Computer monitor

A monitor or display (sometimes called a visual display unit) is a piece of electrical equipment which displays images generated by devices such as computers, without producing a permanent record. The monitor comprises the display device, circuitry, and an enclosure. The display device in modern monitors is typically a thin film transistor liquid crystal display (TFT-LCD), while older monitors use a cathode ray tube (CRT).

Screen size

Main articles: Viewable image size and Computer display standard

The size of a display is usually given as the distance between two opposite screen corners. One problem with this method is that it does not distinguish between the aspect ratios of monitors with identical diagonal sizes, in spite of the fact that a shape of a given diagonal span's area decreases as it becomes less square. For example, a 4:3 21 inches (53.3 cm) monitor has an area of about 211 square inches (1,361 cm²), while a 16:9 21" widescreen has an area of only about 188 square inches (1,213 cm²).

For any rectangular section on a round tube, the diagonal measurement is also the diameter of the tube

This method of measurement is from the first types of CRT television, when round picture tubes were in common use. Being circular, they only needed to use their diameter to describe their tube size. When round tubes were used to display rectangular images, the diagonal measurement was equivalent to the round tube's diameter. This method continued even when CRT tubes were manufactured as rounded rectangles.

Another problematic practice was the direct measurement of a monitor's imaging element as its quoted size in publicity and advertising materials.Especially on CRT displays, a substantial portion of the imaging element is concealed behind the case's bezel or shroud in order to hide areas outside the monitor's safe area due to overscan. Seen as deceptive, widespread consumer objection and lawsuits eventually forced most manufacturers to instead measure viewable size.

Imaging technologies

19" inch (48.3 cm tube, 45.9 cm viewable) ViewSonic CRT computer monitor.

As with television, many hardware technologies exist for displaying computer-generated output:

• Liquid crystal display (LCD). TFT LCDs are the most popular display device for computers.
  • Passive LCDs are noted for poor contrast and slow response. They were used in laptops until the mid 1990s.
  • Thin film transistor. Nearly all modern LCD monitors are TFTs.
• Cathode ray tube (CRT)
  • Raster scan computer monitors produce images using pixels. These were the most popular display device for older computers.
  • Vector displays, as used on the Vectrex, scientific and radar applications, and several early arcade machines such as Asteroids use CRT displays because of requirement for a deflection system, although a raster-based display may be used.
  • Television sets were used by most early personal and home computers, connecting composite video to the television set using a modulator. Resolution and image quality were limited by the display capabilities of television.
  • Penetron - military aircraft displays
• Plasma display
• Video projectors use CRT, LCD, DLP, LCoS, and other technology to emit light to a projection screen. Front projectors use screens as reflectors to send light back, while rear projectors use screens as diffusers to refract light forward. Rear projectors are often integrated into the same case as their screen.
• Surface-conduction electron-emitter display (SED) and field emission display (FED)
• Organic light-emitting diode (OLED) display
  

Cathode ray tube

Comparison of a 21" CRT TV monitor with a 17" CRT PC monitor

The CRT or cathode ray tube, is the picture tube of a monitor. The back of the tube has a negatively charged cathode. The electron gun shoots electrons down the tube and onto a charged screen. The screen is coated with a pattern of phosphor dots that glow when struck by the electron stream. Each cluster of three dots, one of each color, is one pixel.

The image on the monitor's screen is usually made up from at least tens of thousands of such tiny dots glowing on command from the computer. The closer together the pixels are, the sharper the image on screen can be. The distance between pixels on a computer monitor screen is called its dot pitch and is measured in millimeters. Most monitors have a dot pitch of 0.28 millimetres (0.011 in) or less.

Two electromagnets around the collar of the tube deflect the electron beam. The beam scans across the top of the monitor from left to right, is then blanked and moved back to the left-hand side slightly below the previous trace (on the next scan line), scans across the second line and so on until the bottom right of the screen is reached. The beam is again blanked, and moved back to the top left to start again. This process draws a complete picture, typically 50 to 100 times a second. The number of times in one second that the electron gun redraws the entire image is called the refresh rate and is measured in hertz (cycles per second). It is common in television or very early computer equipment, to use a technique called interlacing, in which all the odd-numbered lines of an image are traced, and then all the even-numbered lines; the circuitry of such an interlaced display need be capable of only half the speed of a non-interlaced display. An interlaced display, particularly at a relatively low refresh rate, can appear to some observers to flicker, and may cause eyestrain.

Performance measurements

The performance of a monitor is measured by the following parameters:

• Luminance is measured in candelas per square meter.
• Viewable image size is measured diagonally. For CRTs, the viewable size is typically one inch (25 mm) smaller than the tube itself.
• Aspect ratios is the ratio of the horizontal length to the vertical length. 4:3 is the standard aspect ratio, for example, so that a screen with a width of 1024 pixels will have a height of 768 pixels. If a widescreen display has an aspect ratio of 16:9, a display that is 1024 pixels wide will have a height of 576 pixels.
• Display resolution is the number of distinct pixels in each dimension that can be displayed. (It does not mean currently displayed.) Maximum resolution is limited by dot pitch.
• Dot pitch is the distance between pixels of the same color in millimeters. In general, the smaller the dot pitch, the sharper the picture will appear.
• Refresh rate is the number of times in a second that a display is illuminated. Maximum refresh rate is limited by response time.
• Response time is the time a pixel in a monitor takes to go from active (black) to inactive (white) and back to active (black) again, measured in milliseconds. Lower numbers mean faster transitions and therefore fewer visible image artifacts.
• Contrast ratio is the ratio of the luminosity of the brightest color (white) to that of the darkest color (black) that the monitor is capable of producing.
• Power consumption is measured in watts.
• Viewing angle is the maximum angle at which images on the monitor can be viewed, without excessive degradation to the images. It is measured in degrees horizontally and vertically.