TELEVISION 557 



choice of grain above indicated, the disks were perforated with 50 

 apertures. 



For the second element of the problem, the light-sensitive means, 

 the alkali metal photoelectric cell was chosen as possessing the qualities 

 of proportionality of response and quickness of reaction. The currents 

 produced by it are at best quite small, but they lend themselves 

 to the process of amplification by the three-electrode vacuum tube 

 amplifier. 



The problem of securing a large enough signal, which is intimately 

 associated with that of securing enough light from the object, was, 

 in our development work, postponed in the earlier stages, our first 

 experimental work having been done by concentrating light through 

 photographic transparencies.'^ The solution of the problem of securing 

 adequate light was subsequently attained by reversing the light 

 path and projecting a narrow beam of light through the scanning disk 

 upon the object. By this means only the element of the object which 

 was being scanned was illuminated at any one time, thereby reducing 

 the average illumination enormously, and the problem of increasing 

 the signal strength could be attacked by increasing the amount of 

 photosensitive surface as well as by increasing the brightness of the 

 scanning light. ^ 



The problem of amplifying the photoelectric currents to sufficient 

 value for transmission was solved by a practical compromise which 

 at the same time met one of the transmission difficulties. This 

 compromise consisted in amplifying and transmitting only the fluctu- 

 ating or alternating current components of the signal, leaving the 

 direct current component, which determines the general tone value of 

 the image, for empirical reintroduction at the receiving end. By 

 this scheme, stable amplifier constructions were made available, and 

 the transmission channels, particularly the wire channels, could be 

 utilized in their normal working form. 



At the receiving end, the problem of securing a sufficiently bright 

 image was solved, as indicated earlier, by the use of self-luminous 

 surfaces of much higher intrinsic brightness than it is possible to 

 secure by illumination of a surface by any light source which can 

 be rapidly controlled as to its intensity. The self-luminous surfaces 



^ As one step in the development work moving picture film, projected by a com- 

 mercial projector in synchronism with the scanning disks, was successfully trans- 

 mitted. 



* A still further advantage is obtained by limiting the scanning light to the region 

 of the spectrum to which the photoelectric cells are sensitive (blue and violet). 

 This is unnecessary where one-way transmission only is used but is of value where 

 in two-way transmission a transmitted image is to be viewed by a person being 

 scanned. 



