The flare or the stray light in optical systems reduces the image contrast in photography and causes serious errors in photometry. The amount of the flare for small luminous objects was measured by an improved photographic photometry. Generally, the flare originates in two ways, that is, reflections at air-glass interfaces and scatterings by dusts or scratches on glass surfaces. The flare by reflection increases with the area of the illuminated object and attains a saturated value for a large field angle: on the other hand, the flare by scattering slightly depends upon the area. When plotted versus the area or the square of the subtended angle of the target, the amounts of the flare lie on a straight line with several kink points, and the flare by scattering can be determined by extrapolating this line to zero field angle. For a small luminous target, the effect of the flare by reflection is less at a higher relative aperture than at a lower, and the anti-reflection coating greatly reduces the reflected light but slightly enhances the scattered light.
GORO KUWABARA, "On the Flare of Lenses," J. Opt. Soc. Am. 43, 53-53 (1953)