Stray Light Caused by the Presence of a Luminous Body within the Field of an Optical System*

Abstract

The stray light caused by a luminous body within the field of an optical system has been measured for a variety of conditions. Three pairs of telescopic systems and two pairs of photographic systems were studied. Each pair consisted of one coated and one non-coated specimen. The stray-light measurements were made for various angular distances between a black target in a uniformly illuminated surround and a luminous body which could be varied in brightness with respect to the background. These measurements were made for conditions in which the brightness of the luminous body ranged from ten to ten thousand times the brightness of the target. The optical systems were studied under two conditions of alignment with respect to the target. The first was such that no strong ghost image of the luminous body was likely to fall upon the image formed of the target. The second case was such that the strongest ghost image of the luminous body fell upon the image of the target when the angular separation between the target and the luminous body was 2°. The contrast-rendition values were found to range from approximately 90 to 1 percent for the various cases studied. In general there was somewhat less stray light found in the specimens coated with reflection-reducing films than in the non-coated systems. The general trend of the data, however, was found to be the same for all the specimens studied in that contrast rendition started to drop rapidly when angles between the target and the artificial sun became less than a few degrees.