This node describes the basic workings of a Cathode Ray Tube, or CRT, as used in your computer monitor.

Monochrome CRTs follow the same principles as color CRTs, except they're much simpler in design. They have one cathode, or gun, to supply electrons to create the pictures on the screen. Since there is only one gun, alignment is simpler.

Color CRTs translate the information from your computer video card, via on-board circuitry within the monitor, into a usable visual image. The CRT itself is a glass tube with embedded and attached electronics. At the rear of the tube (sometimes called the neck or yoke of the CRT) are three cathodes that each boil off electrons, one cathode for each primary color. These electrons are accelerated towards the front of the tube. After the electrons are emitted by the cathode, the are attracted towards a metal grid called the control grid. This control grid regulates the overall brightness of the image on your screen, and adjusting the brightness control actually changes the voltage on the control grid. After the control grid is the screen grid. The screen grid starts the acceleration of the electrons towards the display area of the CRT. After the screen grid is the focus grid, which narrows the beam of electrons and focuses them. Adjusting your focus control on your monitor changes the voltage of the focus grid.

Once the electron beams are focused, they pass through two thin magnets called the purity magnet and the convergence magnet. The purity magnet helps to fine-tune the electron beam's focus. The convergence magnet adjusts the beams to converge on one point or pixel on the front of the CRT. These are only adjustable by technicians using proper safety procedures.

After the two permanant magnets are the horizontal deflection assembly and the vertical deflection assembly. The two deflection assemblies are large electromagnets that deflect the electron beams to light a particular point on the CRT face. These are actually outside of the CRT tube itself, as are the purity and convergence magnets. Their magnetic field penetrates the glass tube to control the electron beams.

At the front portion of the glass tube is a device called a shadow mask. This device is a very thin metal plate that has thousands of microscopic holes, each corresponding to one pixel, or one dot. In actuality, one pixel is actually three dots - one red, one green and one blue. They are so close together that they appear as one single dot. When you read the specifications of a new monitor, the rating of 28dpi is actually 28 pixel holes in the shadow mask per linear inch.

Between the shadow mask and the glass face of the CRT is the phosphors layer. The phosphors layer lights up whenever the electron beam hits it. It slowly fades away, and it is this screen persistance that allows the human eye to see a solid color screen of information. If the persistance was short, where the phosphors layer lit up when the beam hit it and stopped illuminating when the beam was not hitting the pixel, we'd see a flickering in the image. Also, the image must be drawn incredibly fast, faster than 15 images per second, for the eye's own persistance to interpret the image as one continuous solid picture. You are actually watching the phosphors layer through the glass enclosure when you are reading this.

Sometimes the shadow mask begins to develop a slight magnetic charge. Most monitors now include a degaussing coil around the front of the CRT to demagnetize the shadow mask.

The voltages used inside monitors are extremely high, on the order of 15 to 30 Kilovolts. Always refer servicing to qualified personnel.

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