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BELL SYSTEM TECHNICAL JOURNAL 



the caesium cells were rendered so by placing over them, windows 

 covered with a deep purple gelatin. This choice of illumination color 

 made possible a satisfactory general level of illumination of the booth 

 and the surroundings of the image without introducing spurious signals. 

 The transmissions of the purple filters, the response curves of the 

 potassium and caesium oxide cells, the radiation curve of the incandes- 

 cent lamps used for the scanning beam, and the transmission curves of 

 the glasses used over the lamps for general illumination, are shown 



Fig. 2- 



-Interior of two-way television booth showing location of two caisium cells 

 above and to either side of scanning and viewing aperture. 



in Fig. 3. Comparing these with the response curve of the eye, also 

 shown in the same figure, it will be evident how the general problem of 

 securing photoelectric signals of maximum efficiency without interfering 

 with the general quality of the image, or desirable conditions of illum- 

 ination, has been secured. 



Before going on to describe some of the optical features at the re- 

 ceiving end, we may pause to discuss the improvements in the tele- 

 vision signal which have been introduced by the changes just described. 

 There is, of course, a substantial gain in the steadiness of the image due 



