Hearing Light and Seeing Sound

Radio Record, Volume III, Issue 23, 20 December 1929, Page 4

Hearing Light and Seeing Sound

‘A Recent American Invention OUND hecomes visible and light is made audible by an apparatus invented by an American engineer. A beam of light, propagated by a speci-ally-devised are, travels silently about the room, only to break into music when it encounters a target mirror. The equipment is known as the "photophone," and the transmissions are called "narrow-casting," to distinguish them from broadcasting. In demonstrating the photophone, the inventor used gramophone _ records with the sending apparatus, The energy is sent over the light beam to the transforming and reproducing elements, which are mounted on a tripod some distance away. When the inventor held his hand in the path of the light beam, the music stopped, but immediately he allowed the light to filter between his fingers the sound recommenced and increased in volume. A cardboard dise, with holes of various sizes from a pin-hole to one of an eighth of an inch in diameter, gave varying degrees of sound volume when used to intercept the beam. The light of a burning match was changed into sound by the reproducer. When the match was struck there ‘was a rattling, crashing sound which lasted during the combustion of the chemicals. The burning wood gave off little sound. A similar demonstration utilising the varying light from a small dynamodriven flashlight sounded like a siren. A speaker at a microphone or a gramophone record can serve as the sound source, but the inventor for demonstration purposes usually employs the latter in conjunction with a magnetic pick-up by means of which the recorded music is transformed into electrie current. . This energy is led to a mirror, onethousandth of a square inch in area, delicately suspended in a magnetic field by means of wires... At one side is placed an ordinary automobile headlight lamp, the light from which is focused on the tiny mirror. The mirror, quivering in time with the electric current, focuses the light by a lens into a narrow beam, which. pulsating at the frequency determined by the music on the record, is projected through space to the light-collecting mirror or lens of the receiving apparatus. At this point another tranaformation must take place; the light must he converted to sound. The mirror or lens condenses the light on the photo-elec-trie cell used in the apparatus. This

eell, which responds instantaneously toe every variation in light intensity, translates the light into electric energy, and « this after amplification passés to the loudspeaker. The difference between sending sound over a beam of light and by way of radio is simply one of degree. The physical transmission in both cases is the same, except that different trans--mitting and receiving devices are used. In the case of light, frequencies of several hundred trillions per second give wave-lengths of the order of a fiftythousandth of an inch. Broadcast waves are normally from 600 to 1500ft, in length. The long wave of comparatively low frequency spreads out in all directions, but the beam of the tight is essentially & straight-line affair and with a suitable forming lens or reflector can } conserved and sent over considerahbie distances without a substantial spread. Broadcast waves will bend around obstructions and pass through walls, but the light wave will not, bend, since the wave is so short in comparison with the size of the obstruction. It will pass through only such solids as are transparent, The inventor has been able to transmit music across a street in broad daylight by the medium of a sufficiently powerful light beam; and at night,.successful transmissions over a distance of three miles have been accomplished. The restriction on the distanees over which communication can take place is compensated for by the highly directional effect obtained by use of the. light beam. Thus, the advantage of y this system over radio for purposes of | secret communication in war-time is apparent. Because of its limitations, however, it would probably be used extensively only by. warships and aeroplanes, where inter-vision is less unbroken than elsewhere.