The principal schemes of photoelectric control which may be applied in the design of a recording physical densitometer are: (1) The direct reading method in which the response of the cell to the light passed by the density is used to move the recording stylus. (2) The constant response method in which the cell indicates the attainment of a predetermined intensity level for the light passed consecutively through the density and a light-modulating system to which the recording stylus is linked. (3a) The intensity matching method in which equality of intensity between two beams, one passing through the density and the other passing through a light-modulating system, is indicated by a single cell exposed alternately to the two beams. (3b) An intensity matching system like (3a) in which equality of intensity of the two beams is indicated by equality of response of two cells one in each beam. The merits and shortcomings of each method are discussed and upon the basis of this discussion the last method (3b) is selected as most nearly fulfilling the specifications of an instrument for the sensitometric department of the Eastman Kodak Company Laboratories. Details of the design of this recording densitometer are explained at some length. The chief items of interest are the beam-splitter, the light-modulator, and the photoelectric control system. A discussion of the performance of this instrument and the variables which influence the performance concludes the paper.
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