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BELL SYSTEM TECHNICAL JOURNAL 



while still having most of the sensitiveness in the blue region of the 

 spectrum. Figure 5 shows the response of the photoelectric cells 

 used to the various parts of the spectrum together with the trans- 

 mission of the blue filter and the brightness of the various parts of the 

 spectrum as evaluated by the human eye. The very great efficiency 

 of the photoelectric cells and the inefficiency of the eye to the light 

 used are apparent. 



To amplify the photoelectric current, the cells are filled with argon 

 at a low pressure. Photoelectrons passing from the sensitive film to 

 the anode ionize the gas atoms along their paths and thus cause a 

 greater flow of current. The ionization of the gas does not, however, 

 instantaneously follow sudden variations of the true photoelectric 

 emission from the sensitive film, that is, there is a time lag in the 



If) 



_i 



ffl 



u 



UJ 

 Q 



+ 4 



+ 2 







10 

 (0 

 O 



lU 



> 



i< 



100 



10,000 

 FREQUENCY IN CYCLES PER SECOND 



1,000 



100,000 



Fig. 6 — Loss in response of photoelectric cells at high frequencies. 



ionization of the gas and in the disappearance of ionization. This 

 lag results in a relative loss and phase shift of the high frequency 

 components of a television signal with respect to the low frequency 

 components which become serious in the wider frequency range 

 utilized in the 72 line image. The relative loss in output from a 

 single large photoelectric cell at high frequencies is shown in decibles 

 by curve A of Fig. 6. 



In the television booth, the twelve large cells mounted in the walls 

 of the booth present an area of approximately seven square feet to 

 collect light reflected from a subject. To secure the desired effective 

 illumination, the cells are mounted in three groups, comprising a 

 group of five cells in each of the two side walls of the booth and a 

 group of two cells in the sloping front wall above the subject. The 

 twelve cells are enclosed in a large sheet copper box, provided with 



