Display Devices

A computer display screen is usually classified as an output device because it typically shows the results of a processing task. Some screens, however, can be classified as both input and output devices because they include touch-sensitive technology that accepts input. Display devices used for output offer three technology options: CRT, LCD, and plasma. Gun-like mechanisms in the CRT (cathode ray tube) spray beams of electrons toward the screen and activate individual dots of color that form an image. CRT display devices often simply called “monitors”, are bulky, however, and consume a fair amount of power. An LCD (liquid crystal display) produces an image by manipulating light within a layer of liquid crystal cells. Modern LCD technology is compact in size and lightweight, andprovides an easy-to-read display. The advantages of LCD monitors (or “flat panel displays”) include display clarity, low radiation emission, portability, and compactness. Plasma screen technology creates an on-screen image by illuminating miniature colored fluorescent lights arrayed in a panel-like screen. The name “plasma” comes from the type of gas that fills fluorescent lights and gives them their luminescence. Like LCD screens, plasma screens are compact, lightweight, and more expensive than CRT monitors. They are rather energy consuming, too.

Image quality is a factor of screen size, dot pitch, width of viewing angle, refresh rate,resolution, and color depth. Screen size is the measurement in inches from one corner of the screen diagonally across to the opposite corner.The quality of a screen is often measured by the number of horizontal and vertical pixels used to create it. A pixel is a dot of color on a photo image or a point of light on a display screen. It can be in one of two modes: “ on” or “off”. A larger number of pixels per square inch means a higher res­olution, or clarity and sharpness of the image. The distance between one pixel on the screen and the next nearest pixel is known as dot pitch. Dot pitch (dp) is a measure of image clarity. A smaller dot pitch means a clearer image. Greater pixel den­sities and smaller dot pitches yield sharper images of higher resolution. A monitor’s viewing angle width indicates how far to the side you can still clearly see the screen image. A wide viewing angle indicates that you can view the screen from various positions without compromising image quality. CRT and plasma screens offer the widest viewing angles. A CRT’s refresh rate (also referred to as“vertical scan rate”) is the speed at which the screen is repainted. The faster the refresh rate, the less the screen flickers. Refresh rate is measured in cycles per second, or Hertz (Hz). The number of colors a monitor can display is referred to as color depth or “bit depth”. Most PC display devices have the capability to display millions of colors. The number of horizontal and vertical pixels that a device displays on a screen is referred to as its resolution.

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