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XIII. PKOJECTIOX AXD PKOJECTORS
ized light. For most purposes, beaded screens are likewise to be avoided. The only screen recommended for general use is the diffusive type, which is fully satisfactory only if the ambient light level in the viewing space is 0.1 foot-candle or less. Where extraneous light does enter a viewing space, a beaded screen is advantageous if most of the audience is located along the axis between projector and screen. In this case the nearest viewers should be located no closer to the screen than 2.5 w, and viewers farthest to the side should not be located more than 12° from the central axis.
Distance of Projector from Screen
The distance of the projector from the screen may be calculated readily if the focal length of the lens is known. To account for all possible sizes of screens, Table XXVII is given in terms of screen widths.
TABLE XXVII Projector Distance vs. Focal Length
Focal length of projector, in.
Projector distance, w
Focal length of projector, in.
Projector distance, w
1
2.65
3
7.95
1.5
3.98
3.5
9.28
2
5.30
4
10.60
2.5
6.66
Uniformity of Screen Illumination
Illumination on the screen should be free from objectionable bands or patches differing in color or brightness from the adjacent parts of the screen. The average illumination at the four corner points should be not less than 65% of the illumination at the center. At no corner should the illumination be less than 50% of the illumination at the center. Better grade projectors in good adjustment can ordinarily meet this requirement.
The Projector
The 16-mm sound motion picture projector is in reality two separate mechanisms combined within a single assembly, one mechanism for the projection of picture and one for the projection of sound. It is the projection of picture that is of interest at the moment.
Figure 108 shows a typical 16-mm sound projector. "A" is the feed