Latensification of four photographic emulsions by post exposure to radiation in the spectral region from 0.36 to 0.68 microns
Roberts, Lieutenant George Hoffman
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The chief problem of this research was to study by standard sensitometric procedures the effects of intensity, time and wavelength on the amount of speed increase which might be obtained from four representative slow, regular, high, and extra high speed emulsions (Royal Pan, Aerial Recon type 1-B Class L, Plus X and Microfile) by latensification. Latensification has been defined simply as a process applied after exposure and before development to increase the speed or sensitivity (reciprocal of the exposure required to produce a given result) of an emulsion. A secondary purpose was to examine latensification of a very slow, fine grain emulsion for possible use in small high quality aerial camera systems, in which the emulsion graininess is the limiting factor when used with conventional high speed emulsions. The latensification method in this case was post-exposure to radiation from 0.36 to 0.68 microns. This spectral range was divided into four bands by the Wratten Filters #25A, 58 and 47B, (red, green and blue respectively) and the Corning 7-54, ultraviolet. White light was also used. A tungsten lamp of constant intensity, combined with a ground glass and two inch opal glass disk diffuser produced a source of uniform illumination. Normally exposed sensitometric strips were then subject to laten sification by exposure to radiation in which the spectral region, intensity, and time were varied. Each emulsion was subject to four spectral regions plus white light and four intensity levels for each region. The exposure was then extended over a range of time believed sufficient to reach a peak speed increase for that intensity. Reciprocity failure was sufficient to need considerable attention. The characteristic curves were analyzed by measuring ASA, halfgamma and inertia (H & D) speeds. The speed ratios (latensified speed/control speed) obtained for each spectral region, intensity, and time of exposure were compiled. Curves of speed ratio against time of exposure and fog level against required exposure time for each intensity and spectral region were plotted. These experiments have shown that white light may be used most efficiently in latensification with the panchromatic emulsions tested. In addition to this it has been shown that the short wavelength region is less effective in producing speed gains than is the long wavelength region. It has been explained that these effects may be due primarily to the greater scattering of the blue and ultra-violet light by the emulsion even though the Gurney-Mott theory of latent image formation would indicate otherwise if these other influences were n ot considered. The overall evaluation attests to the fact that the effect of wavelength is a secondary consideration as compared to the intensity of radiation used to latensify the emulsion. Optimum intensity, when applied with the proper length of exposure time to produce a fog lievel approximately double the normal fog, is the major factor leading to optimum conditions of speed gain. The maximum speed ratio gains achieved for each emulsion were Royal Pan 2.24, Aerial Recon 1.85, Plus X 2.08, and Microfile 7.35. Generally speaking the slower the emulsion the better the gains which may be achieved. Royal Pan, however, was an exception and produced unexpected results in speed gain . The results obtained with Microfile indicate this fine grain emulsion may possibly be employed in miniature camera systems for use in aerial photography. A less active developer than normally used D-11 must be used to reduce contrast to a comparable degree ordinarily used in aerial photography. The effect of forced development in D-11 was found not to produce speed gains as high as the less active development in D-23, Development in Microdol (1:4) proved of little interest as speed loss was excessive. This speed loss is characteristic of most true fine-grain developers.
Thesis (M.A.)--Boston University
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