Abstract

An objective method has been developed for measuring the stray light in optical systems. Stray light reduces the contrast of image and falsely appears to augment the light transmission of optical system. The method utilizes a photoelectric apparatus to compare the contrast of an image formed by an optical system with the contrast of the object under examination. This comparison, when expressed on a percentage basis, is referred to as “the contrast rendition” of the optical system. The contrast rendition of telescopic, photographic, and microscopic systems has been studied under a variety of conditions. It has been found that the contrast rendition values range from 3 to 100 percent for the systems studied. The contrast rendition of an optical system has been found to depend upon its optical and mechanical design, the use of reflection reducing films, surface defects, the cleanliness of its optical surfaces, the use of lens shades, and the brightness condition of the region surrounding the object under examination. The contrast rendition, for large targets and for a uniform surround, has been found to be independent of the brightness level at which the measurements are made, the contrast of the object, the field angle, and the optical aberrations present in the systems examined. The importance of contrast rendition in telescopic systems has been evaluated using the methods and data of Hardy, Duntley, and Blackwell. This evaluation was made by comparing the computed loss in range that would be caused by an imperfection in contrast rendition with the loss caused by an equal imperfection in light transmission and in magnification for telescopic systems. It was found, for all brightness levels, that the contrast rendition was generally more important than the other two variables. Because of the indicated strong dependence of the range at which targets are visible upon contrast rendition, ranges were computed for specific objects, instruments, and observation conditions, and compared with the results of an extensive visibility field test conducted from aboard ship. The computed and observed ranges were found to be in close agreement. This indicates that the contrast rendition of an optical system is a major factor in determining its performance.

© 1947 Optical Society of America

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A Method of Measuring the Contrast Rendition of Telescopic Systems*

Howard S. Coleman, George W. Arnold, and Walter D. Luedecke
J. Opt. Soc. Am. 39(10) 864-869 (1949)

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