Refresh Cathode-Ray Tubes
Fig.below illustrates the basic operation of a CRT. A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam towards specified position on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called a refresh CRT.
The primary components of an electron gun in a CRT are the heated metal cathode and a control grid (fig.below). Heat is supplied to the cathode by directing a current through a coil of wire, called the filament, inside the cylindrical cathode structure. This causes electrons to be “boiled off” the hot cathode surface. In the vacuum inside the CRT envelope, negatively charged electrons are then accelerated toward the phosphor coating by a high positive voltage. The accelerating voltage can be generated with a positively charged metal coating on the in side of the CRT envelope near the phosphor screen, or an accelerating anode can be used, a in fig below . Sometimes the electron gun is built to contain the accelerating anode and focusing system within the same unit.
Spots of light are produced on the screen by the transfer of the CRT beam energy to the phosphor. When the electrons in the beam collide wit the phosphor coating , they are stopped and there are stopped and their kinetic energy is absorbed by the phosphor. Part of the beam energy s converted by friction into heat energy, and the remainder causes electron in the phosphor atoms to move up to higher quantum-energy levels. After a short time, the “excited” phosphor electrons begin dropping back to their stable ground state, giving up their extra energy as small quantums of light energy. What we see on the screen is the combined effect of all the electrons light emissions: a glowing spot that quickly fades after all the excited phosphor electrons have returned to their ground energy level. The frequency ( or color ) of the light emitted by the phosphor is proportional to the energy difference between the excited quantum state and the ground state.
Different kinds of phosphor are available for use in a CRT. Besides color, a major difference between phosphors is their persistence: how long they continue to emit light ( that is, have excited electrons returning to the ground state ) after the CRT beam is removed. Persistence is defined as the time it take the emitted light from the screen to decay to one-tenth of its original intensity. Lower-persistence phosphors require higher refresh rates to maintain a picture on the screen without flicker. A phosphor with low persistence is useful for animation ; a high-persistence phosphor is useful for displaying highly complex, static pictures. Although some phosphor have a persistence greater than 1 second, graphics monitor are usually constructed with a persistence in the range from 10 to 60 microseconds.