Transactions of the Society of Motion Picture Engineers (1916)

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CONDENSERS, THEIR CONTOUR, SIZE, LOCATION AND SUPPORT C. FRANCIS JENKINS Washington, D. C. Surprisingly little literature has been written on the subject of condensing lenses, and none at all with regard to their use in motion picture projecting machines. Investigators outside our own art have so far failed to observe that the problem is not the same in a motion picture projecting machine that it is in a stereopticon lantern, a difference resulting from the necessity for the use of a shutter with the motion picture projector. I shall confine my remarks practically wholly to the problems of the illumination of the picture aperture of a motion picture projecting machine, and its result on the canvas, rather than to the subject, generally. Condensing lenses are employed because it is practically impossible to illuminate the film directly. When we get a cold light it may perhaps be feasible, though this is debatable. But for the present, condenser lenses, for gathering the diverging rays of the luminant and converging them on the picture aperture, continue to be used. Two lenses are usually employed in combination for the reason that to get the same gathering power with the same convergence in a single lens there would be too great a loss of light by reflection from the curved surfaces. So lantern makers usually take two plano-convex lenses and mount them with the curved surfaces together, which puts a flat side next the light source. As a simple but rather interesting experiment showing the loss of light by reflection, hold up your next glass of water and try to look upward through the surface of the water at about 45° angle. You will not be able to see anything above the water until it actually touches the surface. This total reflection phenomena is usefully employed in many ways, e. g., in engraving plants to reverse the picture for etching; in binoculars to give a large field and long-range-telescope in short, compact form ; in periscopes to see without being seen ; in the Graphoscope for mechanical simplicity and convenient operation. But this same reflection when from the surface of a single condenser, is a very decided loss. For this reason it is usual to employ two lenses in a condenser system. The first lens, the lens next the source of light, an electric arc usually, is popularly described as gathering the diverging rays and paralleling them, the second lens then converging them on the aperture plate. Lenses of 6^ and 7^inch focus are usually employed for short projection distances, with the arc lamp 2^ to 3 inches from the surface of the arc lens.