How Do Computers Produce Images on the Monitor?
Nearly everyone has used a monitor at some point, and has seen the lights display on a screen to show information or video for a program, game or movie. Many people sit in front of monitors day in and day out as part of their job. Few people understand how computers actually produce the images on the monitor, though. While not vital knowledge to most people, it's an interesting technology that has come a long way.
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From Computer to Monitor
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A monitor cable connects a computer's video card to the monitor. The most common types of monitor cables are VGA and DVI. This cable transfers the display information---analog for VGA, digital for DVI---from the video card to the monitor's display controller, a type of small processor, which decodes the signal and orders its components to correctly display the image. How this happens varies by the types of monitors: CRT and LCD.
CRTs
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CRT monitors, or cathode ray tube monitors, are sometimes called tube monitors because they contain a vacuum tube, which displays pictures. CRTs have largely been replaced by cheaper and more efficient LCD monitors, but many are still in use.
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How CRTs Work
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After decoding the computer's video signal, the controller fires what is known as a cathode, sometimes also called an electron gun; this sends negatively charged electrons to the front of the vacuum tube. On their trip, each electron passes a magnetic anode, which sends the electrons to the right portion of the screen. The electrons also pass through a mesh known as a shadow mask or aperture grill; this determines the individual pixel and resolution information. When the electrons finally hit the front of the tube, which is coated in phosphor on the inside of the screen, they light up according to the phosphor they hit.
Fun Fact
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Due to the nature of CRT monitors, if a single image is repeatedly sent to the front of the tube it can become "burned" into the screen. Screen savers were invented to combat this flaw by displaying constantly changing images. They serve no purpose on newer LCD screens, but their legacy lives on for the enjoyment they provide. Keep in mind that allowing a monitor to shut down rather than display a screen saver when not in use cuts down on its power consumption.
LCD Components
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Liquid Crystal Display monitors, commonly called LCD monitors or flat-panel monitors, get their name from exactly what the name suggests: crystals that exist in a liquid form. As with CRT monitors, a video signal travels from the computer's video card to the monitor, where it is decoded by a display controller inside the monitor.
Unlike the complex CRT monitors, which perform complex calculations with magnetic anodes and mesh filters to determine resolution and pixel information, an LCD monitor is made up of a set number of pixels, each containing sub-pixels with three color filters---red, green and blue. Each sub-pixel contains liquid crystals suspended between transparent electrodes. These can be manipulated in different ways to display different colors. A light source known as a cold cathode behind each pixel shines through the first filter, which whites the light, through the liquid crystals and transparent electrodes, and finally through the last filter, which is black.
How LCDs Work
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When the controller decodes the video data, it sends orders to each pixel for how to manipulate its liquid crystals and controls the brightness of the cold cathode light sources. The pixels are manipulated by the transparent electrodes supplying small levels of electrical current to the liquid crystals within, which will ultimately determine the color to be displayed for each pixel.
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References
- Photo Credit monitor image by Nicemonkey from Fotolia.com
