The Concept of Hardware (2)


 A display device is an output device that usually conveys text, graphics, and video information. Information shown on a display device often is called soft copy, because the information exists electronically and is displayed for a temporal period of time.

Display devices include CRT monitors, flat-panel displays, and high-definition televisions.

  1. CRT Monitors: Cathode-Ray Tube, an electron tube, or evacuated glass container, having at one end a cathode, or negative electrode, and a device called an electron gun that projects a beam of electrons against a luminescent screen at the opposite end of the tube. A bright spot of light appears wherever the electrons strike the screen. Cathode-ray tubes, or CRTs, are used as picture tubes in television receivers and as visual display screens in radar-receiving equipment, computer installations, and oscilloscopes.

Electrons are emitted from a heated cathode in the electron gun. A series of grids having a positive potential with respect to the cathode accelerate the electrons as they pass. The electrons next pass through a series of doughnut-shaped anodes that focus the stream of electrons so that they strike the luminescent screen as a fine point. Between the electron gun and the screen are either two sets of electric deflecting plates or two sets of magnetic deflecting coils. Electric deflecting plates are used in small CRTs, whereas magnetic deflecting coils are used in large CRTs in which a large deflection is required, as in television tubes.

In CRTs containing electric deflecting plates, a horizontal pair of plates controls the up-and-down motion of the electron beam, and a vertical pair controls the left-to-right motion of the beam. In each pair of plates, one plate has a negative charge of electricity, and the other plate has a positive charge. If the charges are equal in value, the beam will strike the center of the luminescent screen. If the charges are unequal, the electron beam will be deflected. The amount of deflection depends on the voltage applied to the plates. As the signal applied to the horizontal plates varies, so will the spot of light on the face of the tube, which will move up or down in response to the changes in voltage. If the voltage of the vertical plates is varied, the beam of electrons can be made to sweep horizontally across the face of the tube.

Magnetic deflecting coils work in an analogous manner, except that the electron beam is deflected by variations in the strength of the magnetic fields through which it must pass.

A.Flat-Panel Displays:

     A flat-panel display is a light-weight, thin screen that consumes less power than a CRT monitor. Two common types of flat-panel displays are LCD and gas plasma.

  • LCD Displays: Are commonly used in laptop computers, handheld computers, digital watches, and calculators because they are thinner and more lightweight than CRT monitors. While most LCD displays are color, even those on newer models of handheld computers, some devices use monochrome LCD displays to save battery power.

Unlike a CRT monitor, an LCD display does not use a cathode ray tube (CR1’) to create images on the screen: it instead uses a liquid crystal display (LCD). A liquid crystal display (LCD) has special molecules (called liquid crystals) deposited between two sheets of material. When an electric current passes through them, the molecules twist, causing some light waves to be blocked and allowing others to pass through, this then creates the desired images on the screen.

  • Gas Plasma Monitors: For even larger displays, some large business or power user prefer gas plasma monitors, which can measure more than 42 inches and hang directly on a wall. Gas plasma monitors use gas plasma technology, which substitutes a layer of gas for the liquid crystal material in an LCD monitor. When voltage is applied, the gas glows and produces the pixels that form an image. Gas plasma monitors offer larger screen sizes and higher display quality than LCD monitors but are much more expensive.

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