Moving Picture World (Jan-Feb 1927)

January 29, 1927 MOVING PICTURE WORLD 385 Bluebook School Answer No, 551 Note:— This “School” is designed to arouse interest in the study of those many problems which constantly arise in motion picture projection, AND to cause men to really study the Bluebook and assimilate the vast amount of knowledge contained within its covers. Question 551 — This is a question submitted by the editor. It is worded as stated to me by Otto Kafka, president, American Projection Society, who informed me that it was causing much discussion among the members. It is as follows: “How many times do light rays cross between the light source and the screen.’’ When the question was printed I said I would myself comment upon it at the time of answering. I shall therefore do so. The question as stated has very little meaning. In a projector light beam millions upon millions of light rays cross and recross at millions upon millions of places. For example : Take two pin points upon the light source, one at the top and one at the bottom. Were it possible to segregate these two points and cover all others it would be found that each of these points was sending forward light rays to every part of the collector lens surface, and that the rays from the one all cross the rays of the other, except those reaching the extreme top and bottom of the lens. Remembering that there are, Heaven only knows, how many thousands of these pin points on the surface of a light source, it is seen that there must be a huge number of crossing points in this section of the beam alone. And in this, as I understand it, each pin point upon the surface of the converging condenser element must also be treated as a separate light source for the purpose of the question. However, what was evidently meant was this : If we select two light rays emanating from different sections of the surface of the light source, how many times will these two rays cross between the light source anil the screen? Even in this question I believe that in order to be absolutely correct one must specify exactly what points of the light source the rays emanate from, and what point of the collector lens they are incident upon, because the rays selected might or might not cross between the light source and the collector lens, as, for example, a ray from the lower half of the light source incident upon the lower half of the lens and one from the upper half of the light source meeting the upper half of the collector lens would not cross between the light source and collector lens, whereas, rays from the same points incident upon reverse points of the lens would cross. However, again I think this was not the real purpose of the question. I have only cited these facts as evidencing how exactly a question of this sort must be worded in order to enable one to make an exactly correct answer. The following answered three times: Charles E. Curie, Chattanooga, Tenn. ; C. H. Hanover, Burlington, N. Y. ; G. L. Doe and John Doe, Chicago, 111.; T. R. Bankerton, Wentzville, Mo.; Allan Gengenbeck, New Orleans, La.; D. G. Henderson, Quincy, 111.; Charles C. Colby, Santa Fe, New Mexico, and Gilbert Adkindon, Cleveland, Ohio. I shall print the answer and most excellent drawings of John Griffiths. He says: The question, as put, has no particular meaning, though I think I understand what it is intended to mean. The light beam of a motion picture projector contains an inconceivable number of light rays and the number of crossing points of rays is utterly beyond any possibility of calculation. There is, however, a certain set of rays, Which, in a single lens system is known as “secondary axis rays.” These rays are presumed to pass through the optical center of the lens without any angular deviation whatever. However, in a double lens system, such as we have in a motion picture projector, these rays do not pass anywhere near the optical center of the second (projection) lens. There are, however, certain exceptions to this rule, one of which is where a Mazda lamp light source is used. This is so because of the fact that the projector aperture is located so close to the condenser that the group of rays from each point in the film photograph fill the aperture of the projection lens, and the lens selects the rays incident upon its axis as a new secondary axis, thus, in effect, creating two oomplete lens systems, instead of one double one. The second (projection) lens of a double system receives the secondary axis rays diverging from a point on the axis at the entrance plain of the first lens, hence it will focus them at the image of the first lens. In Figure 1, I have eliminated all but the secondary axis rays by placing an ipaque plate over the piano face of the collector lens, the same having a pin bole at the optical axis. By this plan we may readily follow the path of the secondary rays, and see that they cross twice between the light source and the screen; first at the first surface of the collector lens of the condenser, and again at the image of the first crossing plane, viz., the aerial image of the condenser. This may be viewed by establishing the plate with pin hole and blowing smoke into the resultant beam. That is how the photographs were made, figures 38, 39, -10, 16, 48 and 49 of the Blue Book. Ed.) In order to get a clear mental picture of the action of the optical system of a motion picture projector, we must visualize each of these secondary axis rays as being surrounded by a group of supplementary rays which radiate from the same point of the light source. (John, I think you have worded that poorly, unless you mean that no hole could be made small enough — in the plate — to permit the passage of but one ray. If you mean that the point on the light source has appreciable area, then you wouldbe correct, but area must, it seems to me, be presumed either in the point or in the pin hole if supplementary rays surround the one central ray. Ed.) For the want of a better name we willdesignate these rays as group rays. Beginning at the light source they diverge away from the secondary axis until the first surface of the collector lens is reached. Their volume is limited only by the effective diameter of the condenser opening. Those that, get through the second condenser element (converging lens) converge to a point at or near the aperture or film plane on the secondary axis ray. From that point they again diverge until they reach the projectionlens, which overcomes the divergence and again converges them to a point at the screen surface. At all times these group rays are equally distributed around the secondary axis ray,, which originates at the same light source pin. point. If, therefore, we know the path, of the secondary axis rays we may readily trace that of all other rays.” I wish it were possible to talk personally with Brother Griffith before this is sent tothe printer, but it can’t be done. 1 think. I see what he means now. He has drawntwo separate propositions in figures I and. 2. Figure 1 is the diagram of the entirelight source acting through a pin hole in a plate, while Figure 2 is a point light source acting through the entire condenser. That Figure 1 is absolutely correct you may learn, for yourself by the use of a plate (pin hole) and smoke. Figure 2 illustrates what twoisolated rays would do up to the projection, lens. They would cross at the film plane,, or thereabouts. However, if John is right, then my own ideas have been wrong because I have always believed that two rays; would cross — um, yes, I see. I’m right too, by heck. I was going to say they would cross at the film plane and at the aerial image — twice. I see John has it that way too, the black representing ONLY the rays, from ONE point of the source. A ray from another — oh***!!!! I know what I mean, but it’s very hard to explain. Two separate rays, one from the bottom and one from the top, one incident upon the top and the other upon the bottom of the condenser would cross TWICE — once at or near the film plane and one at the aerial image. There! I’m through. Whew! Phew and then some!!!! 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