The picture tube is the largest component of a television set, consisting of four basic parts. The glass face panel is the screen on which images appear. Suspended immediately behind the panel is a steel shadow mask, perforated with thousands of square holes. (Connected to the mask is a metal shield to neutralize disruptive effects of the Earth's magnetic field.) The panel is fused to a glass funnel, which comprises the rear of the picture tube. The very rear of the funnel converges into a neck, to which an electron gun assembly is connected.

The inside of the panel is painted with a series of very narrow vertical stripes, consisting of red, green and blue phosphors. These stripes are separated by a narrow black graphite stripe guardband. When struck by an electron beam, the phosphors will illuminate, but the graphite will not. This prevents color impurity by ensuring that the electron beam only strikes the phosphor stripes it is intended to light.

The electron beam is generated by the electron gun assembly, which houses three electron guns situated side-by-side. Each of the three guns emits an electron beam (also called a cathode ray) into the tube, through the mask and onto the panel.

Because the three beams travel side-by-side, the holes in the mask ensure that each beam, because of its different angle of attack, will hit only a specific phosphor stripe - red, green or blue. The three phosphors, lighted in different combinations of intensity, can create any visible color when viewed from even a slight distance.

The three electron beams are directed across the screen by a series of electromagnets, called a yoke, which draw the beams horizontally across the screen a line at a time. Depending on the screen size, the beam draws about 500 lines across the entire screen. Each time, the phosphors light up to produce an image.

The electron guns and the yoke are electronically synchronized to ensure the lines of phosphors are lighted properly to produce an accurate image. The image lasts only for about a 30th of a second. For that reason, the picture must be redrawn 30 times a second. The succession of so many pictures produces the illusion of movement, just like the frames on movie film.

Color combinations produced by a picture tube
Red Green Blue
Red X
Yellow X X
Blue X
Green X
White X X X
Magenta X X
Cyan X X

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