An apparatus was constructed for measuring the flare light caused by photographic lenses for an infinitely large object of uniform luminance. The object was the inner surface of a large sphere, painted mat white and illuminated uniformly, on various positions of which could be placed the dark opening of an artificial black body. The opening was imaged by the lens under test, and the illuminance of the image was measured photoelectrically. The ratio of the illuminance to that of the image of the inner wall is called the flare factor. The flare factor of a definite lens measured for black openings situated near the optical axis of the lens varies usually with the diaphragm of the lens, having larger values as the lens was stopped down. The phenomenon was -considered to be caused mainly by the flare light due to the multiple reflections of light at air-to-glass interfaces of the lens. The distribution of the flare factor on the image plane of a simple lens system was deduced theoretically, which showed good agreement with the experimental result for the system.
