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LIGHTING OF SOUND FILMS*
LOUIS DUNOYER
Summary. — The author examines different types of illumination apparatus. He discusses their conditions of operation and describes t}ie apparatus devised by himself in greater detail. This apparatus obtains an extremely fine exploring zone simply by projecting the image of a rectilinear incandescent filament on the film by means of a good objective. In order to correct the aberrations rigorously, the light coming from this filament goes through the walls of the lamp in a place where they form parallel faces worked optically.
In conclusion, the author describes some comparative tests made on apparatus with a slit and on the rectilinear filament apparatus. The flux emitted by the latter is superior to the flux emitted by the apparatus with the slit, with a consumption approximately one-tenth as great. At the same time the disadvantages of the slits (dust, defective uniformity of illumination, delicacy of centering, etc.} are eliminated.
INTRODUCTION
1 . Review of the Principle of Sound Films. — It is well known that on a sound film the section set aside for sound production is a small straight band only 3 millimeters wide, in general located between one of the series of perforations for moving the film and the edges of the picture images which are to be projected on the screen. On this small band the sounds first have been recorded by either of the two following processes which we shall review briefly to make them clearer.
In the constant density** process the sound band is divided in two regions, each of which has a uniform photographic density, one clear and the other dark, and whose common boundary is a line which is more or less indented or wavy. The bends of this line correspond to the recorded sound vibrations. In most cases this recording is performed by means of an oscillograph which receives the current from the receiving microphone after amplification by a triod. In vibrating, the spot of the oscillograph, which consists of a small luminous line perpendicular to the length of the band,
* Translated from Revue d'Optique, 10 (Jan.-Feb., 1931), Nos. 1-2, pp. 1-21, 57-68.
** I.e., variable width.
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