TELEPHOTOGRAPHY, the electrical transmission of pictures, is an art about as old as the automobile. As early as 1881 Bidwell in London transmitted pictures; Korn in Germany in 1902; Belin in Paris in 1907; Jenkins in Washington and Ranger in New York in 1924; and in 1925 Ives and other Bell System engineers, using telephone lines, set a record by "the transmission of the first really technically perfect photographs." Since 1925 the Bell Telephone Laboratories have effected fur ther improvements particularly as to size of picture and speed of transmission. The apparatus now employed in the United States handles pictures up to I1 x 17 inches, and scanning is carried out at a maximum rate of 12 square inches per minute. The systems in present use in Europe, principally those developed by Belin in France and Siemens & Halske in Germany, are rather similar in principle to the Bell apparatus. The picture size, how ever, is somewhat smaller and the speed not so great.
To explain the principles of operation, reference will be made to the accompanying diagram, which shows in the upper left cor ner a schematic layout of the transmitter and in the lower left the receiver. The cylinders around which the original picture and the sensitive receiving film are wound are indicated; also the light source of the transmitter which originates the scanning beam. Before reaching the picture, this beam of light passes through an aperture of the light valve, the size of which is varied electromagnetically at the rate of 2,400 cycles per second. There results a sinusoidal variation in the intensity of the scanning beam before it strikes the picture. By reflection from the picture, this rapidly varying beam is further modulated in conformity with the light and shade of successive elementary picture areas.
After reflection, the light travels to the photoelectric cell, where an electric current is generated, which follows exactly the varia tions in the intensity of the light beam. This current, therefore, contains a 2,400 cycle component and, at the same time, varies at a lower rate corresponding to the details of the picture being scanned. As explained under radio telephone transmission, such a current comprises an upper and a lower side band in addition to the "carrier" frequency of 2,400 cycles. In the present instance each side band is about 1,200 cycles wide, i.e., the picture fre quencies vary between zero and about 1,200 cycles. From the photoelectric cell, this signal current travels to an amplifier and thence to an electric wave filter of the band pass type which accepts only the carrier frequency and the lower side band. The output from the filter is ready for transmission either by wire circuits or by radio.
At the receiving station, the signal current is demodulated— that is, divested of the carrier current—and the result is another picture current which exactly embodies the lights and shades of the picture. This current operates the receiving light valve which is interposed between a steady source of light and the unexposed film. Through the resultant action of the light valve, the amount of light which falls on the film, and which is rotating in exact synchronism with the picture at the sending station, is varied from instant to instant in accordance with the light and the shade of the picture. Exact synchronism of the motors which rotate the cylinders of the transmitter and receiver is insured by a 300 cycle tuning fork controlled current which is available at both stations. (F. B. J.)