A technique for viewing film, in which the film is sandwiched between two partially transmitting reflectors, can greatly increase the apparent optical contrast and the information available in low-density regions of the film. The contrast in the toe region of the photographic characteristic curve may be increased by a factor of 7 or more while the contrast in intermediate and high density regions remains unchanged. This "stretching out" of the characteristic curve is useful in salvaging underexposed transparencies. Characteristics the film and reflectors must have to produce the contrast enhancement are discussed.
Osmar A. Ullrich,
Paul G. Andrus,
"Simple Contrast-Enhancement Viewer for Film", Proc. SPIE 0096, Application of Optical Instrumentation in Medicine V, (23 December 1976); doi: 10.1117/12.965414; https://doi.org/10.1117/12.965414