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LIGHT INTENSITIES FOR MOTION PICTURE PROJECTION
J. T. GARDWELL FOR R. P. BURROWS
I have been invited to address you on the subject, "Light Intensities for Motion Picture Projection" — a subject which is ot veryreal importance but concerning which there is a scarcity of data. The best I can do, therefore, is to discuss the factors affecting light intensities and draw from this discussion tentative conclusions for your consideration.
First of all, it is of interest to note that the eye sees a picture on the screen not by the light which strikes the screen, but by the light which the screen reflects to the eye. If we imagine a perfectly black screen, one which reflects no light at all — we see at once that no matter how strong the beam we project on the screen we will obtain no picture at all. Screens used in practice vary in reflection factor; that is, in their power to reflect light, through very wide limits ; hence it is obviously necessary to consider the reflection factor of different screens when discussing screen intensities. Nor is it correct to assume that the screen which reflects the highest percentage of light striking it is necessarily the best screen to use, for the manner in which the reflected light is distributed is also a factor which affects the brightness of the picture. For example, it is possible to conceive of a screen which reflects a very large proportion of the light striking it, yet which, due to the fact that it distributes the reflected light far out to the sides, is actually less satisfactory than one which reflects a lower percentage of the light striking it but which confines its reflected light within useful angles.
From the standpoint of their reflection characteristics, screens in common use may be divided into two classes : Diffuse reflecting screens; Spread reflecting screens.
Of the first class, white cloth screens and plaster screens are typical. A white cloth screen when clean can be made to reflect as high as 70 to 75% of the light which strikes it; and a plaster screen 80 to 88%. The light is reflected at wide angles, as shown in the slide. Such screens are well adapted to theatres in which the position of the seats with respect to the screen is such that the picture must be viewed at relatively large angles, for no matter from what angle the screen is viewed, the brightness is the same because of the way in which the light is reflected.
Aluminumized screens and ground-mirror screens are examples of the spread reflecting class. A clean aluminumized screen can be designed to reflect about 60 to 65 per cent of the light striking