188 BELL SYSTEM TECHNICAL JOURNAL 



The account of the picture transmission system which follows is 

 intended to give only a general idea of the work as a whole. A num- 

 ber of engineers ha\e collaborated in this work, and it is expected 

 that later publications will describe various features of the system 

 and its operation in greater detail. 



Gkneral Scheme of Pictlre Tr.vnsmission 



Reduced to its simplest terms, the problem of transmitting a pic- 

 ture electrically from one point to another calls for three essential 

 elements: The first is some means for translating the lights and 

 shades of the picture into some characteristic of an electric current; 



Fig. 1 — Sending end optical system in section: (L) light source; (D) condensing lens; 

 (A) diaphragm; (S) projection lens; (C) transparent picture film in cylindrical 

 form; (P) photoelectric cell 



the second is an electrical transmission channel capable of trans- 

 mitting the characteristic of the electric current faithfully to the 

 required distance; the third is a means for retranslating the electrical 

 signal as received into lights and shades, corresponding in relative 

 values and positions with those of the original picture. 



Analyzed for purposes of electrical transmission, a picture consists 

 of a large number of small elements, each of substantially uniform 

 brightness. The transmission of an entire picture necessitates some 

 method of traversing or scanning these elements. The method used 

 in the present apparatus is to prepare the picture as a film trans- 

 parency which is bent into the form of a cylinder. The cylinder is 

 then mounted on a carriage, which is moved along its axis by means 

 of a screw, at the same time that the film cylinder is rotated. A 

 small spot of light thrown upon the film is thus caused to traverse 

 the entire film area in a long spiral. The light passing into the 



