An investigation to determine the effect of an infrared post-exposure on the latent image in blue sensitive 3M Dry Silver film 7842 was performed. The film was first given a step tablet exposure using the unfiltered output of a tungsten source at 2856 K. A Wratten #87C filter was used with the same source to provide the second, infrared exposure over one half of the first exposure area. Absolute exposure and exposure reciprocity were tested. Processing was by immersion into a bath of 3M fluorocarbon FC-43 maintained at 127C/260F. The original objective, to cause Herschel effect bleaching of latent image, was not achieved. All IR exposures caused either a density increase, or, at low irradiance, no density change. A quanta ratio of 3*106 between the infrared and actinic exposures was obtained using an IR exposure time of 96 hours. The lack of any density decrease due to the Herschel effect apparently gives evidence to the theory that the Herschel effect is a rehalogenation process as reported by Farnell and Birch. The presence of halogen acceptors in the emulsion is believed to prevent the loss of latent image silver atoms due to rehalogenation. Further, the emulsion carrier and developing agents are IR absorbers and reduce the number of photons available for latent image destruction.
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Cardinali, Thomas J., "An Investigation of the effect of an infrared post-exposure latent image in a 3M dry silver film" (1982). Thesis. Rochester Institute of Technology. Accessed from
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