The picture became much sharper when the 625-line system was introduced in the 1960s. Although the lines form too quickly to see with the naked eye, they show up in a photo. In future, high definition TV may use 2000 or more lines, giving a picture that looks almost as sharp as film or photographs. To this is added the sound signal from the microphones and a sync pulse to time the signal properly. The combined signal is broadcast in the form of radio waves which are picked up by the TV aerial and fed to the set, where an electron beam scans across the screen making it glow in the right places.
Shadow mask Behind the TV screen is a special grille called a shadow mask with tiny holes in it.
The beams for each of the three colours can only pass through at a certain angle to strike the screen on the correct coloured dots.