International projectionist (Jan-Dec 1951)

THE theory of projection optics, simple at first thought, complex in fact, has been made bewildering by misconceptions entertained even in high projector-manufacturing circles. It seems incredible that any manufacturer should be guilty of crass ignorance concerning a technical subject which has been reposing on his own doorstep for quite a number of years. A sound knowledge of projection equipment is built upon an acquaintance with the mechanical, optical, and electrical fundamentals involved in the functioning of that equipment. Now that the art of projection has passed the halfcentury mark, it would seem that the optical theory of projection would be an old, old story to everyone. Recent contributions touching upon this subject, however, lead us to believe that such is not the case. As Larry Davee* points out (IP for October, 1950, p. 12) a motion picture lens, even though complicated by several glass elements inserted for the purpose of correcting chromatic and spherical aberration, works exactly the same as a simple lens. Because this is true, we can employ single element lenses in diagrams intended to illustrate the principle of projectors and cameras. Pickup From Every Point Figure 1 illustrates optical projection under the most simple conditions. The "object" (film-photograph or lantern slide) is evenly illuminated by light from an ordinary bulb. A ground-glass plate is interposed between the bulb and the transparent picture in order to insure perfect diffusion of the light. It will be readily appreciated that light from every point of the picture is picked up by the entire surface of the projection lens and thrown onto the screen to form an enlarged image. It may be seen also that the lens inverts the image, hence the picture must be placed upside-down in the projejctor in order to show right-side up on the screen. The paths of the light rays which determine the boundaries of the beam emerging from the lens are indicated * Century Projector Corp. Frosted glassy °*J>' Top of picture FIG. 1. Optical projection under the simplest conditions. Because the illumination is diffused by the frosted glass, no secondary image is formed between the lens and the screen. Inexpensive photographic enlargers have this type of optical system. This 'Mysterious' Aerial Image By ROBERT A. MITCHELL by straight lines. The beam, therefore, has its smallest diameter close to the lens. Figure 1. however, does not represent the optical conditions peculiar to the standard motion picture machine! Factual Optical Conditions In motion picture projection, the illumination is furnished by a mirror FIG. 2. The formation of a reduced image of the mirror — an "aerial image" — in front of the lens of a motion picture projector. or condenser of limited size placed at a considerable distance behind the projector aperture. Although we seldom give it a thought, the mirror, itself, is an "object" which is "imaged" by the projection lens. Because the distance between ; mirror and projection lens is greater than the focal length of the lens, the image of the mirror must be a reduced image. And it must lie somewhere between the lens and the theatre screen. The projector, therefore, forms two optical images at the same time: an enlarged image of the film on the distant screen, and a reduced image of the mirror only a few inches in front of the lens. In forming an image of the mirror, the projector acts just like a snapshot camera pointed at the moon. The camera lens brings all the rays it receives from the moon to a focus on a film or plate. And because the distance of the moon from the camera lens is much greater than the focal length of the lens, the image of the moon is vastly smaller than the actual size of the moon. All this is very obvious, indeed; but it must especially be emphasized that the camera lens (if it be a good one) does not scatter and lose any of the moon's rays, but collects all of them into an image of the moon. No moonlight will fall anywhere on the plate except within the boundaries of the little moon image. Orienting the Aerial Image So it is with the movie projector. All of the light which reaches the lens from the mirror must be collected into the little mirror image which hangs unseen in space from 2 to 4 inches in front of the lens barrel. To see this "aerial image" of the mirror one need only hold a piece of cardboard — preferably dark — in the plane where the aerial image is formed. The hole in the mirror and the positive carbon support will be clearly visible, though the image is upside-down. The cardboard is then strictly analogous to the plate of the camera photographing the moon. Fig. 2 shows what the light-beam would look like if all of the projector except the arc mirror, carbons, and projection lens could be made invisible. The optical diagram immediately below this picture indicates how the rays form the aerial image in front of the lens. Is it not as plain as day that no light reaches the theatre screen from the projector except that which has passed through the aerial image? Figure 1 cannot represent the actual paths of the light-rays because no aerial image is formed under the conditions of perfectly diffused illumination. We cannot merely substitute an arc mirror for the bulb and frosted glass, as in Fig. 3. and get a true diagram. In fact, Fig. 3 is completely false, as may be gathered from the various question-marks which indicate our skeptical state of mind. From what source of illumination, we ask, do the rays indicated by the heavy dotted lines come? Certainly not from the arclamp mirror, because the mirror sub( Continued on page 28) c/ln " Optical fiooby Trap " FIG. 3. You won't win a prize by finding the errors in this diagram, but you'll find out how even experts have been tripped up by projection optics. See text for the solution of this puzzle. INTERNATIONAL PROJECTIONIST • January 1951 15