. The Bell System technical journal . by the scanning beamor the neon lamp image rendered difficult of observ^ation. It hasbeen necessary for the solution of this problem to reduce the visible in-tensity of the scanning beam considerably below the value formerlyused and to considerably increase the brightness of the neon lamp. The means adopted consists first, in the use of a scanning lightof a color to which the eye is relativ^ely insensitive but to whichphotoelectric cells can be made highly sensitive. For this purposeblue light has been used, obtained by interposing a blue filter in the 452
. The Bell System technical journal . by the scanning beamor the neon lamp image rendered difficult of observ^ation. It hasbeen necessary for the solution of this problem to reduce the visible in-tensity of the scanning beam considerably below the value formerlyused and to considerably increase the brightness of the neon lamp. The means adopted consists first, in the use of a scanning lightof a color to which the eye is relativ^ely insensitive but to whichphotoelectric cells can be made highly sensitive. For this purposeblue light has been used, obtained by interposing a blue filter in the 452 BELL SYSTEM TECHNICAL JOURNAL path of the arc light beam, and potassium photoelectric cells speciallysensitized to blue light and more sensitive than those previouslyused have been developed. The number of these cells and their areahas also been increased over those used in the earlier televisionapparatus so that the necessary intensity of the scanning beam isdecreased. The second half of the problem, namely that of securing a max-. Fig. 3—Interior of the television booth. imum intensity of the neon lamp, has been attained by the develop-ment of water-cooled lamps capable of carrying a high net result of the use of the blue light for scanning, of moresensitive photoelectric cells, and of the high efficiency neon lamps is thatthe user of the apparatus is subjected only to a relatively mild blue IMAGE TRANSMISSION SYSTEM 453 light sweeping across his face, which he perceives merely as a blue spotof light lying above the incoming image. Figure 3 shows the in-terior of the television booth with the frame through which theobserver sees the image of the distant person. A second optical problem is the arrangement of the photoelectriccells required in order to obtain proper virtual illumination of theobservers face. As we have previously pointed out in discussingthe beam scanning method,^ the photoelectric cells act as virtuallight sources and may be manipulated both as
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