American cinematographer (Apr 1937)

134 American Cinematographer • April, 1937 DODD DESCRIBES B-M'S NEW TYPE 24-INCH SUNSPOT By L. E. Dodd, Ph. D. Professor in Charge of Geometrical Optics, Department of Physics, A S WE ALL KNOW, improvement in studio lighting has taken great strides in recent years. What is photographically important is that the improvement came in the technique of delivering more light, with better distribution to the field before the cameras. This writer can make no claim to practical experience as an operator of light projectors in the studios. He could not but notice, however, on a visit to one of the studios in 1935, that in the 24-inch Sunspots (or “inkies,” as they have come to be called, not in- appropriately) considerable ineffi- ciency of performance in two respects, quantity and distribution, still re- mained. These 24s, I am informed, have been and still are, from the standpoint of the studios generally, the best all- purpose lamps yet developed. For at least ten years, nevertheless, there has been no substantial improvement in them. Only at spot is the performance of the 24s really satisfactory both as to intensity and uniformity of distribu- tion. But the spot position is used far less often than the flood and partial flood ranges. Over that range, rather indiscriminately, they have suffered from the limitations mentioned. They have failed to give a really satisfac- tory degree of uniformity in the dis- tribution of the light. More specifically, they have not yielded enough light in the center of the field, relative to that out 10, 20, or more degrees from the center. This will be evident from the comparative material given in this article. In a talk with the writer, Mr. Bard- well of Bardwell & McAlister, Inc., stated his belief, based on long ex- perience with studio lighting, that it was quite practicable to improve the 24s. As an outgrowth of this con- ference the writer was retained as a consultant on light projection prob- lems, pointing especially toward an improvement of the 24. At the outset of such an investiga- tion one is confronted with a peculiar- ly complicated problem, characteristic of such an optical system on which wide demands are made. Numerous, and to a considerable extent conflict- ing, factors enter into the design and performance of a device for projecting variable illumination (with reference to amount of light and size of field) that is at the same time, at all work- ing distances, desirably controlled as to distribution. Firmly believing improvement could be made, research was begun. Analy- sis, calculations, and experimentation under the writer’s general supervision gradually converged toward an im- provement in design. After many months of activity in the firm’s labora- tory, participated in by its technicians and Mr. Bardwell, the latter was struck with the idea of combining the optical parts contained in the inven- tion of a new design, the “Triple-5” 24-inch Sunspot. In the latter part of 1936 demon- stration units were in the field. A patent is now pending. The amount of improvement has exceeded that ex- pected at the start of the special studies. While the distribution has been markedly improved, a point discussed in more detail later, there has been ?n increase of approximately 75 per cent in total light projected. This holds over a wide range of focus, from f ood to quarter-flood. The improvement in central illumi- nation of the field, over the older 5KW 24-inch lamp, is evident in detail in Table I. TABLE I Improvement in Performance of the B-M “Triple-5” 24-inch Projector, Over the Older Type 5KW-24 Fig. 3—New B-M “Triple-5” 24-inch Sunspot (photograph). In pairs of curves, A and B, of : Figure 1, the ironing out of the [ notorious black spot in the center of ii the field is seen to be accomplished j by the new T-5. It is important, from the practical standpoint of perform- ance, to note that the intensity of the center of the field has been raised to meet the intensity of the outside edge. There is a pronounced smoothness of the curves by the new design, es- pecially when backed by the metal- lic mirror (Curve A). The compara- tive performance at quarter-flood and ^ up to spot is essentially similar, ex- cept that in the older lamp with the metallic mirror the black spot in the j center of the field becomes shallower. ' In Figure 2 the comparative per- ' formance of the old 24-inch lamp with the new T-5 is vividly shown by a , comparison of photographs of the field in each case. These photographs simply confirm the results of detailed measurements of the illumination as ■ shown in the curves of Figure 1. i A photograph of the new T-5 as- , sembly is shown in Figure 3. One principle contributing to superior per- formance is the coupling of the specially designed lens and the 5KW globe, so that their motion with re-