Theory of CRT Operation

The CRT display dates back to the invention of television and has been in use for about 70 years. CRT displays designed for use with computers are built to higher specifications than those designed for television. Since computers are commonly used for word processing and other detailed work, computer displays must be able to present users with readable text at comparatively small font sizes without causing eyestrain that would limit their effective use.

A CRT display is composed of a vacuum tube with an electron beam generator inside. In a color CRT, several magnetic fields precisely focus three electron beams, shot from this electron “gun,” to strike specific points on a luminescent screen. The electron beams hit luminescent phosphors painted on the screen, energizing them so they light up for a short time and create a visible pattern. The target and intensity of the beams are varied rapidly to produce the pattern we see on the screen.

The electron beams repeatedly “paint” the entire screen area, refreshing its phosphorescent glow, from top to bottom, left to right. The number of times the screen is repainted each second is the CRT's scan, or refresh, rate, expressed in hertz (Hz). Higher scan rates result in images that appear stable; lower scan rates result in images that seem to flicker, which can cause eyestrain. You can adjust the scan rate in the Displays pane in System Preferences, but you must select a rate that is compatible with the display for best results.

The large glass vacuum tube itself makes up most of the size and weight of a CRT display.

Detailed commands sent from the CRT display's circuit boards to the cathode, the deflection coils, and the grids control the image's shape, color, brightness, and focus. The cathode's job is to convert the red, green, and blue signals into beams of electrons; the deflection coils steer the beams; and the grids attract or repel the electron beams. Just in front of the screen, an aperture grill or shadow mask controls which phosphors the electrons strike on the screen.