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178 CINEMATOGRAPHIC ANNUAL
Both of these processing methods are in practical use today, and it is the purpose of this paper to investigate the results obtained in the reproduction of sound by the two methods, incidentally establishing the sensitometric foundation more precisely than was possible some years ago.
It will be assumed that we have a light source capable of being modulated by electric currents so that the wave of light variation shall be a distortionless copy of the current wave modulating the light source. For the sake of definiteness in the discussion, we shall limit ourselves to the case where the light intensity is constant and the time of exposure varied by the current, although it will later be pointed out that the conclusions reached apply as well to the case of constant time of exposure and varied light intensity.
The two processing technics to be examined are popularly known as straight-line recording and toe recording, and it will be shown in this paper that excellent sound records can be obtained from each, provided certain requirements are met.
Sensitometric Scales
In any processing where uniformity of results is desired, sensitometric control of development of both positive and negative is necessary, whatever the type of sound record. The same control is necessary for uniformity of pictorial results, although in this case the trained eye has been used as a measuring instrument with fair success. For the sound record more impersonal measurements are required. Let us examine the sensitometric scales and the kinds of measurements involved.
Sensitometers are classified as time scale or intensity scale. In the time scale sensitometer the sensitometric strip is made by exposing successive areas of the film to successively increasing intervals of time. In the intensity scale sensitometer the successive areas of the strip are exposed for a constant time to increasing intensities of light. This light variation is accomplished by interposing between the light source and the film to be exposed a variable mechanical shutter or a step tablet of varying density.
The sensitometer scales which we take into account in film processing must be carefully distinguished as to the class in which they belong and the kind of light source used in the sensitometer. Furthermore, the kind of density measurements to be made on the developed sensitometer strip must be specified.
We have to distinguish the following types of sensitometric exposure:
(1) The daylight time scale of the Eastman Kodak sensitometer. This instrument uses as a light source the acetylene flame screened to daylight quality, and the sensitometer strip is exposed to a constant light of rather low intensity for intervals of time which become minutes at the maximum exposure. The exposure times increase in square-root-of-2 steps.
(2) The light-valve time scale of the Western Electric recording machine which exposes the negative to a light intensity of 100,000 meter-candles or more for times varying from 1/20,000 to 1/200,000 second. This recording machine uses a tungsten light source having a color temperature of about 2800°K.