TELEVISION, by virtue of usage, has come to mean the transmission, to a distant point, usually by electrical means, of moving scenes, to be viewed at the distant point practically sim ultaneously with their original occurrence. Television is thus differentiated from the viewing of distant events by a telescope, from the electrical transmission of still pictures, and from the projection of motion pictures from films transported to a distance.
The problem of electrical tele vision consists essentially in the production of electrical signals by light, the transmission of these signals to a distant point, and the recreation of light from them. If these three processes can be carried out with adequate speed and accuracy the transmission of complete moving images may be attained. The process of obtaining an electrical signal from light at one place and of using this signal to control light at another place is a comparatively simple matter as long as a single light signal is all that is required. In the case of image transmission, however, it is necessary to transmit not one but a vast number of signals in a very brief time. Any image or picture may be considered as made up of a very large number of small uniform areas or elements each of different brightness. When an image is viewed by the eye, the rods and cones of the retina perform this analysis of the image into small elements, and the many fibrils of the optic nerve simultaneously transmit to the brain the impulses set up by light in each of the rods and cones. It would be theoretically possible to build a tele vision system in which a large number of photosensitive cells were simultaneously exposed to an image, and a separate electrical circuit connected with each one carried the electrical signals to a distant point, where again a bank of lamps was controlled by the signals. Such a multi-channel scheme is not, however, practical, and all successful methods of television have resorted to a process of scanning, by means of which the whole of an image is traversed point by point and the signals corresponding to the various degrees of light and shade are transmitted in sequence instead of simultaneously. This is the method which is used in the trans
mission of still pictures over telephone lines, but television is differentiated from still picture transmission by the much greater speed which it requires. While the transmission of a still picture may take several minutes, in television the entire image must be traversed in a small fraction of a second (about 1/2o), in order that, through persistence of vision, the image when reconstructed may appear complete, and may change its character sufficiently rapidly to reproduce motion of the object without flicker.
Various devices, such as vibrating mirrors and wheels of lenses have been proposed for the rapid scanning of an image for television purposes. The means which thus far has been found most practical is a flat circular disc pro vided with a spiral of small holes, first proposed by Nipkow in 1884. By the rotation of this disc each hole of the spiral passes in turn across the field of view, so that upon each complete rota tion of the spiral, the whole image has been covered. The most straightforward way of utilizing such a disc is shown in Fig. 1, where an image of the scene to be transmitted is formed, by means of a lens, upon an aperture in front of the disc. When the disc is rotated at a speed of 15 to 20 revolutions a second, the image transmitted through the holes appears virtually complete and steady, although actually built up of a sequence of light signals.
The Nipkow disc, while simple and convenient for the analysis of an image into a small number of elements, is inherently in efficient, and becomes impractically large when really fine grain images are required. For this reason other scanning means of more compact form, or means which may dispense altogether with mechanically moving parts, in particular devices employing cathode ray beams, are receiving considerable attention.