Noise spectrum of photographic emulsions
George, Ralph U
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It is the purpose of this thesis to measure the energy spectrum, referred to in this thesis as the noise spectrum, of the granularity of photographic emulsions. This was accomplished by using an optical diffraction device as a spatial spectrum analyzer. A uniformly exposed developed emulsion is illuminated by a plane wave, and the diffraction pattern obtained gives the grain energy distribution; i. e., the noise spectrum, as a function of spatial frequency. The analyzer is composed of a point source which coherently illuminates a uniformly exposed developed emulsion in the aperture of a positive lens, with monochromatic light, and a device for photographing the diffraction pattern at the Fraunhofer plane. The targets were chosen so as to cover the films commercially available and also obtain data on coarse, medium and fine grain emulsions. These films were Royal X Pan, Super XX and Micro-File (5402). Three uniform average densities of 0.28, 1. 00, and 1. 76 of each film type were used. These targets were placed in the target plane (lens aperture) and a picture of the diffraction pattern was taken by placing a sheet of Royal Pan film in the Fraunhofer plane. A picture was taken of the diffraction pattern of each of the nine targets. These pictures were then reduced to density vs. distance traces then converted to energy distribution curves. Some inherent reflected and stray light was discovered present in the analyzer. This reflected and stray light was caused by lens surface reflections and low quality lens surfaces. The energy spectrum of the granularity noise was reduced by the amount of this inherent reflected and stray light which was transmitted through the target. A historical development of previous work done on the problem is presented in two parts, the earlier approaches, and the later application of Communication Theory to this problem. Then the new approaches to the problem developed as a result of this Communication Theory outlook on optics are discussed. The analytical considerations of the Fourier approach to the problem used in this thesis are also discussed. The results of this investigation indicated that the fine grain emulsion had a narrow noise spectrum as compared to the coarse grain emulsion. This proved to be in direct opposition to theory. The theory is presented in the appendix. It was finally concluded that the approach used in this case is invalid because the inter-reflections between the multiple grain layers in the emulsion destroy the true diffraction pattern. A means of verifying this conclusion is also presented.
Thesis (M.A.)--Boston University
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