Computer Monitors : CRT Monitors

Display Technologies

Since the beginning of computer era, there have been a number of technologies used for the display of output. The major technologies are CRT, LCD, Plasma, LED and OLED displays.

1. Cathode Ray Tube (CRT) Monitors

These monitors employ the CRT technology to create a display. The CRT (also known as picture tube) receives the How CRT Monitors work

signals through a cable and the signal is decoded by the display controller which finally appears on a phosphor screen. The detailed working is as following:

As shown in the image CRTs have a conical shape and there is an electron gun or cathode ray gun at the back end of the monitor and a phosphor screen in the front. The electron gun fires a stream of electrons towards the display screen through a vacuum tube. This stream of electrons is also known as cathode rays. At the middle of the monitor, there are magnetic anodes which are magnetized in accordance with the instruction from the display controller. When electrons (cathode rays) pass through the magnetic anodes, they are pushed or pulled in one direction or other depending on the magnetic field on the anodes. This directs the electrons towards the correct part of phosphor coating inside the display glass. When electrons strikes the phosphor coated screen passing through a mesh (shadow mask or aperture grill), the phosphor lights up making a displayable dot on the computer screen. There are three different colored phosphors (Red, Green and Blue) for each pixel and the color of the pixel depends on the phosphor on which the electrons strike.

This image shows the color combination schematic for phosphor particles. The monitor that has a single electron gun

has three different phosphors for each pixel. A cathode ray strikes to one or more of these phosphors and the corresponding colored pixel appear on the screen. However high quality monitors use individual electron gun for each color which improves the image quality. Distance for two same colored phosphors (for single electron gun monitors) is known as dot pitch. Lesser the dot pitch higher is the quality of monitors.

Aperture Grille v/s Shadow Mask

CRTs incorporate a metal sheet behind the display screen which affects the pixels on the screen as well as

brightness on the screen. Shadow mask is an obsolete technology in which there is a metal sheet with millions of holes to pass electrons in order to hit the phosphor coating. The shadow mask covers the entire screen thereby protecting the phosphors from stray ions (due to vacuum) and also limits the strength of the rays reducing the brightness on the monitor.

Aperture grille is a mesh of wires rather than any metal sheet with holes in it. Although the grill is fragile, it allows a brighter display.

What is the resolution of the screen? – Resolution of a monitor tells how densely pixels are arranged on the screen. A combination of dot pitch and the viewable image area defines the maximum resolution of the screen. For example if a 21 inch monitor screen with a viewable area of 401mm x 298mm has a dot pitch of 0.26 mm, then its resolution is 1843 x 1370 pixels derived from a formula.