A display is a computer output surface and projecting mechanism that shows text and often graphic images to the computer user, using a cathode ray tube ( CRT ), liquid crystal display ( LCD ), light-emitting diode, gas plasma, or other image projection technology. The display is usually considered to include the screen or projection surface and the device that produces the information on the screen. In some computers, the display is packaged in a separate unit called a monitor . In other computers, the display is integrated into a unit with the processor and other parts of the computer. (Some sources make the distinction that the monitor includes other signal-handling devices that feed and control the display or projection device. However, this distinction disappears when all these parts become integrated into a total unit, as in the case of notebook computers.) Displays (and monitors) are also sometimes called video display terminals (VDTs) . The terms display and monitor are often used interchangably.
Most computer displays use analog signals as input to the display image creation mechanism. This requirement and the need to continually refresh the display image mean that the computer also needs a display or video adapter . The video adapter takes the digital data sent by application programs, stores it in video random access memory ( video RAM ), and converts it to analog data for the display scanning mechanism using an digital-to-analog converter ( DAC ).
Displays can be characterized according to:
- Color capability
- Sharpness and viewability
- The size of the screen
- The projection technology
Color CapabilityToday, most desktop displays provide color. Notebook and smaller computers sometimes have a less expensive monochrome display. Displays can usually operate in one of several display modes that determine how many bits are used to describe color and how many colors can be displayed. A display that can operate in SuperVGA mode can display up to 16,777,216 colors because it can process a 24-bit long description of a pixel . The number of bits used to describe a pixel is known as its bit-depth . The 24-bit bit-depth is also known as true color . It allows eight bits for each of the three additive primary colors - red, green, and blue. Although human beings can't really distinguish that many colors, the 24-bit system is convenient for graphic designers since it allocates one byte for each color. The Visual Graphics Array ( VGA ) mode is the lowest common denominator of display modes. Depending on the resolution setting, it can provide up to 256 colors.
Sharpness and ViewabilityThe absolute physical limitation on the potential image sharpness of a screen image is the dot pitch , which is the size of an individual beam that gets through to light up a point of phosphor on the screen. (The shape of this beam can be round or a vertical, slot-shaped rectangle depending on the display technology.) Displays typically come with a dot pitch of .28 mm (millimeters) or smaller. The smaller the dot pitch in millimeters, the greater the potential image sharpness.
The actual sharpness of any particular overall display image is measured in dots-per-inch ( dots per inch ). The dots-per-inch is determined by a combination of the screen resolution (how many pixel s are projected on the screen horizontally and vertically) and the physical screen size. The same resolution spread out over a larger screen offers reduced sharpness. On the other hand, a high-resolution setting on a smaller surface will product a sharper image, but text readability will become more difficult.
Viewability includes the ability to see the screen image well from different angles. Displays with cathode ray tubes ( CRT ) generally provide good viewability from angles other than straight on. Flat-panel displays, including those using light-emitting diode and liquid crystal display technology, are often harder to see at angles other than straight on.
The Size of the ScreenOn desktop computers, the display screen width relative to height, known as the aspect ratio , is generally standardized at 4 to 3 (usually indicated as "4:3"). Screen sizes are measured in either millimeters or inches diagonally from one corner to the opposite corner. Popular desktop screen sizes are 12-, 13-, 15-, and 17-inch. Notebook screen sizes are somewhat smaller.
The Projection TechnologyMost displays in current use employ cathode ray tube ( CRT ) technology similar to that used in most television sets. The CRT technology requires a certain distance from the beam projection device to the screen in order to function. Using other technologies, displays can be much thinner and are known as flat-panel displays . Flat panel display technologies include light-emitting diode (LED), liquid crystal display ( LCD ), and gas plasma. LED and gas plasma work by lighting up display screen positions based on the voltages at different grid intersections. LCDs work by blocking light rather than creating it. LCDs require far less energy than LED and gas plasma technologies and are currently the primary technology for notebook and other mobile computers.
Displays generally handle data input as character maps or bitmap s. In character-mapping mode, a display has a preallocated amount of pixel space for each character. In bitmap mode, it receives an exact representation of the screen image that is to be projected in the form of a sequence of bits that describe the color values for specific X and Y coordinates starting from a given location on the screen. Displays that handle bitmaps are also known as all-points addressable displays.