The effect of striae on image quality.
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Striae are defined as fine streaks in optical glass that have an index of refraction different from that of the surrounding glass mass. The formation of striae occurs during the melting process and is attributed to a number of causes. Striae in optical glass result in an angular deviation of the light passing through the glass. The purpose of this study is to determine the amount of light that is deviated in the image plane for a given amount and grade of striae. This is an important problem since many demands are being made to improve the informationgathering ability of a modern photo-reconnaissance system. Five optical glass samples were selected and prepared so that defects other than striae were at a minimum. Selections were made so that there was· a variation of the amount and grade of striae in each of the samples. The amount of the characteristic striae grades of A, B, C, and D was determined with the aid of a simple grader suggested by the National Bureau of Standards. The population of the various grades of str iae is shown in shadowgraphs made fo r each of the five glass samples. The per cent of light deviated in the image plane by striae was measured in an instrument called the scatte·rometer. This per cent of light deviated for a given image angle is a measure of the effect of striae on image quality and is presented in graphical form. Results of this work show that the per cent of light deviated depends on the grade as well as on the amount of striae present in the glass sample. For finer striae, such as Grades A and B, light is. deviated at a small angle. It is concluded that the striae in any of the single samples tested will have little effect on the image quality of a photographic system. However, with an increase of -striae population by the use of several of these samples, deterioration of the image quality may result. A criterion for the amount of image information lost as a function of the per cent of light deviated at a given image angle has not been established; however, a method is presented which shows how the data obtained can be used to- determine the effect of striae on image quality for specific camera systems. It is found that the effect of striae on image quality becomes more serious with an increase in angular resolution.