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MANAGERS AND PROJECTIONISTS 219
The Screen
THE sole and only function of the screen is that of reflecting "picture light." The eye sees the picture precisely for the same reason it sees any other visible object. Exactly as light rays are reflected from any visible object to the eye, so in projection light rays are reflected by the screen surface to the eye. The picture appears plainer, sharpr and in every way better if the picture light be abundant— the screen brilliantly illuminated — than if it be dull, or if light other than picture light be also reflected.
The term "picture light," as used here, may be understood as meaning light projected to the screen from or by the projection lens ; "other light" is 4ight reaching the. screen from sources other than the projection lens, which latter, being undirected by the lens, shines indiscriminately upon both whites and shades of the picture, thus dulling the contrast and causing the blacks to appear gray.
There is a great difference in screen surfaces and the results had from different ones with a given intensity of picture light. This is likely to be especially true when the screen be viewed from various parts of an auditorium, but until quite recently there has been no dependable, authoritative data setting forth the characteristics of various surfaces used for projection. The author had in mind the making of certain measurements and tests for 'this edition of the handbook, and had in fact already done considerable preliminary work thereon when the Society of Motion Picture Engineers made available tests of screen surfaces which made that work on our part unnecessary. We were well pleased with this, partly because of the tremendous amount of work involved in the making of such tests, but even more because such data should come from just such a source as the Society. We shall present the tests and the conclusions arrived at, page 259-267. They have real value in that they enable exhibitors and projectionists to work intelligently in the matter of selecting screen surfaces for auditoriums of various depths and widths.
A FALSITY. — In the past many exhibitors and projectionists have based their judgment of the efficiency of various