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248 PROJECTION SCREENS COMMITTEE [J. S. M. P. E.
we do not know that this value would have been preferred to a higher one, which our facilities did not permit.
The results obtained with this reel and the high intensity lamp are in fair agreement with those for the low intensity. A brightness of 17 foot lamberts is too great for such an indoor scene projected in a darkened auditorium. A value between 8.5 and 11.5 is indicated as perhaps the most acceptable.
Reel 2 consisted of comparatively brilliant outdoor scenes. It was shown after the reel of indoor scenes and it is supposed that the first reaction of the audience was to pronounce the illumination bright. After sufficient time had elapsed for ocular accommodation, a greater brightness was found acceptable and, in the case of the high intensity lamp, preferred. The light intensities on the screen were naturally far below those at which the original scenes would have been viewed.
One conclusion is that it is necessary to vary the light intensity for different types of prints, although it is theoretically possible to select one light intensity and maintain it by recording scenes on a sliding photographic scale, each value of brightness to have a definite constant position on this scale. The optimum value of brightness according to these tests should be a compromise between the extremes of 7 and 17 foot lamberts, the mean of which is 12. This is somewhat higher brightness than is customary.
REAR PROJECTION
Historical. — Rear projection is not new; it has been used for fifteen years in Germany, France, and England. In this country we are all familiar with the small projectors used in public places for advertising, demonstration, and stock quotations. Application to the theater was delayed by two difficulties: one, the lack of a suitable translucent material, and the other, of an efficient distortionless wide angle lens. Within the past six months several small theaters have opened in New York to show newsreels and sliort subjects on a rear projection screen.
Mechanics. — There are several possible materials for use as rear projection screens. The more common are dental rubber, treated silk, ground glass, celluloid, and a gelatin composition. The last is one which is being used on a large scale. Glass screens have a satisfactory transmission characteristic but the large sizes are heavy and difficult to protect. Celluloid screens would be satisfac