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it and will confine the reflected light within an angle of approximately 30°. Ground-mirror screens when clean can be made to reflect approximately 80 to 90% of the light and confine the light within about 30°. Such screens are well adapted to theatres in which the seats are so arranged that the picture does not have to be viewed at large angles.
Allow me to emphasize the point that the reflection factors I have mentioned assume a clean reflecting surface; a very slight accumulation of dust can reduce the screen brightness very materially. I believe that with a schedule of regular and frequent cleaning, theatre managements can in many cases increase their screen brightness by fully 50% of their present average values if they desire to do so.
Thus, it is seen that the intensity of the beam projected determines only relatively the brightness of the picture.
Uniformity of screen illumination is another factor which must be considered in connection with screen intensities. It is not uncommon to find screens upon which the intensity near the center is several times the intensity near the edges of the picture, or where the intensity on one-half of the screen is much higher than upon the other half. When this condition of non-uniformity obtains, certain portions of the picture are, of course, brighter than others and while the eye cannot readily detect small differences in brightness the fact remains that to say a screen is illuminated to a certain average intensity is not definite when a wide difference exists between minimum, average and maximum intensity values. Good projection requires a screen intensity approaching uniformity, and the nearer the intensities at different points come to being equal the better will be the projection from this standpoint. The uniformity of intensity at different points on the screen is affected by the condenser design, steadiness of the light, and by refinement in focusing adjustment.
Another factor which we must consider is extraneous light. Obviously a higher screen intensity is required in a theatre in which daylight is allowed to enter or where lights are kept burning at all times than in one where all the light comes only from the projection apparatus. The effect of extraneous light is to decrease the contrast between the high lights and the low lights on the screen. If for example the intensity of the beam in a low light of the film is, say, 1/50 of a foot candle at the screen, and in high light the intensity is, say 2 foot-candles, the contrast is 1 to 100. If upon the screen image is now superimposed an intensity of 1/10 of a foot-candle, due to extraneous light, the contrast becomes approximately 1 to 17. From this it is apparent that even a very low intensity of extraneous light calls for considerable increase in screen intensity if good contrast is to be secured. In this connection I may say that the organization with which I am connected has under way at present an investigation of motion picture theatre lighting and will be glad to co-operate