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STRAIGHT-LINE AND TOE RECORDS WITH THE LIGHT-VALVE*
Donald Mackensie **
IN ANY discussion of processing tcchnic for sound records on photographic film, it is essential to keep in mind the distinction between the pictorial result desired and the object of sound recording, in order that preconceptions based on pictorial technic shall not be improperly transferred to the technic of sound reproduction.
It is the object of pictorial presentation to reproduce for the eye a wide range between highlights and shadows, together with acceptable gradations of the intermediate values of the scene photographed. The object of the sound film technic is to produce a positive transparency in which the projected transmission from point to point along the length of the film shall be proportional to the exposure of the negative at the corresponding point. This statement is applicable to the variable width as well as to the variable density track, provided we understand by "transmission" the light reaching the photelectric cell from a given position of the scanning line on the sound track divided by the light from the scanning line direct. In the present discussion consideration is given exclusively to the variable density type of sound record and to the processing technics which have been proposed.
The requirements of pictorial reproduction have been thoroughly discussed by L. A. Jones,1 who shows that faithful reproduction of brightness values is attained only when the over-all gamma is unity, with the further restriction that negative exposure and positive printing shall be confined to the straight-line portions of the respective HBD curves. In practice it is common to use an over-all gamma greater than unity and allow the positive print to intrude into the toe of the positive curve, for reasons depending on lens performance and conditions of screen illumination.
The requirement that the over-all gamma shall be unity and the positive and negative exposures shall be confined to the respective straight lines, forms the basis of the classical recommendations for variable density sound records where a light source is available which can give the desired negative exposure. Another processing technic for variable density sound records is based upon a suggestion by F. F. Renwick.2 This involves toe exposure of the negative, developing the negative to a high gamma and printing it on a positive material similar to the negative material, using the positive toe and developing the print to the same high gamma as the negative. Such a procedure makes it possible to use positive emulsion with sources of restricted intensity and has been recommended even for the unrestricted light source.
* Presented in the Symposium on Laboratory Practices at the Spring. 193 1. Meeting at Hollywood. Calif. Printed in the S. M. P. E. Journal. ** Electrical Research Products. Inc.. New York. N. Y.
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