Cinematographic annual : 1930 (1930)

Record Details:

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SENSITOMETRY 123 emulsion had three distinct sections, the under, the correct, and the over exposure regions. Sound records on film must, as far as possible, be restricted to the correct or straight line portion of the H and D curve. For that reason it is customary to study sensitometrically the emulsion on which sound negatives are to be exposed and the developer that has been chosen and determine the limits of the straight line portion of the curve. This is accomplished quite easily in sound recording if consideration is given, for example, to the light valve method of recording, which produces a variable density record. It is known that the ribbons of the light valve operate between certain fixed positions, that is, they close and then open to the limit of the valve. The so-called unmodulated position of the ribbons allow half as much light through them as can be put through them when they are separated to their maximum extent. Therefore, at the unmodulated position it is known that only a factor of two times greater exposure can be obtained. On the H and D curve of the emulsion being studied it is customary to place the density of the unmodulated exposure 0.3 log exposure units below the point where the over-exposure region breaks away from the straight line portion. This is done because 0.3 is a logarithmic value of exposure and a 0.3 difference in log E represents a change of a factor of 2 in exposure. Therefore, between this point and the break of the curve at either end of the straight line there remains sufficient range to record on that straight line the entire functionings of the light valve. The condition of developing the sound negative to a gamma of 1.00 is ideal when the print can also be developed to a gamma of 1.00. If sound records only are considered this procedure is permissible, but when it is necessary to have the release print record contain both the picture and the positive sound track, gamma of unity is not sufficient to give a picture of the desired quality. Therefore, to obtain the desired picture quality it is necessary to raise the positive gamma. To do this, however, it is necessary to develop the sound negative to a gamma lower than 1.00. In other words, in the production of sound records photographically, it is desired that the product of the negative and positive gammas equal unity ( 1.00) . It can be seen readily that a negative gamma of 1.00 and a positive gamma of 1.00 equals 1.00 when multiplied together. When the negative gamma is lowered therefore, to a value of, for example, 0.65 it is necessary to raise the positive gamma to such a value that the product of the two gammas should also equal 1.00. These results expressed numerically would be yN 0.65 x yP "X" = yO 1.00 or yP "X" = 1.54. However, as aforestated, positive gammas to produce good pictures must be higher than the 1.54 determined above. This can be accomplished by still further reducing the sound negative gamma, or better, by increasing the overall gamma to a value somewhat greater than unity. As a matter of fact overall gammas of greater than unity are obtained in actual production, as can be seen from the following example, which represents a condition followed in one of the studios. This studio works at a negative gamma of 0.65 and a positive gamma of 2.00. Therefore, expressed numerically,