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May, 1930] WESTERN ELECTRIC RECORDING SYSTEM 525
increase with an increase of development time up to a certain point sometimes called gamma infinity, where further development causes no increase in density. Practically speaking, where the emulsion and the developer are constant, gamma is almost another name for development time.
It is probably not desirable at this time to go into the theory involved but it is sufficient to say that in order to produce satisfactory sound prints the gamma of the negative development should be the reciprocal of the positive development gamma. In other words, the product of negative and positive gamma, sometimes called overall gamma, should be unity. However, gamma determination is not critical; the overall gamma may vary from 0.8 to 1.2 without producing noticeable distortion. Incidentially the effect of improper development results in sound distortion similar to amplifier overloading, that is, it will cause the sound to be muffled or raspy or full of harmonics which were not present in the original.
The measurement of gamma is really a very simple operation. The film is placed in a sensitometer which is nothing more than a suitably diffused light source with a strip of known densities placed between this source and the film to be measured. After exposure and development the densities on the film are measured and properly plotted against the exposure values derived from the known densities in the sensitometer strip. The slope of this curve gives us gamma.
Densities are measured by means of a densitometer and any type giving diffused measurements may be used. A densitometer is simply an instrument which measures the density of a film by measuring the amount of light transmitted through it compared with the light transmitted through air or clear film. If a bath shows too high a gamma the time of development is decreased until the correct gamma is obtained.
As was said before we must treat sound development in a somewhat different manner from picture development but in doing so we do not want to change what is considered the best practice from a picture standpoint. We record sound and picture on different films, but print both sound and picture on the same film. We must, therefore, make our positive sound development conform to present standard picture development practice. All that is necessary therefore is to measure the gamma of a positive developed pictorially and then develop the sound negative to a gamma which is reciprocal. This usually means that the development time for a negative sound