GB311520A - Means for controlling the intensity of a beam of light for use, for example, in recording sound - Google Patents

Abstract

311,520. Whitaker, A., and Gramophone Co., Ltd. July 4, 1928. Photographic recorders.-Relates to means for controlling the intensity of a beam of light in accordance with mechanical oscillations, for recording sound and for like purposes. According to the invention, a beam of polarized light is passed through a body which, when unstressed, is isotropic, and mechanical stresses corresponding to the sound or other oscillations are applied to the said body, such stresses not being uniformly distributed, and the light beam being passed through the body at or approximately at the position of maximum stress. Variations in the transmitted light produced by this means may be recorded photographically. In the arrangement shown in Fig. 1, a parallel beam of light from a source 1, which may be monochromatic, is directed through two polarizing prisms 3, 4, between which is placed a body 5 which is isotropic when unstressed, and may for example be made of glass, celluloid, or the material known by the registered Trade Mark " Xylonite." One end of this body is fixed at 6, and the other carries a coil 7 arranged in an annular gap between magnetic pole-pieces 9, 10. The prisms 3, 4 may be so arranged that when the body 5 is unstressed, light is wholly or partially prevented from passing through to an optical system 11 which coacts with a slit 12 past which a film 13 travels. When acousticallyvarying currents pass through the coil 7, stresses are produced in the body 5, which is so shaped that the distribution of stress is not uniform, and the position of maximum stress lies in the path of the beam of light. The varying stresses have the effect of enabling light to pass through the prisms with corresponding variations in intensity, and the variations are recorded on the film. Fig. 3 shows a modification wherein two isotropic bodies 14, 15 are used. These parts, which may be of glass, are pressed into point or linear contact with each other by springs 16, the general arrangement and action being similar to that previously described. Instead of applying stresses electromechanically to the isotropic body, mechanical oscillations may be applied directly; or the oscillations applied to the body may be highfrequency oscillations modulated by acoustic or other relatively low-frequency mechanical oscillations. Further, two quartz crystals may be used in combination, the crystals being such that they act in opposite senses as regards rotation of the plane of polarization of the light. The combination thus acts as an isotropic body. In another arrangement, a piezo-electric crystal, or a battery of such crystals, is used, in conjunction with apparatus of the kind described above. Changes of shape in the crystal or crystals, caused by the application to their faces of potential variations, are transmitted to the isotropic body whereby the beam of polarized light is influenced.