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Noise and distortion limit the usefulness of the photographic transparency as a data-storage medium. The noise, which tends to be multiplicative, derives from the random distribution of silver grains in the photographic image, as well as from thickness variations in the developed emulsion. Distortion, on the other hand, results from 1) the nonlinear relation between transmittance and exposure, 2) the finite width of the emulsion's point-spread function and 3) the existence of an adjacency-enhancement function. Although grain noise remains intrinsic and untreatable, nonlinear distortion—both global and local—may be treated by lowering the contrast of the exposure pattern or, preferably, by recording the data in the form of a phase-modulated carrier wave, as in holography. A solution to the remaining difficulty, namely, linear-global distortion, is obtained through the use of high-resolution, Lippmann-type emulsions.
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