Cinematographic annual : 1930 (1930)

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122 CINEMATOGRAPHIC ANNUAL Development of Sound Negative and Positive It is under this general heading that sensitometry has made the greatest progress and taken its rightful place in the motion picture industry. It is quite probable that 90% of the sensitometry practiced pertains to the production of sound negatives and positives. It would be well at this point to depart for a moment from the consideration of the development of a sound negative for a short sensitometric interlude. It has previously been stated that gamma is the tangent of the angle formed by the straight line portion of the H and D curve and the exposure axis. Mathmatically the tangents of various angles expressed in degrees are numerical values greater or less than unity as the angle formed is greater or less than 45°. The tangent of an angle of 45° is 1.00. When gamma values of the order of 0.65 are referred to it is easy to picture that the angle formed by the straight line of the H and D curve and the exposure axis is less than 45°. When gamma values of 2.0 are referred to it is likewise easy to picture an angle of greater than 45°. In Figure V are shown the straight line portions only of three hypothetical sensitometric curves. The gamma values of these straight lines are 0.65, 1.00, and 2.00. It will be observed that the gamma of 1.00 straight line shows equal changes in both density and exposure. In the case of the line representing gamma of 0.65 it will be observed that to produce a given density change an appreciably greater exposure difference must be given. Conversely, to produce a given density change on the gamma of 2.00 line, much less change must be made in the exposure. It therefore follows that if a piece of film is developed to a gamma of 1.00 the changes in density will be directly proportional to the changes in exposure. In other words, equal increments are obtained on both axes. It would seem logical therefore, that if a negative could be produced in which the ratio between exposure and density was unity an ideal condition would be obtained. With this point in mind, consideration can now be given to the production of photographic sound negative records. It has been proved from a consideration of sound waves in conjunction with the exposure and development of light impulses which have been recorded on film, that the development of the negative sound track exposure would be ideal at a gamma of 1.00. Therefore, if the photographic material on which the sound record has been made is developed to a gamma of 1.00 there will then be obtained a series of densities, if a variable density method is considered, which are directly proportional to the exposures which caused these densities. Also the sound wave which has a definite sinusoidal characteristic will not be distorted if this negative is played back in a reproducer. Therefore, it would seem that if this negative were then printed and that print developed also to a gamma of 1.00, a positive would then be obtained which would upon sound reproduction, likewise, reproduce the sound impulses undistorted. Again it is necessary to consider sensitometric characteristics. It was pointed out earlier in this article that the H and D curve of an