Page "Analog television" Paragraph 14

A cathode-ray tube ( CRT ) television displays an image by scanning a beam of electrons across the screen in a pattern of horizontal lines known as a raster.

At the end of each line the beam returns to the start of the next line ; the end of the last line is a link that returns to the top of the screen.

As it passes each point the intensity of the beam is varied, varying the luminance of that point.

A color television system is identical except that an additional signal known as chrominance controls the color of the spot.