CATHODE - RAY TUBES

The basic operation of a CRT:

                           A beam of electrons (cathode rays) emitted by an electrode, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor - coated screen. The phosphor then emits atiny low spot of sunshine at each position contacted by the beam. Because the sunshine emitted by the phosphor fades very rapidly, some method is required for maintaining the screen picture.

A method to stay the phosphor glowing is to redraw the image repeatedly by quickly directing the beam back over the identical points. this kind of display is named a refresh CRT. the first components of an electrode during a CRT are the heated metal cathode and an impact grid. 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 recent cathode surface. within the vacuum inside the CRT envelope, the free, charged electrons are then accelerated toward the phosphor coating by a high positive voltage.

The accelerating voltage is generated with a charged metal coating on the within of the CRT envelope near the phosphor screen, or an accelerating anode is used. Sometimes the electrode is made to contain the accelerating anode and focusing system within the identical unit. Intensity of the beam is controlled by setting voltage levels on the grid, which may be a metal cylinder that matches over the cathode. A high negative voltage applied to the grid will shut off the beam by repelling electrons and stopping them from passing through the little hole at the top of the grid structure. A smaller negative voltage on the grid simply decreases the quantity of electrons passing through.

Since the number of sunshine emitted by the phosphor coating depends on the quantity of electrons striking the screen, we control the brightness of a display by varying the voltage on the grid. We specify the strength for individual screen positions with graphics software commands. The focusing system during a CRT is required to force the beam to converge into atiny low spot because it strikes the phosphor. Otherwise, the electrons would repel one another, and therefore the beam would displayed because it approaches the screen. Focusing is accomplished with either electric or magnetic fields. Electrostatic focusing is often utilized in television and lighting tricks monitors. With electrostatic focusing, the beam passes through a charged metal cylinder that forms an electrostatic lens.

The action of the electrostatic lens focuses the beam at the middle of the screen, in only the identical way that an lens focuses a beam of sunshine at a specific focal length. Similar lens focusing effects is accomplished with a flux founded by a coil mounted round the outside of the CRT envelope. Magnetic lens focusing produces the littlest spot size on the screen and is employed in special- purpose devices.