American cinematographer (Aug 1936)

August, 1936 o American Cinematographer 329 Ultra Violet Recording With ^^Black Light^^ by William Stull, A.S.C. T he origin of many a revolutionary invention may be traced to the fact that the inventor noticed some insignificant detail to which nobody else gave o thought. The development of RCA's new method of re- cording sound with ultra-violet light is based on on engi- neer's ability to notice just such o routine detail. For quite a number of years we have recorded sound on film; on a film coated with a speeded-up positive type of emul- sion, to be exact. Long before talking pictures were thought of, engineers and practical photographers alike had known that such emulsions were most strongly sensitive to the blue, violet and ultra-violet components of light. And we had also known that the emulsion itself forms a sort of filter which prevents the ultra-violet rays from penetroting deeply. But nobody thought of putting these facts ta- gether in the interests af better saund. At least, nobody thought of it until Engineer Glenn L. Dimmick, looking for a means of producing a cleaner sound- track, put twa and twa together and found that they didrv't make three. In recording sound on film, one of the most important considerations is a cleanly-defined sound-track. The vi- brations which make up the basic pitch of any sound may not in themselves be of a particularly high frequency, but the overtones and harmonics, which determine the char- acter of the sound may be of very high frequency. Re- cording sound on film either with the striated track of the variable density systems or with the serrated track of the variable area methods, these high frequencies are repre- sented by microscopically fine differences in photographic density. With RCA's variable area record, the sound is recorded as a double row of fire, saw-tooth serrations; ex- tremely high frequencies record with such minutely fine serrations that the track must be magnified many times to oppear as coarse as a fine-toath comb. To achieve a really clear record, the valleys between the peaks of these fine serrations must be kept clear, for if the serrations ore not clearly defined, the sound record loses its clarity, and the overtones disappear. Perhaps the commonest cause of such trouble is ordinary photographic fogging of the film. The recorder imprints the picture of the high-frequency wave-form on the film, but when the film is developed, the line of demarcation be- tween the exposed and unexposed sections (especially at the base of the serrations) is not clear. As a result, the track does not contain the higher frequencies desired, and the reproduced sound is "fuzzy*', and of imperfect quality. This much was well knawn when Dimmick started his investigations. Assuming that the recorder itself was blameless, he analyzed the actian of the recording light- White Light Ultra Violet Sound Track of 9000 Frequency Tone beam on the film. He soon found that the film's emulsion inherently tended to spread or diffuse the recording light beam, as shown in Figure 1. This beam is focused on the surface of the film, and gives a tiny bar of light which is .00075” wide. Since the surface of the film is the focal point of this beam, it must inevitably start spreading beyond this point, so that the bottom of the emulsion is exposed over a wider area than the top. More- over, the silver grains suspended in the emulsion tend to diffuse the light still more, exposing a yet wider area. That would be bad enough, but there are further compli- catians. Passing through the emulsion, the recording- beam travels through the celluloid support, being bent slightly by refraction at each surface, and continues in the broader beam bounded by lines "a" and "b". At this point, a factor familiar to every photographer enters the problem. This is halation. Not all of the beam passes through the film-base; part of it is reflected up- ward again, as indicated by "a" and "b". When this light reaches the emulsion, it produces a secondary exposure over a broad area, giving an effect very similar to the halation which so long troubled photographers, and cloud- ing the finer grodations of the sound-track. Moreover, the dispersed rays of the beam, spread aport by the diffusing action of the silver grains, travel through the base and also reflect upward as shown by "c" and "d'*, and add a secondary fogging effect. Cleorly, the onswer to this problem, Dimmick reasoned, would be something that prevented the light of the re- cording beam from penetrating through the emulsion. If the beom could be made to penetrate just deep enough to give a clear, well-defined image af the recording-slit, and then stop, all the troubles due to diffusion and halation would be eliminated. The photographed picture of the sound-wave would be cleanly defined; the high frequencies Continued on page 335