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BELL SYSTEM TECHNICAL JOURNAL 



up of 2500 small elemental areas; that is, to scan the view in a series 

 of fifty parallel lines. The second factor is determined by the 

 sensitivity of the potassium hydride photoelectric cell. This cell is, 

 at the present time, the most efficient light-sensitive cell that can 

 follow the rapid variations in light intensity without a time lag. 

 The third factor, the limitation of amplifier systems, results from the 

 extraneous currents that are present in metallic conductors and 



Fig. 2 — Light from a single source is projected as a small moving spot on the subject; 

 the reflected light is received by several photoelectric cells 



amplifier tubes. The thermal agitation of the electrons in any input 

 resistance generates such currents; and rapid variations in the number 

 of electrons emitted from the hot filament of an amplifier tube also 

 generate disturbing voltages. For successful amplification, the initial 

 photoelectric current must be considerably larger than these extraneous 

 currents. Consequently, the optical arrangement must be such that 

 at any one instant it collects enough light from an elemental area 

 of the view to generate this minimum permissible output current from 

 the photoelectric cell. 



The operation and advantages of the scanning method actually 

 used in the present process for transmitting television images may be 

 better understood by first considering a simple and analogous method 

 illustrated by Fig. 1. The subject is illuminated by lights placed in 

 front of it as shown. A lens forms an image of the subject on the 

 rotating disk. This disk is pierced with a series of small holes or 

 apertures arranged in the form of a spiral; and, as the disk rotates, 



