Illumination provided by an apparatus with nine pairs of apertures may be used conveniently to test the linearity of photometers. The apertures increase in area from one pair to the next by a factor of two. Thus the range covered is 512 to 1. The apertures are selected individually or in pairs by holes in a rotatable disk. The photometer reading on each illumination level in a pair is determined, and the numerical sum of these is compared with the reading on the two together. The ratio can be used to correct the photometric scale of the receiver. The advantage of this apparatus is the straightforward method of selecting illumination levels, the wide range of illumination, and the easily arranged equipment.
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