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PHOTOGRAPHIC CHARACTERISTICS OF SOUND RECORDING FILM*
LOYD A. JONES AND OTTO SANDVIK
The various methods and general principles involved in the recording of sound by photographic methods are too well known to require detailed description and discussion. The methods at present in use commercially may be divided broadly into two classes: (l) The variable density type, and (2) the variable width type. The former may be subdivided, with respect to the method used for obtaining the variable exposure, into (a) those using the "light valve," and (b) those employing a "flashing lamp." In variable width recording the film is moved at a uniform linear velocity past a slit, or an optical image thereof. By suitable means the transverse length of this slit image is so modulated that the exposed area varies in lateral dimensions giving a sound record of the so-called "saw tooth" type. At any point within the exposed area the exposure incident on the photographic material is constant, both factors of exposure, namely, intensity (I) and time (t), being constant. This statement requires some modification since with the film moving continuously in one direction past a slit of finite width and the boundary of the illuminated area moving in a direction perpendicular to that of the film there must be a narrow envelop in which the t factor of exposure varies to some extent.
In variable density recording with the light valve the film is moved at a constant linear velocity past an illuminated slit, or an optical image thereof, the width of which is modulated. In this case it is evident that the intensity factor (/) of exposure is constant, while the time factor (t) is variable. In variable density recording with the flashing lamp the film is moved at a constant linear velocity past an illuminated slit of fixed width, the intensity of the illumination being modulated. In this case it is evident that the exposure at any point on the photographic film is variable due to the variation in the inten
* Communication No. 414 from the Kodak Research Laboratories. 180