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PICTUEE PROJECTION AND THE AUDIENCE 483
although design competition in the near future will drive prices downward and performance upward toward the desired goal, the prospects of obtaining a good loudspeaker for the present manufacturing cost of $3 or $4 still seems remote. It would seem reasonable to believe that the cost of a loudspeaker for 16-mm projection should be higher than that for a home radio set due to the greater importance of its more stringent requirements. The importance of these requirements is not often consciously realized, and the availability of means to correct them seems still less often appreciated. A wide variety of loudspeakers is already available at a variety of prices — from about $30 and upward (less enclosure) ; any one of the better-grade units will show a significant improvement in performance over the speaker supplied as regular equipment if the amplifier distortion is not excessive.
Picture Projection and the Audience
Reflective projection is most widely used in 16-mm projection, and some essential conditions for satisfactory operation should be mentioned. The widely used sound projector of the 750-w. lamp type (equipped with a 2-in. lens) will be presumed as a reference. The projector is customarily placed near the rear-center of the projection space, the screen is in front. The audience views the picture that is reflected by the screen.
A matte screen (also called diffusive or flat-white) should be used in every instance possible, since it provides the widest angle of satisfactory viewing. A screen is considered dirty if it appears dark in comparison with a clean sheet of writing paper, and should be replaced for it is wasting from one-quarter to as much as one-half of the light thrown upon it by the projector.
An efficient projector can provide about 250 lumens with a 750-w. lamp operated at rated voltage at its terminals; with good condenser system design in the projector, a 1000-w. lamp of the 10 hour (short life) type can provide about 350 lumens with rated voltage at its terminals. The light output of many commercial machines will fall short of these values, and data on lumens output of a particular make and model of a machine should be obtained from its manufacturer. If 225 lumens is taken as representative value, 10 ft.-lamberts* is obtained if the screen is 5 ft. wide. 10 ft.-lamberts is the value of screen brightness ordinarily sought as a reference standard in 35-mm motion picture theater projection; ASA standard Z22.39-1944 calls for not less than 9
* Assuming a very efficient optical system.