TELEVISION 553 



necessitates, at an early stage, the design of some scanning system 

 by which the image to be transmitted may be broken up into sequences 

 of signals. In the simplest case, where one transmission channel is 

 to be used, the whole image will be resolved into a single series of 

 signals; if more than one transmission channel is to be utilized, the 

 resolution may, by parallel scanning schemes, or their equivalent, be 

 broken up into several series for simultaneous transmission. 



Like the eye, an artificial television system must have some light- 

 sensitive element or elements by means of which the light from the 

 object shall produce signals of the sort which can be transmitted by 

 the transmission system to be used. For a television system to 

 operate over electrical transmission lines this means some photo- 

 electric device. It is obvious that this photoelectric device must be 

 extremely rapid in its response, since the number of elements of any 

 image to be transmitted must be some large multiple of the funda- 

 mental image repetition frequency, that is 16 per second. The 

 response should, of course, be proportional to the intensity of the 

 light, and finally, the device must be sufficiently sensitive so that it 

 will give an electrical signal of manageable size with the amount of 

 light available through the scanning system. This latter requirement, 

 that of sensitiveness, is one which, it was realized, from studies made 

 with our earlier apparatus for the transmission of still pictures over 

 wires,' would be extremely difficult to meet. In the picture trans- 

 mission system a very intense beam of light from a small aperture is 

 projected through a transparent film and on to a photoelectric cell. 

 In practical television, the system must be arranged to handle light 

 reflected from a natural object, under an illumination which would 

 not be harmful or uncomfortable to a human being. Actual experi- 

 ment showed that the greatest amount of light which could be collected 

 from an image, formed by a large aperture photographic lens on the 

 small scanning aperture of the picture transmission apparatus, was 

 less by a factor of several thousand times than the light projected 

 through it for still picture transmission purposes. Assuming the 

 same kind of photoelectric cell to be used, the additional amplification 

 required over that used in the picture transmission system, taking 

 into account also the higher speed of response demanded, would 

 bring us at once into the region where amplifier tube noise and other 

 sources of interference would seriously affect the result. This indi- 

 cated clearly that some more efficient method of gathering light from 

 the object than the commonly assumed one of image formation by a 



*" Transmission of Pictures over Telephone Lines," Ives, Horton, Parker and 

 Clark. Bell System Technical Journal, Vol. IV, No. 2, April, 1925. 



