564 BELL SYSTEM TECHNICAL JOURNAL 



In the system actually used for television transmission, this apparent 

 limitation has been evaded by reversing the entire optical system of 

 Fig. 1 and arranging it as shown diagrammatically in Fig. 2. Instead 

 of scanning an image of the subject, the actual subject is scanned 

 directly by a rapidly moving spot of light. An illustrative laboratory 

 set-up, Fig. 3, shows the arrangement of parts in such a transmitting 

 station. A fifteen-inch disk rotating approximately eighteen times 

 per second carries a series of fifty small apertures arranged in the 

 form of a spiral. A beam of light is condensed by a lens from a 40- 

 ampere Sperry arc to intensely illuminate a limited area in the path 

 of the moving apertures; and a slender, intense beam of light passes 

 through each aperture as it moves across the illuminated area. A 

 frame in front of the disk permits light to emerge from only one 

 aperture at a time and the lens in front of the disk focuses an image 

 of this moving aperture on the subject. As a result of this arrange- 

 ment the subject is completely scanned in a series of successive, 

 parallel lines by a rapidly moving spot of light, once for each revolution 

 of the disk ; and on account of the transient nature of the illumination 

 the subject is scarcely aware that he is being exposed to it. 



As the spot of light traces across the subject, light is diffusely 

 reflected or scattered from the subject in all directions, and some of 

 the light that is reflected forward passes into three large photo- 

 electric cells placed just in front of the person who is being viewed. 

 The current outputs from the three photoelectric cells operate in 

 parallel into a common amplifier system. As the beam of light passes, 

 for instance, across a person's eyebrow less light is reflected to the 

 photoelectric cells, and as the beam passes across his forehead more 

 light is reflected. Since the current output from the photoelectric 

 cells is proportional to the received light, the current follows accu- 

 rately the brightness of the various elemental areas of the subject's 

 features as he is traced over by the scanning beam. This fluctuat- 

 ing current is unidirectional. 



The actual operation of such an optical system, its influence on 

 the lighting effects and quality of the reproduced image, may best be 

 understood by noting that optically the system is identically the 

 same as if all of the rays of light were reversed in direction to give an 

 optical system equivalent to Fig. 1. The television apparatus sees 

 the subject exactly as if rays of light came out of the photoelectric 

 cells to illuminate the subject; the lens formed an image of the subject 

 on the disk; and the apparatus scanned this image and reproduced 

 it at the receiving end. The lights and shadows seen in the image 

 are the same as if the subject were illuminated by three large lights in 



