Television

The Transmission of moving pictures by radio waves (in broadcasting) or by cable (in closed-circuit television). John Logie Baird in Britain developed the first practical television system in 1925 using a mechanical scanning process. But this process was soon replaced by the modern method of electronic scanning, pioneered by Vladimir Zworykin in the United States.

In a simple black and white television camera, a lens focuses an image on a light-sensitive plate in the camera tube. Electrons are emitted from the various parts of the plate according to the amount of light reaching them. They travel to a target plate and make it electrically charged. The pattern of charges on the target is an electrical representation of the optical image formed on the light-sensitive plate by the lens.

The Target is then scanned rhythmically from left to right and line by line, by a beam of electrons from an electron gun. The electron beam interacts with the charges on the target to produce an electric current, which varies according to the amount of charge there - that is, on the brightness of each part of the picture. This variable current comprises the video signals, which are then transmitted, together with a synchronizing signal on a radio carrier wave.

A Television receiver picks up the transmitted picture signals, via an aerial, and first separates the video signals from the carrier wave. The signals are then fed to a cathode-ray tube, where they are made to vary the strength of an electron beam. The beam strikes the fluorescent viewing screen, causing it to glow more or less according to the strength of the beam. At the same time the beam is made to scan the screen side to side and line by line in the same way as the beam did in the camera tube. (625 line are common in Britain.) In this way a pattern of light and dark is built up on the screen, which reproduces the original scene viewed by the television camera.

In colour television, the coloured light entering the camera is split up by filters into different combinations of the three primary colours red, blue and green. Each colour is then fed to a separate camera tube, which produces its own signals. These signals are then transmitted and picked up by the receiver. There they are fed to three separate electron guns in the picture tube. Beams of electrons representing red, blue and green light are then directed onto phosphors on the screen that glow respectively red, blue, and green. The phosphors are so close together that their colours merge to reproduce the original colour.