. The Bell System technical journal . ely into two beams for large angles of deviation. The problemof general illumination of the booth is principally the choice of a colorof light which shall affect neither the potassium nor the casium this purpose, a monochromatic yellow-green was chosen, securedby covering all the lights with a combination of orange and signal greenglasses. The potassium cells are insensitive to this color of light, and 268 BELL SYSTEM TECHNICAL JOURNAL the caesium cells were rendered so by placing over them, windowscovered with a deep purple gelatin. This choice
. The Bell System technical journal . ely into two beams for large angles of deviation. The problemof general illumination of the booth is principally the choice of a colorof light which shall affect neither the potassium nor the casium this purpose, a monochromatic yellow-green was chosen, securedby covering all the lights with a combination of orange and signal greenglasses. The potassium cells are insensitive to this color of light, and 268 BELL SYSTEM TECHNICAL JOURNAL the caesium cells were rendered so by placing over them, windowscovered with a deep purple gelatin. This choice of illumination colormade possible a satisfactory general level of illumination of the boothand the surroundings of the image without introducing spurious transmissions of the purple filters, the response curves of thepotassium and caesium oxide cells, the radiation curve of the incandes-cent lamps used for the scanning beam, and the transmission curves ofthe glasses used over the lamps for general illumination, are shown. Fig. 2- -Interior of two-way television booth showing location of two caisium cellsabove and to either side of scanning and viewing aperture. in Fig. 3. Comparing these with the response curve of the eye, alsoshown in the same figure, it will be evident how the general problem ofsecuring photoelectric signals of maximum efficiency without interferingwith 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 is, of course, a substantial gain in the steadiness of the image due SOME OPTICAL FEATURES IN TWO-WAY TELEVISION 269 100806040200 100 80 I- 5 60O UJ a. 20 0 ( ■\ \ EYE — 1 > 1 V y s ^ j^ s 1 \ 4 MM SIGNALPURPLE / > \ \ / ^ V, / A / 1-1 KS CELL / I L \ —N V \ Cs O CELL \ ^ .-. ^ is— \ w
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