Motion Picture Herald (Jan-Feb 1936)

MODERN PROJECTION PROJECTION • SOUND REPRODUCTION . ACOUSTICS The Screen and Lenses Used in Rear Projection • Examining the two elements peculiar to the system— an article supplementing discussion of the Trans-Lux method in the November 16th issue the trans-lux motion picture screen, when first proposed, seemed a fanciful idea. To project a picture from the rear necessitated a screen without seams, entirely in one piece, and with sufficient diffusing power to prevent the light of the arc from coming directly through as a "ball of fire." The first step necessary to the achievement of such a screen was to discover and perfect a translucent material of which the screen could be made. It had to be strong enough to stand the strain of stretching to a smooth even plane, yet translucent enough to permit a proper image. It had to be immune to changes in temperature and humidity so that it would not sag and throw the picture out of focus. Consequently, a reinforcing fabric was Diagram of the projection lens. the next essential to be sought. It had to be both transparent and seamless and good manufacturing foresight decreed, as well, that it should be of sufficient area to accommodate not only the screen of average size but a large one (the maximum size at the time was 18 feet by 22 feet). Experiments showed that silk was the logical fabric for such reinforcing, but an investigation covering practically every silk manufacturer in America and Europe, disclosed that silk of that width was yet to be loomed, and the suggestion that silk could be made in such an unprecedented width was regarded as incredible. Eventually, however, and quite by accident, a loom maker was found whose pet hobby was the invention of a wide loom, and although the widest silk then made measured only 30 inches, he accepted the unusual commission. The loom was developed slowly, step by step, and after silks of varying widths were woven, a piece of silk 20 feet wide was eventually made. When the screen was finally provided for, there was yet another problem to be solved before rear projection was practical. It was necessary to develop a wide-angle lens requiring but one foot of throw for every foot of picture width projected, since with the projector placed behind the screen instead of in front of it, a maximum throw of 18 or 20 feet was automatically imposed by the depth of the average stage area available. There were many short focus" lenses available, but they had such a small aperture that hardly any light from the arc lamp would pass through. They were satisfactory for camera use where the brilliance of the sun produced sufficient light, but were worthless for projection. The lenses commonly used for ordinary front projection had a throw of at least 3^2 feet for every foot of picture width — in other words, they required 70 feet or The Trans-Lux projection lens. more to project a 20-foot picture. When the lens makers both here and abroad were approached and asked to produce a 1-inch focus projection lens that would pass sufficient light, they ridiculed the idea and said it could not be done— and, of course, they were correct as far as the then known projection lens systems were concerned. Finally, an old-time projectionist and designer of motion picture equipment conceived an idea which accomplished the seemingly impossible. He proposed what was the early Trans-Lux wide-angle lens. There followed many years of experimentation, however, before a really effective design was attained. The optical principle involved in the rear projection lens may be explained in this simplified way: The lens consists of three main optical units (see diagram) calculated in relation to each other and combined to produce the desired results. The odd shape of the lens is due to the first of these units, which consists of a specially designed reflecting prism which bends the light through an angle of about 45°. This accomplishes two purposes; namely, the placing of the now to nun, January 11, 1936 25