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Computer Graphics can use many different output devices, such as monitors, printer, plotter, etc. but the most common display device is the Cathode Ray Tube (CRT) monitor.
In a CRT the focusing system acts like a light lens with a focal length such that the center of focus is the screen. The horizontal and vertical deflectors allow the electron beam to be focused on any spot on the screen. The screen is coated with a special organic compound called a phosphor. For color systems there are groups of three different phosphors, one to produce red shades, one for green shades, and one for blue shades.
Electrons hit the screen phosphor molecules and cause a ground state to singlet excited state transition. Most of the phosphors relax back to the ground state by emitting a photon of light which is called fluorescence. This happens very rapidly so that all of the molecules which fluoresce do so in under a millisecond. Some of the molecules convert from an excited singlet state to an excited triplet state in a process called inter-system crossing. This is quantum mechanically forbidden, which means it still happens but only by a relatively small fraction of the phosphors and is a slower process. These phosphors then emit light, called phosphorescence, that decays slower but still rapidly (in about 15-20 milliseconds) so there is the need to refresh the screen by redrawing the image.
Phosphors are characterized by color (usually red, green, or blue) and persistence, which is the time for the emitted light to decay to 10 % of the initial intensity. High persistence phosphors allow for a lower refresh rate to avoid flicker, e.g., the original IBM PC monochrome monitor had a high persistence phosphor. This allowed it to have good resolution for text with inexpensive electronics. However, this is poor for animation since a "trail" is left with moving objects.
Low persistence phosphors are good for animation but require a high refresh rate to prevent flicker. A refresh rate of 50 - 60 Hz is usually sufficient to prevent flicker, but some systems refresh at even higher rates such as 72-76 Hz.
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