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The Cathode Ray Tube (CRT)
In simple terms, a monitor operates very similarly to how your regular television set works. The principle is based upon the use of an electronic screen called a cathode ray tube or CRT, which is the major (and most expensive) part of a monitor. The CRT is lined with a phosphorous material that glows when it is struck by a stream of electrons. This material is arranged into an array of millions of tiny cells, usually called dots. If you look up close at your monitor (or use a magnifying glass) you can see these dots.
At the back of the monitor is a set of electron guns, which produce a controlled stream of electrons, much as the name implies. To produce a picture on the screen, these guns start at the top of the screen and scan very rapidly from left to right. Then, they return to the left-most position one line down and scan again, and repeat this to cover the entire screen. In performing this scanning or sweeping type motion, the electron guns are controlled by the video data stream coming into the monitor from the video card, which varies the intensity of the electron beam at each position on the screen. This control of the intensity of the electron beam at each dot is what controls the color and brightness of each pixel on the screen. This all happens extremely quickly, and in fact the entire screen is drawn in a small fraction of a second.
There are three electron guns (on a color monitor) that control the display of red, green and blue light respectively. The surface of the CRT is arranged to have these dots placed adjacently in a specific pattern. There are separate video streams for each color coming from the video card, which allows the different colors to have different intensities at each point on the screen. By varying the intensity of the red, green and blue streams, the full rainbow of colors is made possible.
The surface of the CRT only glows for a small fraction of a second before beginning to fade. This means that the monitor must redraw the picture many times per second to avoid having the screen flicker as it begins to fade and then is renewed. This rapid redrawing is called "refreshing" the screen.
The CRT is the heart of the monitor and is discussed in much more detail in its own section.