Image enhancement in a microfilm projection printer utilizing partial coherence and spatial filtering

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Title: Image enhancement in a microfilm projection printer utilizing partial coherence and spatial filtering
Author: Pophal, Donald
Abstract: An optical system was assembled for a study of spatial filtering and partial coherence. It was used to produce enlargements of microfilm images at a magnification of 12-X. Enhancement of fine-detail contrast was obtained by means of a spatial filter that transmitted most of the light diffracted by the microfilm negative and absorbed about 60$ of the undiffracted light. The filter increased the modulation transfer function (MTF) of the enlarging system by a factor of about 1.5 measured at spatial frequencies lying in the region where the eye is most sensitive. At higher frequencies, the enhancement became greater than was expected, possibly because of phase effects. The optical system was partially coherent and produced coherent-noise blemishes in the enlargements. Consequently, a study was made of the effect of decreasing the degree of coherence by increasing the angular size of the light-source aperture. The 0.8 mm aperture being used was replaced successively by larger ones ( 1 , 3 5, 10, and 20 mm) and by a completely diffuse system. The 10 mm aperture (0.04 radians), which gave a coherence interval of 14.3 >um on the negative, reduced the optical noise greatly and produced as good a result in terms of the system MTF as compared to apertures of higher coherence intervals. For the frequency range that is visually important for a 12-X enlargement, it kept the MTF nearly equal to that of a coherent system and made it distinctly better than that obtained with the diffuse system. Spatial filtering with the 10 mm aperture is possible for enhancement at high frequencies that become important when enlargements much greater than 12-X are used.
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Date: 1981-11

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