A monitor (also referred to as a visual display unit) is an electronic visual display for computers. The monitor encompasses the display device, circuitry, and an enclosure. The display device in contemporary monitors is usually a thin film transistor liquid crystal display thin panel, while older monitors use a cathode ray tube.

At first, computer monitors were used for data processing and television receivers for entertainment. It is now becoming more common for computers to be used both for data processing and entertainment as well. Displays that are made exclusively for data use generally have an aspect ratio of 4:3 while ones used solely for entertainment are usually 16:9 widescreen. The size of an approximately rectangular display is generally expressed as the distance between two converse screen corners, or simply the diagonal of the rectangle. However, one problem with this is that it does not take into account the display aspect ratio. For example, a 16:9 21 inch widescreen display is far less high and has less area than a 21 inch 4:3 screen. The 4:3 screen has dimensions of 16.8 × 12.6 inches and an area of 211 sq inches, while the widescreen is 18.3 × 10.3 inches, and is 188 sq inches in area.

The performance of a monitor is measured by many different parameters. Luminance is measured in candelas per square meter; viewable image size is measured diagonally; aspect ratios are the ratios of the horizontal length to the vertical length; display resolution is the number of distinct pixels in each dimension that can be displayed; dot pitch is the space in millimeters between same color subpixels; refresh rate is the number of times in a second that a display is illuminated; response time is the amount of time in milliseconds a pixel in a monitor takes to go from active to inactive and vice versa; contrast ratio is the ratio of the luminosity of white to black that the monitor can produce; power consumption is measured in watts; and the viewing angle is the maximum angle that images on the monitor can be viewed without distorting it.

Different types of monitors such as CRT, LCD, and plasma can be compared to one another with benefits and disadvantages. CRTs have a high dynamic range with outstanding color and can display in almost any resolution and refresh rate with sub-millisecond response times and virtually no color, saturation, contrast, or brightness distortion. CRTs do not have input lag and are much cheaper than other screens. However, they do have a large size and weight, as well as high power consumption and noticeable flicker at low refresh rates. LCD screens are great because they are light and compact, have low power consumption, and little to no flicker depending on backlight technology. On the contrary, they tend to have a limited viewing angle, slow response times, and input lag. Plasma monitors are popular due to their high contrast ratios, high speed response, good viewing angles and colors, as well as the fact that they are light and compact. However, plasma monitors have problems such as large pixel pitch, high operating temperature and power consumption, input lag, and only one native resolution.

Many monitors have different and varying digital displays. Early digital monitors are sometimes known as TTLs due to the voltages on the red, green, and blue inputs being compatible with TTL logic chips. Later digital monitors support LVDS, or TMDS protocols. The disadvantage of TTL monitors, however, was the limited number of colors available because of the low number of digital bits used for video signaling. TTL Monochrome monitors only used five of the nine pins it had. One pin was used as a ground, while two pins were used for horizontal/vertical synchronization. The electron gun was controlled by two separate digital signals, a video bit, and an intensity bit to control the brightness of the drawn pixels, thus making only four shades visible.

CGA monitors used four digital signals to manage the three electron guns used in color CRTs. Each of the three RGB colors can be switched on or off autonomously. The intensity bit intensifies the brightness of all the guns that are turned on. A CGA monitor is only capable of rendering 16 colors. Also, many CGA monitors were capable of displaying composite video through a separate jack. On the other hand, EGA monitors used six digital signals to control the three electron guns. Contrasting CGA, each gun is allocated its own intensity bit. This allowed each of the three primary colors to have four different states resulting in 64 colors. Some monitors are single color screens. In the 1980s, display colors other than white were popular on monochrome monitors because they were more comfortable on the eye. Green screens were the most popular color, with paper white amber displays also available. Most modern computer displays have the ability to show the various colors of the RGB color space by shifting between red, green, and blue analog video signals. Today, digital monitors are being engineered to only have a digital video interface.

Conveniently, more than one monitor can be attached to the same device. Each display can operate in two basic configurations called extension and mirroring. Mirroring is when at least two displays are showing the same image. This is the most straightforward thing and commonly used for presentations. Hardware with only one video output can be tricked into mirroring with an external splitter device. Extension is the more sophisticated of the two. It allows each monitor to exhibit a different image to form adjacent areas of arbitrary shape. Along the same lines, multiple devices can be connected to the same monitor using a video switch, usually in the form of a Keyboard Video Mouse switch, which is made to simultaneously switch all of the user interface devices for a workstation between different computers.

Most modern monitors have other convenient features, such as a power-saving mode (if no video-input signal is received for a period of time), glossy displays (for eliminating glares), and touch screens (used for alternate input). Major manufacturers of monitors include Acer, Apple Inc., Dell, Gateway, Hewlett-Packard, Samsung, Toshiba, and ViewSonic.

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