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RELATION BETWEEN ILLUMINATION AND SCREEN SIZE FOR NON-THEATRICAL PROJECTION*
D. F. LYMAN**
Summary. — Methods of measuring the illumination output of 8 -mm. andl6-mm. projectors are discussed, with particular reference to some of the many variables that affect the results. This output value, expressed in screen lumens, permits determining the screen size for any desired foot-candle level. But because screen brightness depends also upon the reflection characteristics of the screen material, there is presented a classification of various types of screen material. For each class of screen, maximum and minimum illumination values are suggested, with the object of keeping the screen brightness within the limits necessary for good picture quality. Charts illustrate the relation between these factors and between the screen size and projection distance.
During the development of 8 and 16-mm. projectors, the screen illumination has been increased steadily by the adoption of more and more efficient optical systems and lamps of higher wattage. This increase has introduced another projection problem, a tendency toward excesssive screen brightness, which may be nearly as objectionable as under-illumination. It is the purpose of this paper to suggest flexible limits for screen brightness, taking into consideration the light flux from the projector and the size and reflection characteristics of the screen.
LIGHT FLUX FROM THE PROJECTOR
The luminous flux from a projector is expressed in terms of screen lumens. One method of determining its value is as follows : The projector is run at about normal speed without film, and the light beam is projected to a wall or screen upon which can be measured the magnified image of the gate aperture. For this purpose the lens is focused so that the image is sharply defined, and the aperture is framed properly. Since the illumination varies slightly with the position of the lens, the image is made about average screen size, at least three or four feet wide. Then, by means of an illuminometer,
* Presented at the Spring, 1935, Meeting at Hollywood, Calif. ** Eastman Kodak Co., Rochester, N. Y.
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