CRT (cathode ray tube) monitors are a now redundant type of PC monitor which were popular even in the early years of the 21st century. A cathode ray tube contains one or more electron guns, which fire electrons through a vacuum onto phosphor “pixels”. Three colours of phosphor “pixels” are present (red, green and blue), and which are lit depends on the deflection of the electrons by a magnetic field. Although colour 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 colour 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. For more information about cathode ray tubes and CRT monitors.
LCD (liquid crystal display) monitors 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 generally consist of liquid crystal filled pixels sandwiched between two polarising filters. A backlight creates light which passes through the first filter, whilst electrodes regulate a current which passes through the liquid crystals and causes them to align in a certain way. The alignment of these crystals, as regulated by the electrodes, determines the intensity and colour of light which will pass through to the second filter and will be outputted onto the screen. Together, these liquid crystal filled pixels make up the image on the screen of the PC monitor.
Almost all modern LCD monitors use TFT (thin-film transistor) technology, and most are of the Twisted Nematic (TN) panel type. For more information about TFT technology and TFT LCD monitors, including PVA and IPS panels.
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, colour 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 (featured in the video below) feature OLED technology.
For more information about OLED and other future technology, please read our future technology article. If you would like more information than you’ll ever need to know about OLED technology, specifically, please read the PC Monitors OLED article.
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. A recent extension to this technology, which has seen use mainly in high-end professional displays is so-called “WCG-CCFL” or wide colour gamut CCFL backlighting. This broadens the colour gamut and is therefore desireable for work requiring a broad colour range (from around 75% of NTSC to up to 96% of NTSC colour space).
An alternative type of backlight involves the use of coloured (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. The use of RGB LEDs also broadens the colour gamut considerably, taking it beyond NTSC (up to about 114% NTSC). The improved efficiency of LED backlights over CCFL lamps, lower heat emissions and mercury-free status are all attractive benefits.
Similar environmental benefits are also bought about at reduced cost by using ‘white’ LED strips around the bezel of the monitor and reflecting the light inwards – rather than placing RGB LED units behind the screen. This technology is referred to as ‘edge-lit’ and is becoming increasingly popular in TV screens and PC monitors. 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 colour gamut is not typcially extended beyond that of regular CCFL lamps. It is important to note that although the backlighting itself can provide a “wide colour gamut”, the accuracy of these colours and what will actually be seen by the user is still determined by the quality of the filter (i.e. the underlying technology of the panel).
The colour gamut of a PC monitor describes the range of colours the monitor is capable of producing, from a given predefined subset of colours (i.e. the visible spectrum). The image below shows the colours of the visible spectrum, with triangles representing NTSC (national television system committee; i.e. the theoretical maximum colour gamut of images broadcast on TV) and the typical colour 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 colour gamut.
The colour depth represents the bits (i.e. number of colours) displayed by the monitor. Most modern LCD monitors are based on a technology called twisted nematic (TN), which is capable of producing 6-bits per pixel (6 green plus 6 red plus 6 blue= 18-bit colour or 262,144 colours). This colour depth is artificially enhanced using a dithering method which allows a slightly different shade of a colour to be displayed per refresh of the screen. This allows for an apparent colour depth approaching 24-bit colour (16, 777, 216), or ‘truecolor’ to be displayed by an LCD PC monitor. PVA and IPS LCD panels can output 8-bits or even 10-bits per pixel, and can therefore product transition 24-bit or 30-bit colour (although actual output will depend on the quality of the backlight as well). Since OLED pixels emit light directly and can be very precisely controlled; PC monitors of the future will almost certainly surpass this colour depth without using any sort of dithering method.
N.B. 32-bit colour, as used in Windows, is not a true colour depth. It represents 24-bit colour with an additional 8-bits of non-colour data (alpha, z, bump data etc.).
The contrast ratio is a measure of the relationship between the intensity of the brightest white and the darkest black a monitor can display. A higher contrast ratio tends to indicate deeper blacks but also reflects, by definition, relatively bright whites. Although there are other factors at play as well the distinctiveness of bright colours vs. dark colours is also enhanced by strong contrast and certain colours will tend to ‘pop out’ more.
When it comes to measuring contrast ratio and comparing the figures that the manufacturers provide there are some important things that must be taken into consideration. Because manufacturers of PC monitors seem to use their own ‘unique’ way of measuring the contrast ratio figures are often overstated and not comparable to the figures used by other manufacturers. To make matters worse, manufacturers have recently been stating ‘dynamic contrast ratios’ as well as ‘static contrast’ ratios. 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 (at the same backlight intensity), dynamic contrast is a measure of the difference in recorded luminance over time – the backlight intensity is allowed to vary. Under a dynamic contrast operating mode the manufacturer will record the white point luminance (backlight goes to full intensity) and the black point luminance (backlight goes to practically off) of an entirely white scene and entirely black scene, respectively. This effect and the resulting contrast ratios are often amplified by the use of LED backlighting as they can be rapdidly dimmed to a state of ‘practically off’ without flickering for the black scene. In real world usage you are rarely presented with entirely dark scenes or entirely white scenes and you are instead met with a complex intertwining of the two extremes, so really adjusting the backlight intensity to suit the overall image is an unwelcome compromise. The main problem with how dynamic contrast ratios have been implemented so far is that either; the overall effect is unnatural and the backlight intensity shifts very noticeable or the intensity shifts are so subtle that they can’t keep pace with the changing scenes and end up doing the whole experience more harm than good. To that end, most users will disable the dynamic contrast option on their monitor.
With upcoming technologyies that do away with the backlight, such as OLED monitors, the static contrast ratio will technically be the same value as the dynamic contrast ratio and it should be then that we see high contrast ratios worth paying attention to. In our reviews the contrast ratio referred to will be that stated by the manufacturer, but contrast will also be used more broadly as a relative term (i.e. a monitor has excellent contrast or poor contrast compared to other monitors tested).
Measured in candelas per metre squared (cd/m2), this is a measure of the amount of light a monitor is capable of emitting. Most modern PC monitors have values of around 250-300cd/m2, which are very respectable and more than adequate. LED-backlit monitors may exceed this value and approach 350cd/ m2 and PC monitors of the future, such as OLED monitors, will undoubtedly increase this figure further.
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. Because display resolution is not relative to a fixed dimension (for example the DPI or dots per inch printed by a printer), resolutions for screens of different sizes cannot be directly compared.
For a more detailed look at display resolution, and how PC monitors compare to TV monitors in this regard.
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 a measure of the time it took for a PC monitor to transition from “on” (white) to “off” (black) and then back again (the Tr + Tf or ISO response time). 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 (or colour) to another. This is more representative of a real-world scenario whereby a pixel will rarely switch from an on to an off state and back again. Confusingly, however, some manufacturers still state the Tr + Tf response time for some new panels and grey to grey for others.
By applying a surge of extra voltage when needed (a technique called ‘Overdrive’), the response times (grey-to-grey) of modern PC monitors can go as low as around 2-5ms. It’s worth remembering that these are still usually ‘best case’ response times for a particular colour transition that a monitor is good at performing – there is no industry-wide standard of measurement so these figures are often very poor indicators of real-world responsiveness. The Tr + Tf response time may go up to 30ms on a monitor monitor (but is typically below 14ms on a TN panel) and can help give an indication of how likely the monitor is to display trailing in dark scenes in particular – Overdrive techniques can not improve such transitions as they generally request the largest voltage surge possible ‘by default’. Although things have already come a long way; response times are being pushed even further as technology evolves. OLED monitor response times, for example, are expected to be around 0.01ms or lower.
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. The measurement of viewable area is shown in the image below.
The viewing angle refers to the angle, in degrees, around which the monitor can be viewed without the image becoming “considerably distorted”. Early LCD monitors suffered from fairly poor viewing angles, meaning that unless you were directly in front of the screen colours would become more washed out and faded and text would become unreadable. Modern LCD monitors have much wider viewing angles, usually around 120-170 degrees (perhaps slightly higher for PVA and IPS panels) and future PC monitors should be viewable from pretty much anywhere in front of the monitor (above, below and to the side) without distortion.
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.
A thin metal screen filled with very small holes is referred to as a shadow mask. This technology is found in PC monitors which contain three electron guns, which fire electrons through the holes to focus on a specific point on a CRT monitor’s phosphor surface. Unwanted electrons are therefore shadowed and the phosphors which are lit up are precisely controlled.
The once popular Sony Trinitron series of PC monitors use aperture grills, as opposed to the shadow mask technology described above. Miniscule vertical wires make up the aperture grill, through which electrons are fired to illuminate the phosphor screen. This technology allowed a flat screen to be used so that the image is less distorted and is generally kinder on the eyes.
Slot masks are a combination of aperture grills and shadow masks, and were a less common PC monitor technology. Slot mask monitors employ vertically aligned slots rather than small holes, increasing electron transmission and thus improving screen brightness.
The dot pitch, measured in millimetres, is a measure of the distance between two phosphors of the same colour. A PC monitor with a lower dot pitch therefore gives a sharper image. The way in which dot pitch is measured differs between aperture grill and shadow mask monitors. For aperture grills it is the horizontal distance between two like-coloured phosphor “stripes” (and therefore sometimes referred to as ‘stripe pitch’), whereas for shadow mask monitors it is the diagonal distance between two like-coloured phosphor dots. Modern LCD monitors have a similar measurement between pixels known as the ‘pixel pitch’.