TELEVISION >555 



Assuming the undistorted transmission of the signals to a distant 

 point, the next fundamental problem of television is the reconstruction 

 of the image, or the translation of the electrical signal back into 

 light of varying intensity. Just as at the sending end we have seen 

 that the production of a useful electrical signal with the amount of 

 light available from a naturally illuminated object is a major problem, 

 so at the receiving end the converse problem, that of securing an 

 adequately bright light from the electrical signal, presents great 

 difficulty. The nature of the problem may be understood by assuming 

 that it is to be done by projecting the received image on a screen 

 similar to an optical lantern projection screen. If the spot of light 

 which is to build up this image scans the whole area in the same way 

 that the object is scanned, we find that the amount of light which can 

 be concentrated into a small elementary spot will, when distributed 

 by the scanning operation over the whole screen, reduce the brightness 

 of the screen in the ratio of the relative areas of the elementary spot 

 and the whole screen. The amount of this reduction will, of course, 

 depend upon the number of elements into which the picture is divided, 

 but will in any event be a factor of several thousand times. It is 

 doubtful whether any light source exists of sufficient intensity such 

 that an image projected by it can be spread out by a scanning operation 

 over a large screen and give an average screen brightness which would 

 be at all adequate. It is possible to imagine optical systems by which 

 such a thing as the crater of an arc could be projected upon the screen, 

 but the motion of this image and its variation in intensity would 

 involve the extremely rapid motion of lenses, mirrors and apertures 

 of a size such as to render the operation mechanically impracticable. 

 It appears from these considerations that the only promising means 

 of reconstructing the image would be those in which a light source, 

 whose intensity can be controlled with great rapidity, is directly 

 viewed. 



Another element of a television system upon whose solution success 

 depends as much as any other is that of synchronization; the recon- 

 struction of the image, postulated in the last paragraph, is only possible 

 if the reconstructed elements fall in exactly the right positions at 

 the right times, to correspond with the signals as generated at the 

 analyzing end. The criterion for satisfactory synchronization will be 

 expressed in terms of variation from identity of speed by figures which 

 will depend on the fineness of grain of the image which it is planned 

 to send. No element of the image must, of course, be out of place 

 by a considerable fraction of the size of the element. 



