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Main » 2011 » February » 11 » Gurus Monitor Info
5:02 PM
Gurus Monitor Info

MONITOR INFORMATION

The size of an approximately rectangular display is usually given as the distance between two opposite screen corners, that is, the diagonal of the rectangle. One problem with this method is that it does not take into account the display aspect ratio, so that for example a 16:9 21 in (53 cm) widescreen display is far less high, and has less area, than a 21 in (53 cm) 4:3 screen. The 4:3 screen has dimensions of 16.8 × 12.6 in (43 × 32 cm) and area 211 sq in , while the widescreen is 18.3 × 10.3 in (46 × 26 cm), 188 sq in (1,210 cm2). For many purposes the height of the display is the main parameter; a 16:9 display needs a diagonal 22% larger than a 4:3 display for the same height.

This method of measurement is inherited from the method used for the first generation of CRT television, when picture tubes with circular faces were in common use. Being circular, only their diameter was needed to describe their size. Since these circular tubes were used to display rectangular images, the diagonal measurement of the rectangle was equivalent to the diameter of the tube's face. This method continued even when cathode ray tubes were manufactured as rounded rectangles; it had the advantage of being a single number specifying the size, and was not confusing when the aspect ratio was universally 4:3.

A problematic practice was the use of the size of a monitor's imaging element, rather than the size of its viewable image, when describing its size in publicity and advertising materials. On CRT displays a substantial portion of the CRT's screen is concealed behind the case's bezel or shroud in order to hide areas outside the monitor's "safe area" due to overscan. These practices were seen as deceptive, and widespread consumer objection and lawsuits eventually forced most manufacturers to instead measure viewable size

Performance measurements

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

  • Luminance is measured in candelas per square meter (cd/m2 also called a Nit).
  • Viewable image size is measured diagonally. For CRTs, the viewable size is typically 1 in (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. Maximum resolution is limited by dot pitch.
  • Dot pitch is the distance between subpixels of the same color in millimeters. In general, the smaller the dot pitch, the sharper the picture will appear.
  • Refresh pitch  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 image. It is measured in degrees horizontally and vertically.

Comparison

CRT

Pros:

  • High dynamic range (up to around 15,000:1), excellent color, wide gamut and low black level. The color range of CRTs is unmatched by any display type except OLED
  • Can display natively in almost any resolution and refresh rate
  • No input lag
  • Sub-millisecond response times
  • Near zero color, saturation, contrast or brightness distortion. Excellent viewing angle
  • Usually much cheaper than LCD or Plasma screens.
  • Allows the use of light guns/pens

Cons:

  • Large size and weight, especially for bigger screens (a 20-inch unit weighs about 50 lb (23 kg))
  • High power consumption
  • Generates a considerable amount of heat when running
  • Geometric distortion caused by variable beam travel distances
  • Can suffer screen burn-in
  • Produces noticeable flicker at low refresh rates
  • Normally only produced in 4:3 aspect ratio (though some widescreen ones, notably Sony's FW900, do exist)
  • Hazardous to repair/service
  • Effective vertical resolution limited to 1024 scan lines.
  • Color displays cannot be made in sizes smaller than 7 inches (5 inches for monochrome). Maximum size is around 24 inches (for computer monitors; televisions run up to 40 inches).

LCD

Pros:

  • Very compact and light
  • Low power consumption
  • No geometric distortion
  • Little or no flicker depending on backlight technology
  • Not affected by screen burn-in
  • No high voltage or other hazards present during repair/service
  • More reliable than CRTs
  • Can be made in almost any size or shape
  • No theoretical resolution limit

Cons:

  • Limited viewing angle, causing color, saturation, contrast and brightness to vary, even within the intended viewing angle, by variations in posture.
  • Bleeding and uneven backlighting in some monitors, causing brightness distortion, especially toward the edges.
  • Slow response times, which cause smearing and ghosting artifacts. However, this is mainly a problem with passive-matrix displays. Current generation active-matrix LCDs have response times of 6 ms for TFT panels and 8 ms for S-IPS.
  • Only one native resolution. Displaying resolutions either requires a video scaler, lowering perceptual quality, or display at 1:1 pixel mapping, in which images will be physically too large or won't fill the whole screen.
  • Fixed bit depth, many cheaper LCDs are only able to display 262,000 colors. 8-bit S-IPS panels can display 16 million colors and have significantly better black level, but are expensive and have slower response time
  • Input lag
  • Dead pixels may occur either during manufacturing or through use.
  • In a constant on situation, thermalization may occur, which is when only part of the screen has overheated and therefore looks discolored compared to the rest of the screen.
  • Not all LCD displays are designed to allow easy replacement of the backlight
  • Cannot be used with light guns/pens

Plasma

Pros:

  • High contrast ratios (10,000:1 or greater,) excellent color, and low black level.
  • Virtually no response time
  • Near zero color, saturation, contrast or brightness distortion. Excellent viewing angle.
  • No geometric distortion.
  • Softer and less blocky-looking picture than LCDs
  • Highly scalable, with less weight gain per increase in size (from less than 30 in (760 mm) wide to the world's largest at 150 in (3,800 mm)).

Cons:

  • Large pixel pitch, meaning either low resolution or a large screen. As such, color plasma displays are only produced in sizes over 32 inches.
  • Image flicker due to being phosphor-based
  • Heavy weight
  • Glass screen can induce glare and reflections
  • High operating temperature and power consumption
  • Only has one native resolution. Displaying other resolutions requires a video scaler, which degrades image quality at lower resolutions.
  • Fixed bit depth. Plasma cells can only be on or off, resulting in a more limited color range than LCDs or CRTs.
  • Can suffer image burn-in. This was a severe problem on early plasma displays, but much less on newer ones
  • Cannot be used with light guns/pens
  • Dead pixels are possible during manufacturing

Problems

Phosphor burn-in

Phosphor burn-in is localized aging of the phosphor layer of a CRT screen where it has displayed a static image for long periods of time. This results in a faint permanent image on the screen, even when turned off. In severe cases, it can even be possible to read some of the text, though this only occurs where the displayed text remained the same for years.

Burn-in is most commonly seen in the following applications:

  • Point-of-service applications
  • Arcade games
  • Security monitors

Screensavers were developed as a means to avoid burn-in, which was a widespread problem on IBM Personal Computer monochrome monitors in the 1980s. Monochrome displays are generally more vulnerable to burn-in because the phosphor is directly exposed to the electron beam while in color displays, the shadow mask provides some protection. Although still found on newer computers, screen savers are not necessary on LCD monitors.

Phosphor burn-in can be "fixed" by running a CRT with the brightness at 100% for several hours, but this merely hides the damage by burning all the phosphor evenly. CRT rebuilders can repair monochrome displays by cutting the front of the picture tube off, scraping out the damaged phosphor, replacing it, and resealing the tube. Color displays can theoretically be repaired, but it is a difficult, expensive process and is normally only done on professional broadcasting monitors (which can cost up to $10,000).

Plasma burn-in

Burn-in re-emerged as an issue with early plasma displays, which are more vulnerable to this than CRTs. Screen savers with moving images may be used with these to minimize localized burn. Periodic change of the color scheme in use also helps.

Glare

Glare is a problem caused by the relationship between lighting and screen or by using monitors in bright sunlight. Matte finish LCDs and flat screen CRTs are less prone to reflected glare than conventional curved CRTs or glossy LCDs, and aperture grille CRTs, which are curved on one axis only and are less prone to it than other CRTs curved on both axes.

If the problem persists despite moving the monitor or adjusting lighting, a filter using a mesh of very fine black wires may be placed on the screen to reduce glare and improve contrast. These filters were popular in the late 1980s. They do also reduce light output.

A filter above will only work against reflective glare; direct glare (such as sunlight) will completely wash out most monitors' internal lighting, and can only be dealt with by use of a hood or transreflective LCD.

Color misregistration

With exceptions of correctly aligned video projectors and stacked LEDs, most display technologies, especially LCD, have an inherent misregistration of the color channels, that is, the centers of the red, green, and blue dots do not line up perfectly. Sub-pixel rendering depends on this misalignment; technologies making use of this include the Apple II from 1976, and more recently Microsoft  and XFree86 

Incomplete spectrum

RGB displays produce most of the visible color spectrum, but not all. This can be a problem where good color matching to non-RGB images is needed. This issue is common to all monitor technologies that use the RGB model. Recently, Sharp introduced a four-color TV (red, green, blue, and yellow) to improve on this.

Display interfaces

Computer terminals


TTL monitors


Virtual displays

Much software and video hardware supports the ability to create additional, virtual pieces of desktop, commonly known as workspaces. Spaces is Apple's implementation of virtual displays.


Category: Hardware Blog | Views: 466 | Added by: seniorkoa | Gurus Rating: 5.0/1
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