Monitor Technology Explained


Monitor types

CRT (cathode ray tube) monitors

CRT (cathode ray tube) monitors are a now archaic display technology that was popular even into the 21st century. A cathode ray tube contains multiple electron guns, which fire electrons through a vacuum onto phosphor "pixels". Three colors of phosphor "pixels" are present (red, green and blue), and deflection by a magnetic field determines which ones will be lit. Although color reproduction and contrast were excellent in later models of CRT monitor, modern LCD monitors (see below) are vastly thinner and lighter, whilst providing outstanding contrast ratios, good color reproduction and response times. A list of now-redundant terms related to CRT monitors can be found in the final section of this PC Monitors article.

LCD (liquid crystal display) monitors

LCD screens are the current standard of display for most PC monitors, TV screens and electronic devices such as digital cameras, mobile phones and MP3 players. LCD PC monitors usually contain two polarising filters with liquid crystal filled cells or pixels. A backlight creates light which passes through the first filter, whilst electrodes regulate a current which passes through the liquid crystals and determines their alignment. The electrodes regulate the alignment of the crystals, determining the light color and intensity of the image.

OLED (organic light emitting diode) monitors

OLED (organic light emitting diode) is an emerging screen technology which is yet to make it into the PC monitor mainstream due mainly to high costs currently associated with OLED monitor manufacture. OLED monitors use the principle of electroluminescence; using materials which glow when a current is applied, rather than relying on a backlight. This means that the monitors are much thinner and lighter, have an unmatched contrast ratio, color reproduction and response time and can even be made flexible. Although this technology isn't currently used on PC monitors, smaller screens such as those on high-end touch screen phones, digital cameras and the beautiful 11-inch Sony XEL-1 TV feature OLED technology.

PC monitor terminology

Monitor backlight

Backlights are used in LCD monitors to illuminate the liquid crystals, as explored previously. There are two main types of backlight. The most common type in the PC monitor is a CCFL (cold cathode fluorescent lamp) backlight, which does a very good job at illuminating the screen to various intensities.. CCFL lamps are good at illuminating the screen to various intensities, and in the case of WCG (wide color gamut) CCFL lamps - provide a broad color gamut of up to 96% NTSC color space.

Some manufacturers use an alternative type of backlight, involving the use of colored (red, green and blue) LEDs (light emitting diodes) to illuminate the screen. Because the intensity of LEDs can be individually controlled (as well as with high precision and evenness), variable contrast can be used across the screen and superior contrast can theoretically be obtained. LED backlights are also considerably more efficient, are mercury free and provide lower heat output than CCFL lamps. Using RGB LEDs also broadens the monitor's color gamut considerably (taking it to up to 114% of the NTSC color space).

An alternative technology is the use of highly efficient 'white' LEDs around the border of the screen (usually referred to as 'edge-lit'). A real advantage of edge-lit LED technology over any behind-the-screen backlighting is that you can create screens that are considerably thinner and lighter. The purity of light and responsiveness to various light intensities can also provide a contrast and perceived luminance advantage over CCFL backlighting, although the actual color gamut is not typcially extended beyond that of regular CCFL lamps. Without a suitably high color depth (as provided by PVA/IPS and other more expensive panels), it is worth nothing that a broad-gamut backlight offers little advantage.

Colour gamut

This is the range of colors, from the visible spectrum, that a PC monitor is capable of producing. The image below shows the colors of the visible spectrum, with triangles representing NTSC (national television system committee; i.e. the theoretical maximum color gamut of images broadcast on TV) and the typical color gamut of CCFL backlit monitors, white LED backlit monitors and red-green-blue (RGB) LED backlit monitors. Although not shown in the image, typical future OLED PC monitors will most likely be represented by a significantly larger triangle - representing an exceptionally broad color gamut.

Colour depth

The color depth refers to the number of colors a monitor can process and display, based on internal processing.Most modern monitors are twisted nematic (TN) panels.TN panels are capable of processing 6-bits per pixel (3x6= 18-bits in total, or 262,144 colors). Dithering is used to display a slightly different shade of a color each refresh of the screen, and therefore the apparent color depth approaches 24-bit color (16.7 million colors). PVA and IPS LCD panels are capable of outputting 8-bits or even 10-bits per pixel, and can therefore transition 24-bit or 30-bit color (although actual output will depend on the quality of the backlight as well). In the future, OLED monitors should surpass this color depth, as the pixels emit light directly.

N.B. 32-bit color, as used in Windows, is not a true color depth. It represents 24-bit color with an additional 8-bits of non-color data (alpha, z, bump data etc.).

Contrast ratio

The contrast ratio is a measure of the relationship between the intensity of the brightest white and the darkest black a monitor can display. Because manufacturers of PC monitors seem to use their own 'unique' way of measuring the contrast ratio, however, figures are often overstated and not comparable to the figures used by other manufacturer. Additionally, some manufacturers have begun flinging about huge "dynamic contrast ratio" numbers as well. Whereas the static contrast ratio is a measure of the ratio of the darkest black to brightest white displayed on the monitor at any given time, dynamic contrast is a measure of the temporal intensity difference. This effect is often amplified by varying the intensity of individual backlighting elements. Different areas of the screen will therefore be illuminated to different intensity (this is especially effective with LED backlighting), resulting in a much broader contrast ratio. The main problem with how dynamic contrast ratios have been implemented so far is that the overall effect is unnatural and even painful to look at - most users will disable the dynamic contrast option on their monitor. Most monitors have poorly implemented their dynamic contrast ratios, making the viewing experience unpleasant - users will often disable this feature. Since OLED monitors have no backlight and the pixels emit light directly; insane contrast ratios will actually be meaningful and will look more natural without a backlight interfering.

Luminance or brightness

Luminance is measured in cd/m2 (candelas per sq. meter) and is an expression of the level of light emitted by a PC monitor Usual values for modern moderns are around 250-300cd/m2, which are very respectable and more than adequate. LED-backlit monitors may have a luminance exceeding 350cd/m2 and PC monitors of the future, such as OLED monitors, will surely increase this figure further.

Display resolution

The display resolution of a PC monitor generally refers to the number of pixels displayed in the horizontal by vertical dimensions. For a CRT monitor, this number can be varied mechanically by the monitor itself and therefore the display resolution is variable. For an LCD or OLED monitor, the resolution is fixed by the number of pixels laid out horizontally and vertically inside the monitor, and is referred to as the optimal or native resolution.

Response time

The response time is an indication of the time, in milliseconds, for the pixels of an LCD or OLED monitor to transition from one state to another. A faster response time in a monitor means a more fluid image with less trailing or "ghosting", which used to be a problem with the earlier generations of LCD displays. Traditionally, the response time was indicative of the time it took for a PC monitor to transition from "on" (white) to "off" (black) state and then back again. In 2005, however, it was deemed more useful for manufacturers to state a "grey to grey" response time; the time it takes to transition from one shade of grey to another. This is more representative of a real-world scenario as a pixel will rarely switch from an on to an off state and back again.
The response times (grey-to-grey) of modern PC monitors are usually around 2-5ms. This makes them perfect for watching movies and playing games, and you'd be hard-pushed to notice any distracting trailing or ghosting. Despite this, the response times are being pushed even further as technology improves. OLED monitor response times, for example, are expected to be around 0.01ms or even lower.

Screen size

The screen size refers to the diagonal size of the screen, usually in inches, from the top of one corner to the opposite bottom corner. For CRT monitors, this measurement includes the casing of the PC monitor and another (lower) figure for the "viewable area". For LCD monitors, this figure traditionally only referred to the viewable area of the screen (i.e. inside the bezel) - but many manufacturers have reverted to measuring the entire screen size to bump up the numbers.

Aspect ratio

A measure of the horizontal by vertical screen size. Traditional square monitors have an aspect ratio of 5:4, whereas most widescreen PC monitors have an aspect ratio of 16:9 or 16:10.

Viewing angle

This is the angle around which the screen can be viewed without the image becoming considerably altered.. Early LCD monitors suffered from fairly limited viewing angles, and even small off-centre viewing was impossible. Modern LCD monitors have much wider viewing angles, usually of around 120-170 degrees (perhaps slightly higher for PVA and IPS panels|even higher for PVA/IPS panels}) and screens of the future should be viewable from most angles in front of the screen without distortion.

Outdated technology

Refresh rate

For CRT monitors, refresh rate can determine the likelihood of your monitor giving you eyestrain and/or a headache. It is a measurement, in Hz, of the number of times pixels on the screen are drawn in a second. If the refresh rate is too low (generally below 85Hz) flickering ensues and the associated headaches and eyestrain follows. Although LCD monitors still have a "refresh rate", it is only important for specialist applications (for example 3D viewing using shutter glasses) and if you wish to disable v-sync without the commonly associated tearing. Because LCD monitors contain liquid crystals which merely act as shutters against a backlight, this flickering phenomenon does not occur, even if the refresh rate of the monitor is a seemingly low 60Hz.

Shadow mask

A sheet of thin metal with tiny holes in it is referred to as a shadow mask. This was a popular technology for traditional CRT monitors, where three electron guns would fire electrons through the holes and focus them at a specific point on the phosphor surface. Unwanted electrons are therefore shadowed and the phosphors which are lit up are precisely controlled.

Aperture gril

This was the technology of choice for Sony Trinitron CRT monitors. Electrons are fired through tiny vertical wires (which make up an aperture grill) and are focussed on the phosphor screen to illuminate it. This technology allowed a flat screen for a flat screen that is less likely to produce eyestrain.

Slot mask

Slot masks are a combination of aperture grills and shadow masks, and were a less common PC monitor technology. Slot mask monitors consist of vertically aligned slots rather than small holes, which improves brightness by increasing electron transmission.

Dot pitch

This is the distance, in millimetres, between two phosphors of the same color. A screen with a lower dot pitch therefore gives a sharper image. The method of which dot pitch is measured differs between aperture grill and shadow mask monitors. For aperture grills it is the horizontal distance between two {like-colored phosphor "stripes"|phosphor stripes of the same color (and therefore sometimes referred to as 'stripe pitch'), whereas for shadow mask monitors it is the diagonal distance between two phosphor dots of the same color.

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