Movietone Bulletin (September 1928)

Projector Optical System (Conunued from last issue) HIS week we will start to extL... into the chronometer balance and cylinder escapement of the projection lens, possibly with results beneficial, since if it is, as so often stated, true that the intermittent movement is the “heart” of the projector, then surely the projection lens is the stomach, gizzard, or what have you. When the picture is in focus on the screen, the rear end of the projection lens will be a certain distance from the film, the distance varying with the make and the focal length of the lens. That distance is known as the “working distance,” and it is a very important item indeed, particularly when using a plano convex condenser. With a plano convex condenser the light beam diverges more or less sharply after passing through the aperture. The amount of divergence will depend upon the distance of the condenser from the aperture. The greater this distance the greater the divergence. This does not hold true with the parabolic condenser. If the beam diverges sufficiently between the aperture and lens so that it will not all enter the lens, then you will not only have light loss, but also unevenness of screen illumination. This latter is illustrated in figure 47, page 182, Vol. 1 of your Bluebook of Projection. It therefore is up to you to find out whether or no the beam is all entering the lens, which may be done by removing the projection lens and, having drawn a circle the diameter of the rear factor of your projection lens in the center of a piece of white paper, and covered the entire circle with black ink, (not really necessary, but better), support the paper inside the mechanism at exactly the same distance from the aperture as was the face of the rear factor of the projection lens. Next turn on the light, center the circle in the beam, and if any portion of the beam falls outside the circle, that much light is being wasted and there is uneveness of screen illumination. Yes, I know it is quite a job to get a paper in exactly the right position. It also is a lot of trouble for a stationary engineer to take a card reading of his engine cylinders, but if he is worth his salt he takes them occasionally just the same. It was truly said by some ancient: “There is no excellence without great labor.” That applies to motion picture projection as well as to the other affairs of life. Light Struck Screen “Our manager,” writes a Pennsylvania movietone projectionist, “won't listen to reason on one thing and we want your help, brother Richardson, in convincing him of the damage being done. “The trouble is that the front of the auditorium is pretty brightly lighted and a lot of it reaches the screen. Also the light from a light under the upper boxes on each side strikes the screen badly. The manager won’t let anything be done. Says it doesn’t do any harm and we are silly to think it aces.” There is a lot more, some of it kind words for the Bulletin, too, but those are the essential facts and space in this sheet is at a premium. I am glad to help in the remedying of so obvious and serious a fault. Here are the facts: When light passes through the photograph of the film located over the projector aperture, it is absorbed or stopped by the photographic emulsion exactly in proportion to the density of the emulsion covering all the thousands upon thousands of pin points on the photograph. The photograph upon the screen is nothing more or less than the variations in light density thus set up. Light reaching the screen from any source other than the projection lens shines with equal brilliancy upon all portions of the screen photograph which it reaches, hence it shines on both the light portions, which it is too feeble to make appear any more white, and if it be of a yellow tone may actually make appear less brilliant. However, its effect is more pro | FLICKERS | If you think projection worries are too trying and you lead a hard life and there is no relief from the strain, just consider the poor bungalow. It only gets plastered once in a lifetime. Things are not adjusted just right. The worm will turn, but the dollar never chases anybody. Bird science is confusing. Many an old bird has taken a chicken out on a lark and ended up on a bat. A lot of fellows can’t stop rocking the boat long enough to paddle their own canoe. Etiquette books tell us what is bad form, but then so do one-piece bathing suits. Synthetic gin is a funny thing. It either makes you go blind or makes you see double. Movietone records everything but the voice of conscience. The best kind of tired business man to be is a retired business man. nounced with regard to the darker shades of the photograph. It makes them more or less grey. It lightens them and in so doing injures their contrast with the lighter portions, which dulls and “flattens” the picture. The manager is in error, and in serious error, too. That “stray light” as it is called, is needlessly injuring the beauty of the picture. It should so far as possible be eliminated, which may be accomplished by a judicious shading of the lights. You will find this matter fully dealt with on pages 224-225 Vol. 1, Bluebook of Projection. Suggest you let your manager read this and then show him the above reference. You will there learn how to test for stray light, which is one important function of the projectionist.