Some Optical Features in Two-Way Television * 



By HERBERT E. IVES 



A comprehensive description of the two-way television system now being 

 demonstrated between the American Telephone and Telegraph Company 

 building, and the Bell Telephone Laboratories, in New York City, has been 

 published elsewhere.^ Part of that account gives the essential features of the 

 optical arrangements whereby the users of the apparatus are appropriately 

 lighted, and are assured against visual discomfort from the scanning opera- 

 tion. Since the apparatus was first installed, however, some important 

 changes have been made in the distinctively optical features, whereby the 

 performance of the system has been notably improved, and its operation 

 considerably simplified. These changes deserve description, and the pres- 

 ent account is mainly concerned with them, although for the sake of com- 

 pleteness some details previously described are included. 



IT IS an inherent feature of the two-way television system that either 

 user is continuously scanned as he views the image from the distant 

 station. The beam scanning method,^ by which a beam of light sweeps 

 over the subject's face, enables the scanning operation to be performed 

 with a minimum amount of light. Even so, because of the relatively 

 low intensity of the television image, it is necessary to reduce the in- 

 tensity of the scanning beam in every way possible. In the two-way 

 apparatus as first operated, advantage was taken of the fact that the 

 photoelectric cells employed, which were of potassium, were principally 

 sensitive to blue light. The scanning beam derived from a high power 

 arc lamp was accordingly passed through a deep blue filter, which 

 reduced the photoelectric efficiency of the beam very little, but because 

 of the relatively low visual value of blue light, effectively reduced the 

 brightness of the beam many times. The user of the apparatus saw, 

 above the incoming image, merely a mild blue spot of light, which did 

 not interfere with his vision. 



A disadvantage of the use of blue light, which was anticipated, and 

 found in practice to be quite real, was that dark, tanned, or ruddy 

 complexions were rendered as altogether too dark, in comparison with 

 whites such as the ordinary linen collar. The effect is precisely that 

 encountered in the earlier photographic processes before color sensitive 

 plates and color filters were available. While this defect was mini- 

 mized by the use of a dark background, and to some extent by chopping 

 off the highlights by electrical means, it was recognized as undesirable. 



* Jour. Optical Soc, Feb. 1931. 

 ^Bell Sys. Tech. Jour., July 1930. 

 2 Jour. Optical Sac, March 1928. 



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