IMAGE TRANSMISSION SYSTEM 453 



light sweeping across his face, which he perceives merely as a blue spot 

 of light lying above the incoming image. Figure 3 shows the in- 

 terior of the television booth with the frame through which the 

 observer sees the image of the distant person. 



A second optical problem is the arrangement of the photoelectric 

 cells required in order to obtain proper virtual illumination of the 

 observer's face. As we have previously pointed out in discussing 

 the beam scanning method,^ the photoelectric cells act as virtual 

 light sources and may be manipulated both as to their size and 

 position like the lights used by a portrait photographer in illum- 

 inating the face. In the present case, it is desired to have the whole 

 face illuminated and accordingly photoelectric cells are provided to 

 either side and above. One practical difficulty which is encountered 

 is that eyeglasses, which often cause annoying reflections in photog- 

 raphy are similarly operative here. For this reason, it is important 

 that the photoelectric cells be placed as far to either side or above 

 as possible. The banks of photoelectric cells shown in Fig. 3 are 

 accordingly much farther removed from the axis of the booth than 

 were the three cells used in the first demonstration. In the position 

 which has been chosen for the cells, reflections from eyeglasses are 

 not annoying unless the user turns his face considerably to one side 

 or the other. 



The number of cells has been so chosen as to secure a good bal- 

 ance of effective illumination from the three sides and it has been 

 found desirable to partly cover the cells on one side in order to aid 

 in the modelling of the face by the production of slight shadows in 

 one direction. 



Another optical problem is the illumination of the interior of the 

 booth. There must, of course, be sufficient illumination for the 

 user to locate himself, and it is also desirable that the incoming 

 image and the scanning spot be not seen against an absolutely black 

 background. The illumination of the booth is by orange light, to which 

 the cells are practically insensitive, and so arranged that the walls and 

 floor are well illuminated. In addition to the wall and floor illumination, 

 a small light is provided on the shelf bar in front of the observer so 

 as to cast orange light on the front wall surrounding the viewing 

 frame. This light contributes materially to reducing the glaring 

 effect of the scanning beam, and to the easy visibility of the incoming 

 image. 



In addition to the optical features which are visible to the person 

 sitting in the booth, there are very necessary optical elements which 



^Jul. optical Soc. of America, Mardi, 1928, p. 177. 



