Scanning and Persistence of Vision

When you look in to a television picture you are merely watching a tiny spot of light traversing the screen from side to side 405 times (405 line)  in one fiftieth of a second, at a speed of about 7,000 miles an hour in the case of a 12 inch CRT, and a correspondingly higher speed in the case of larger tubes.  

This spot of light traces out the rectangular area on which the picture is seen and which is known as the raster. The optical illusion, which the picture undoubtedly is, is due to the way our eyes function, which is called persistence of vision.

Briefly this means that the eye sees as a continuous moving picture any series of pictures each of which is moved at a constant speed and then brought to a stop at a frequency of 16 pictures (frames) per second or more. 

For example When you visit the cinema what you see as a continuously moving picture is a series of “stills” which are jerked behind the projector lens at a frequency of 24 frames per second. Each picture is Halted in front of the lens for a fraction of a second and then moved on to give place to the next picture in the sequence, slightly different from the preceding picture,thus creating the illusion of moving pictures. The eye will not respond to a frequency in excess of 16 per second.

Persistence of Vision

When light rays impinge upon the retina of the eye the impression which they make does not cease immediately the light rays stop. On the contrary, it persists for an appreciable time afterwards, this effect being known as “persistence of vision,” or “visual persistence.” It is upon this “lag of retina,” as persistence of vision is sometimes called, that we are able to build up a reproduction of motion on the television or cinema screen. In both instances a series of successive pictures (each differing slightly from the preceding one) is projected on to the TV or cinema screen so rapidly that the eye is not able to get rid of the impression made by the one picture or image before the next one arrives.
Persistence of vision lasts for approximately one twelfth of a second. Hence, if a series of varying images are projected    upon a screen at a minimum rate of 12 per second the effect of motion will be obtained, although the picture will seem   somewhat jerky. For a smooth continuous picture a frequency of at least 24 per second is required. It should be noted that persistence of vision is a phenomenon which is attached to the actual retention of the image on the retina of the eye. The perception of the image by the retina in the first place is, so far as we can tell, instantaneous. A television picture is, however, unlike a photograph. In the latter the whole of the picture is visible, and is impressed upon the negative,  whereas a television picture depends entirely upon persistence of vision.
With a moving picture it is only the illusion of movement which depends upon persistence of vision. With television, both  persistence of vision and the building up of a picture are combined. Thus, in the television studio the cameras photograph    the scene being portrayed by a scanning process which breaks the picture up into tiny pieces. It was Paul Nipkow, a  scientist, who in 1884 first demonstrated the principle of scanning by his crude shadowgraph transmitters. It was with the Nipkow system that Baird experimented, and it is only fair that the credit should be given to Nipkow for this basic invention. It is right, also, to set on record that the Baird system was proved to be a failure, since it was based on a low-definition system which gave very crude and coarse pictures. High-definition TV owes nothing to and was not invented by Baird. He must, however, be given the credit for drawing public attention to the fact that television was possible, even though he knew that his own system was a failure. 

The Scanning Spot and Raster

The area traced out by the scanning spot, and on which the picture appears is known as the raster. The spot of light which    traces out the picture is called the scanning spot, and it is made to sweep continuously over every portion of the picture to be televised, thus enabling the picture to be split up into a large number of small areas or picture  elements. Other factors being equal, the smaller the scanning spot the finer in detail will be the televised image, for it will enable the light and the shade of the picture to be picked up and transmitted with precision, a task which becomes more  difficult as the size of the scanning spot is increased. 
The BBC System
The television system employed by the BBC and ITA transmits two sets of pictures, each of 202 5 horizontal lines at 25 per second. These lines are interlaced and the frame and flicker frequency is 50 per second, scanned from top to bottom, and the two frames mentioned above, are interlaced to produce the 405 lines and a complete picture speed of 50 per second. 
To be continued .......