Movietone Bulletin (July 1928)

Loss of Light HIS letter comes from a movie tone projectionist in the Northeast: “Dear Mr. Richardson: When this theatre installed movietone we were just plain scared stiff. We certainly did wish we had studied more, but it then was too late. We got through though, and now that the scare has worn off, find our interest in the work of projection greatly increased. The Bulletin is great. It has helped us a lot. “Yesterday an argument arose between myself, my partner and two projectionists who came in to see the apparatus. It was concerning the amount of light lost between the lamp and the screen. We remained half an hour after the show was out, arguing. Finally it was decided to put it up to you and accept your decision in the matter. The question is how much of the light is lost between the lamp and the screen. Will you decide the matter? Oh yes. We were discussing the straight are and condenser system.” Almost the exact question came in the same mail from a local union in Lawton, Oklahoma, so you see great minds run together. I cannot tell you precisely what the necessary losses are, because to a considerable extent it depends upon local conditions. However, I can point out to you the various losses and where they occur. Any unnecessary distance between the are crater (which is the light source, not the “lamp’”—use correct nomenclature, brother) means serious loss of light. See Fig. 36H, page 162, Vol. 1, Bluebook of Projection or same Figure, same page, 4th edition. The light loss here may be very great indeed, as you will observe by. studying the drawing referred to. Another source of very heavy loss of light is unnecessary spot diameter. This is graphically illustrated in Fig. 41, page 173 of either fifth or fourth edition Bluebook of Projection. Of course you well know that the rotating shutter cuts off approximately 50 per cent. of the total light reaching its plane. This is unavoidable, though it may be controlled to some extent by the projectionist. If the condenser lenses be spaced further apart than just sufficient to avoid physical contact of the lenses, there will be some unnecessary loss of light, since the collector lens sends forward to the converging lens a diverging beam of light; see page 170 of editions of the Bluebook of Projection before referred to. I refer to these books because you there will find the matter fully explained and illustrated. If the projection lens has_ insufficient diameter to admit the entire light beam, then there will not only be loss of light, but also uneveness of screen illumination will be set up. All these various elements are under the control of the projectionist in so far as has to do with waste of light. Great waste, with resultant high inefficiency in operation may result from wrong adjustment of any one of the elements named, or a slight error in each may, in the aggregate, cause very heavy loss. In addition to these various possibilities for loss, there is a further one at each surface the light encounters, either in passing from air to glass or vice versa. This loss is (see the Professors Gage, page 141, Vol. 1, the Bluebook of Projection) from 4 to 5 per cent. at each such surface, if the glass be well polished, as all projector optical train lenses now are, and perfectly clean. If the lenses be dirty this loss may be very much greater In a projector optical train using a 2-lens condenser, there are six surfaces in all at which light must pass from air to glass or from glass to air, so if all surfaces be clean, the loss will total at least 24 per cent. of the total light, to which a small loss must be added because of the slight light absorption in passing through the glass itself. This latter will probably not exceed a total of more than two per cent. for the entire optical train. I believe we may safely assume that not to exceed five (5) per cent. of the total light emanating from the crater of the straight electric arc reaches the screen, and that in ordinary practice this is considerably diminished. It has been placed, by competent authorities, as low as three per cent. I believe, however, that when it sinks as low as that, there is high inefficiency somewhere in the optical train. I am glad this question has arisen because, whereas a loss of one per cent. of the total light emanating from the light source might be a very small matter, still when only a possible five per cent. is reaching the screen, if one per cent. of that amount be removed by inefficient management, it becomes serious indeed. Intimate knowledge of the various sources of possible light loss is highly -essential to efficient work on the part of the projectionist because when he does understand, it becomes a relatively simple matter to control and eliminate them. Without competent knowledge the losses may be very great indeed, and there should be none whatever, because in all conscience under the best possible conditions they are great enough. Watch Your S prockets Movietone projectionists will do well to watch their sprockets closely, more especially the sound sprocket. Just a wee bit of dirt adhering to the intermittent sprocket may and probably will cause the screen image to move up and down once to every revolution of the intermittent sprocket. A bit of dirt adhering to the face of the rim of the sound sprocket will set up trouble with the sound, since every time that sprocket revolves the film will be slightly displaced. Moral: Keep every sprocket perfectly clean, but especially the sound and intermittent sprockets. As I have before advised you, get a ten cent tooth brush, or some other small stiff bristled brush, and hang it from a string in convenient location. Or I think you may lay it in the base of the mechanism. Anyhow, have it real handy and use it at least on the sound sprocket every time you thread. Anybody can make a mistake. But only a fool can’t figure out a different way to make it the next time.