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Sept., 1935] ILLUMINATION AND SCREEN SIZE 231
mission. Moreover, there is a slight decrease of efficiency with an increase of wattage of the lamp and the size of the source.
Eight-mm. projectors of different types have delivered 0.02 to 0.15 screen lumen per watt. Here again, the maximum is attained with offset-filament construction. With centered monoplane lamps and favorable design, 0.06 to 0.08 screen lumen per watt represents average efficiency, whereas biplane filaments give 0.07 to 0.09.
Classifying projectors according to luminous flux output has an additional advantage. It is quite easy to determine the screen size for an assumed illumination, or the illumination for a given screen size by rearranging the formula:
to W = -V//TS.5 or E =
0.75W2 CLASSIFICATION OF SCREENS
Before proceeding with recommendations for screen brightness, it will be necessary to consider the reflection characteristics of the several kinds of screen materials.
Class 1 (Fig. 1) is an arbitrary designation for a matte or diffuse screen, a surface that appears equally bright through a viewing angle of at least 30 to 40 degrees from the optical axis. This surface should be as white as possible, especially for color pictures. Its reflection factor should be at least 70 per cent. A white unglazed paper similar to typewriter paper is a good example of this class.
Class 2 is a semi-matte screen, which, through an angle of plus and minus 30 degrees from the optical axis, has an average reflecting power about twice as great as that of a Class 1 screen. Slightly glossy white surfaces, some beaded screens, and rough surfaces with an aluminum coating fall into this class.
Class 3 is a semi-specular screen, which, as the number indicates, has an average reflecting power about three times as great as that of a Class 1 screen, but only through a 60-degree angle. Many aluminum coated surfaces of a fairly fine texture are included in this group.
Class 4 is about as specular as can be used for projection. It has a high reflection factor on the axis or at an angle of reflection equal to the angle of incidence, but the brightness decreases rapidly until at 30 degrees it is less bright than a Class 1 screen, assuming that both classes are subjected to the same illumination. This kind of