Motion Picture Herald (Nov-Dec 1934)

November 17, 1934 Motion Picture Herald 21 MOBEBN PBOJtCTION PBOJECTION • SOUND REPRODUCTION • ACOUSTICS SUPERVISION OF THE OPTICAL SYSTEM TO AVOID LIGHT LOSS A phase of projection routine often neglected, with the result that maximum illumination values are not attained — thus resulting in a waste of money TWO ELEMENTS of proper projection routine which, though pretty well understood by well trained projectionists, are yet frequently lost in the shuffle — neglected, that is to say, enough to cause avoidable projection costs. These are: 1. Proper determination and maintenance of the distance of the light source from the collector. 2. Control of the diameter of the spot. In many cases it is probably just because these matters are routine that they do not receive the supervision their importance merits, and as a result light losses are incurred which are entirely avoidable. To use a graphic representation with which many projectionists are familiar (Figure 1, taken from F. H. Richardson's Bluebook of Projection), the reader is asked to observe that if a collector having a free opening of 4.25 inches, is located at F, the angle of light AB will be picked up. On the other hand, if the collector were to be moved back to position C, a lens with a diameter of 7.25 inches would be required to pick up exactly the same light angle — that is to say, the same amount of light. It is evident from even this cursory examination that the projectionist who wishes to handle his equipment efficiently, getting everything from its operation that it is designed to give, he must study his projector optical system as a whole and by each individual element and be able to select the condenser, if it is used, that will be able to work at a permissible minimum distance from the light source so that the maximum amount of light can be available for use. With respect to mirrors, the distance is of course automatically fixed and should never be altered in any degree. A mirror of the wrong focal length means loss of light plus improper distribution of light. The only way of being sure that the mirro'r is of exactly the proper focal length is to follow the instructions of the manufacturer of the equipment concerning adjustment. If results are then not satisfactory, the projectionist may take accurate measurement of the distance from the crater floor to the mirror vertex, and the distance of the mirror vertex to the cooling plate and diameter of the spot, then sending these measurements to the lamp manufacturer. Having adjusted conditions so that the maximum amount of light is collected, attention should be turned to the spot, which is the next great waster of light (see Figure 2, also taken from the Bluebook of Projection, based on old aperture, but still demonstrating the effect of over-size). It is of great importance that the spot be just as small as possible. The projectionist who depends entirely upon the arc control is not likely to be getting, at least safely, as small a spot as possible. On the other hand, the projectionist who makes a practice of watching his picture consistently will be in a position to get a small spot without incurring discoloration. DIAMETER OF SPOT 15" AREA OF SPOT 1.76 SQ.IN. Percentage of total light passing through apertures Spot A 13% Percentage of total light passing through apertures Spot B 43% Percentage of total light passing through apertures Spot C 19% Percentage of total light loss using Spot A ^l°/o Percentage of total light loss using Spot B 5770 Percentage of total light loss using Spot C ^\°/o Figure 2