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MONITORS

MONITORS  


  Monitors are vital part of user interface and the video sub systems of a PC which Provides instant feedback by showing you text and graphics. To display the output processed by the processor three type of display technology is used which are as follows 
1. CRT Technology 
2. TFT-LCD 
3. Gas Plasma 
    CRT is a very old display technology and with the coming of LCD and Gas Plasma day by day becoming obsolete. The main component of a CRT is the "picture tube” which is main part of the CRT. 
Parts of a CRT monitor:
1. Vacuum tube enclosed in glass 
2. Electron gun assembly 
3. Screen with phosphorous coating

Working of CRT 
    In a CRT television, a gun fires a beam of electrons (negatively-charged particles) inside a large glass tube. The electrons excite phosphor atoms along the wide end of the tube (the screen), which causes the phosphor atoms to light up. The television image is produced by lighting up different areas of the phosphor coating with different colors at different intensities .so in brief the working of the CRT can be summarized as described below. 

    Cathode ray tubes produce crisp, vibrant images, but they do have a serious drawback: They are bulky. In order to increase the screen width in a CRT set, you also have to increase the length of the tube (to give the scanning electron gun room to reach all parts of the screen). Consequently, any big-screen CRT television is going to weigh a ton and take up a sizable chunk of a room. So in brief the working of the CRT can be summarized as described below.
When heated 
a. The electron gun emits a stream of high-speed electrons. 
b. Electrons get attracted to the other end of the tube 
c. A focus control and deflection coil steer the beam to a specific point on the phosphorous screen, the phosphor glows. 
d. Three layers of phosphors are used: red, green, and blue. 
e. A metal plate (shadow mask) align the electron beam
f. Yokes around the collar of the tube bends the electron 

LCD Monitors 
Liquid Crystal Display is a digital display technology that produces images on a flat surface. LCDs are common because they offer some real advantages over other display technologies. They are thinner and lighter and draw much less power than cathode ray tubes (CRTs).

    What is a Liquid Crystal- liquid crystals are closer to a liquid state than a solid. It takes a fair amount of heat to change a suitable substance from a solid into a liquid crystal, and it only takes a little more heat to turn that same liquid crystal into a real liquid. This explains why liquid crystals are very sensitive to temperature and why they are used to make thermometers and mood rings. It also explains why a laptop computer display may act funny in cold weather or during a hot day at the beach. One feature of liquid crystals is that they're affected by electric current. A particular sort of nematic liquid crystal, called twisted nematics(TN), is naturally twisted. Applying an electric current to these liquid crystals will untwist them to varying degrees, depending on the current's voltage. LCDs use these liquid crystals because they react predictably to electric current in such a way as to control light passage . 

Working of LCD 
The combination of four facts makes LCD technology possible 
a. Light can be polarized. 
b. Liquid crystals can transmit and change polarized light. 
c. The structure of liquid crystals can be changed by electric current. 
d. There are transparent substances that can conduct electricity.

An LCD is a device that uses these four facts in a surprising way. 
    An LCD works by trapping a liquid crystal solution between two sheets of polarized glass. When an electric current passes through the liquid crystals, they rotate causing a change in the polarization of the light passing through them in response to the applied voltage. This results in more or less light passing through the polarized glass to reach the face of the display. LCD panels do not generate light - rather they filter or subtract light produced by a backlight source to create the image on the panel surface. Generally LCD has a lifespan of approximately about 20 - 25 years of daily 8-hour usage .In colour LCD each pixel is made up of three liquid crystal cells where each cell is fronted by a red, green, or blue filter and when light through filtered cells creates colors on LCD. 

Features:
a. LCDs have low-glare 
b. Completely flat screens 
c. Low power requirements 

Types of LCD: 
a. Active Matrix 
b. Passive Matrix 

Plasma Display 
Plasma television is a flat, lightweight surface covered with millions of tiny glass bubbles. Each bubble contains a gas-like substance, the plasma, and has a phosphor coating. Think of the bubbles as the pixels. Each picture element in a plasma TV display acts as a miniature light source. Essentially millions of Neon signs. Now, think of each pixel-bubble as having three sub-pixels - one red, one green, one blue. When it is time to display an image signal (RGB or video), a digitally controlled electric current flows through the flat screen, causing the plasma inside designated bubbles to give off ultraviolet rays. This light in turn causes the phosphor coatings to glow the appropriate color making your Plasma TV provide the best video image anywhere.

    When the intersecting electrodes are charged (with a voltage difference between them), electric current flows through the gas in the cell. As we saw in the last section, the current creates a rapid flow of charged particles, which stimulates the gas atoms to release ultraviolet photons. 

    The released ultraviolet photons interact with phosphor material coated on the inside wall of the cell. Phosphors are substances that give off light when they are exposed to other light. When an ultraviolet photon hits a phosphor atom in the cell, one of the phosphor's electrons jumps to a higher energy level and the atom heats up. When the electron falls back to its normal level, it releases energy in the form of a visible light photon. When the intersecting electrodes are charged (with a voltage difference between them), an electric current flows through the gas in the cell. As we saw in the last section, the current creates a rapid flow of charged particles, which stimulates the gas atoms to release ultraviolet photons.

    The released ultraviolet photons interact with phosphor material coated on the inside wall of the cell. Phosphors are substances that give off light when they are exposed to other light. When an ultraviolet photon hits a phosphor atom in the cell, one of the phosphor's electrons jumps to a higher energy level and the atom heats up. When the electron falls back to its normal level, it releases energy in the form of a visible light photon. 
    Millions of RGB bubbles glowing and dimming to make a rich, vivid image. The plasma display is one of the most exciting consumer electronics products to debut in the past decade. It's literally changing the shape of television, from the familiar CRT-type TV's that have been around for 50 years, to a sleek, nearly flat display that can hang on a wall. These new displays deliver high-definition television, and they serve both as TV's and computer monitors. The main advantage of plasma display technology is that you can produce a very wide screen using extremely thin materials. And because each pixel is lit individually, the image is very bright and looks good from almost every angle. The image quality isn't quite up to the standards of the best cathode ray tube sets, but it certainly meets most people's expectations.



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