave-lengths, and therefore the spectral sensitivity curve derived from the intensity-scale characteristic curves is independent of any reciprocity law failure which the emulsion may exhibit.
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Charles H. Evans (1940) studied this question.
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