PRODUCTION OF TELEVISION SIGNALS 



565 



the positions of the photoelectric cells and looked at from the position 

 of the lens. It also follows from the above considerations that, 

 within its range of resolving power, this scanning method will not 

 only reproduce a plane subject, such as a drawing, but that it will also 

 faithfully reproduce three-dimensional figures with sharp edges and 

 elevations and depressions, just as well as they could be reproduced in 

 a photograph. 



In addition, because the light passes in an approximately parallel 

 beam through a disk aperture, the slender beams of light sweeping 

 across the region in front of the transmitter just barely overlap each 

 other even at a considerable distance from the apparatus. Conse- 

 quently, it is not necessary that the subject be at the exact positions 

 at which the small apertures are sharply focussed; and within wide 

 limits no confusion results as the subject moves toward or away from 

 the apparatus. The brightness as well as the size of the received 

 image decreases as the subject moves away from the photoelectric 

 cells; and for good transmission of the human features, which reflect 

 very little blue light to which the photoelectric cells are sensitive, a 

 person should not be more than a few feet away from the cells. 



Fig, 4 — Large photoelectric cell. The cell presents forty square inches of photo- 

 sensitive surface to receive light reflected from a subject 



This method of scanning permits two very large gains to be made 

 in the amount of light available for producing photoelectric currents. 

 The transient nature of the light permits a very intense illumination 

 to be used without inconvenience to the subject. Furthermore, the 

 optical efficiency of the system is not limited by the apertures of 

 available lenses; but can be increased by using large photoelectric 

 cells and more than one cell connected in parallel. 



The photoelectric cells of the potassium hydride, gas-filled type 

 used in the transmitting stations, were specially constructed for the 

 purpose and are probably the largest photoelectric cells that have 

 ever been made, Fig. 4. Three of these cells present an aperture of 

 120 square inches to collect the reflected light. 



