Projection engineering (Jan-Dec 1931)

DECEMBER, 1931 Page 23 so that it becomes incident upon the sensitive surface a second time, this time from the rear. In the case of glass plates where the thickness of the emulsion support is appreciable, halation is very serious, while in the case of the thin film base the effect is so reduced that in ordinary photography it is negligible. Indeed, one of the advantages that has always been claimed for film, as compared with plates, is the great freedom from halation which the former provides. The importance of halation, however, is a relative matter. A spreading of light which is utterly negligible in an 8x10 picture may be serious in a smaller picture, and very serious indeed in a motion picture frame. For this reason negative film has for years been made with a double coating of emulsion, so that in addition to other benefits conferred by this procedure, halation is reduced to a minimum. When working with glass plates photographers have been accustomed to reduce the halation by coating the backs of plates with a "backing" which prevented the reflection of light from the rear surface of the glass and which could be removed during or after development. "Backing" is a messy operation and the dissolving of the backing in the developer is not at all desirable, but its effect in diminishing halation has led to a wide use of this method. In the motion picture industry it has not seemed desirable to place a soluble backing on negative film, but fortunately the same result can be obtained in another way. For many years the Research Laboratories of the Eastman Kodak Company have been experimenting with backings, as a result of which they have evolved a special light gray backing which can be applied to negative film and which, in conjunction with the double coating, obviates halation uo.e OU Dev. 9MIN. INBffRMC 00 as 0.6 0.9 l,t IS 1.8 Z.\ ZH J.7 3.0 Relative Loo Exposure Fig. 5. Curves of white and gray base films. completely. This gray base does not change during processing and so involves no danger of chemical trouble. It is of importance to note that antihalation backings have no direct chemical effect upon the emulsion characteristics. However, a decided improvement of photographic quality is effected by avoiding the scattering of light by the base side of the film. The absence of scattering provided an apparent change in the characteristics of the emulsion, although this effect is reached indirectly and not as an actual alteration of the emulsion. Results obtained by making motion pictures with the new film show a marked improvement in sharpness, detail in the highlights, and an over-all improvement of photographic quality. The effects of halation are far-reaching, and a film which avoids these effects should be of the utmost value to the cameraman. Discussion , Mr. Mole : May not the present film be used for making night shots in the daytime, where light green fields are often encountered as a background? The regular panchromatic film required a great deal of care when making night shots during the daytime due to the fact that there was a tendency for the green to show white. I wonder if in this more sensitive type of negative such a con dition might not be corrected? Mr. Huse : I believe that a great deal of "over-correction" is in reality under-exposure. The old type of film was not sufficiently fast, particularly in the red, where you needed the speed most when making night shots in the daytime, using deep red filters. But the new film has sufficient speed to permit most satisfactory work of this kind. It is possible to get a much better timed exposure and to do away with any sharp contrast which m'ight appear to be over-correction and which in reality is partial overcorrection combined with under-exposure. Mr. Palmer: Will you explain the physical difference between non-halation film and ordinary film ? Mr. Huse: Non-halation film contains a neutral gray dye which prevents halation. This will cause an increase of about 1 to \y2 printer points in printing; otherwise, there is no change required in the composition nor are any other difficulties found in printing through it. This film was presented to a group of camera and laboratory men last week and all the laboratory men present thought that this feature would not introduce any difficulties whatsoever. Mr. Kains: Is any special handling of this super-speed film required for use in the tropics? Mr. Huse: No more precaution is necessary in using the supersensitive film than in using any other type of film. Any type of film will deteriorate under high temperature and high humidity and the physical characteristics of this film are identical to those of any panchromatic film. As a matter of fact, our films are being shot in the tropics, and I heard only the other day that we had some films sent back to us from Europe which were shot in the tropics not more than a month ago. SOME ASPECTS OF A MODERN SOUND RECORDING SYSTEM (Concluded from page 15) pendable. There are no ribbons to replace and tune ; it will not break down on percussion sounds as ribbon valves sometimes do and it is not affected by temperature. The frequency characteristics of recording lamps are inherently superior to the mechanical type owing to less attenuation of the higher frequencies and complete absence of second and third harmonics. Operation of the lamp is extremely simple. Exposure is regulated by the current flowing through the lamp. Light from the lamp passes through a quartz slit .8 mil wide which is in contact with the film. The slit is mounted in the end of the lamp holder which is equipped with micrometer adjustments. Focussing the light from the record QUESTIONS NOS. 21 TO 25 21. What is a condenser?, (a) optical? (b) electrical? (c) chemical? 22. Do the Western Electric and RCA Photophone both employ amplifiers on the projector? If not, how is the energy from the photoelectric cell amplified ? 23. What is the difference between a starting box and a speed regulator ? 24. What current is required for a 2,000 c.p. arc lamp ? 25. Why is the Edison three-wire system economical of copper? ing lamp upon the film by an optical system was discarded because of the distortion produced by the lens with attendant loss of light. To overcome the loss introduced by the lens the recording lamp must be worked at a higher brilliancy resulting in short life. Needless to say, frequent replacement of recording lamps runs up the operating costs. Fortunately, in the interests of quality and economy the direct slit is superior. With a little care the quartz slit will last indefinitely. The lamp will handle a level of plus 12 db. without overloading. An impedance matching network is interposed between output of amplifier and recording lamp. This up-to-date, dependable equipment is manufactured by the Canaday Recording Equipment Co., 714 Leader Bldg., Cleveland, Ohio.