International projectionist (Jan-Dec 1958)

Projection CLINIC Projector Ports and Picture Quality ALL OF US are familiar with the effect of soiled glass in the projector ports — dim, "fuzzy" images marred by such contrast-degradation effects as dulled highlights, fogged grays, and washed-out blacks. But what of improperly positioned port glasses? Even plate glass of the highest optical quality will injure the clarity of the projected pictures if the light rays pass through the glass at substantial angles. Glass bends (refracts) light when angles are involved; and even when these angles are small, reflections taking place inside the glass result in a serious displacement of some of the light rays and make a sharp focus on the screen utterly impossible. And the shorter the focal length of the projection lens, the worse is the effect of internal reflections in the port glass! Figure 1 illustrates what happens to light rays passing through plate glass obliquely. It is fortunate that the image-blurring internal reflections may be eliminated by positioning the glass perpendicularly to the optical axis of the projector. This fact may be demonstrated by holding a pane of plate glass in the projection beam and tilting it at various angles while a sharply focused picture is being projected. We have found that blurring by the port glass is just perceptible at an angle of six degrees, and is quite serious at angles greater than 10 or 12 degrees. For this reason, therefore, the projectorport glasses should not be set flush with the projection-room wall in theatres having projection angles greater than five degrees. In all such theatres, the bottom of the projector-port glasses should be brought out from the wall so that the glass tilts upward at an angle equal to the projection angle, as shown in Fig. 2 where an angle of 15 degrees is assumed for illustrative purposes. This done, the projected light rays will impinge upon the glasses squarely and pass through them undeviated by refraction or internal reflections. The trouble of constructing special tilted holders for the port glasses is well worth while as regards improved picture quality. And, of course, only colorless optical plate glass of the highest quality should be used in projector ports. Damaged Reels Endanger PROJECTIONISTS are reluctant to use "shipping reels" in the projectors, and rightly so. They are frequently bent and battered, and hardly fit even for shipping film to and from the exchanges. The 1,000-foot shipping reels are especially bad because the hub is so small (1%-inch diameter). Film will Incident Surface ~ reflected ray jf' Internal reflections FIG. 2. In order to avoid the image-blurring caused by internal reflections, the optical axis of the projector should pass through the port glass squarely. In theatres where a substantial projection angle prevails, therefore, the port glass should be tilted out from the bottom at an equal angle to minimize adverse optical effects. The projected pictures will then have maximum clarity. not "take up" on them when used in the lower magazines of many projectors. The set of film reels owned by the theatre should naturally be of the very best quality and kept in good condition to prevent damage to the prints. "Projection reels" are usually made of cast aluminum or welded iron wire, although 2,000-foot spring-steel reels similar to those used by the exchanges are preferred by some. All of these reels (or "spools," as they are called abroad) have disadvantages which must be carefully considered. Cast aluminum reels are brittle, and may crack if roughly handled. Weldedwire reels bend out of shape rather eas(Continued on page 20) iPlate glass Refracted ray Transmitted ray FIG. 1. Rays of light passing through plate glass at an angle are refracted (bent) and internally reflected. The reflections from inside surfaces produce unavoidable blurring of the picture when image-forming rays from a projector pass through the port-hole glass obliquely. INTERNATIONAL PROJECTIONIST • JANUARY 1958 B