Contrast Ratio
The Contrast Ratio of a TFT is the difference between the darkest black and the brightest white it is able to display. This is really defined by the pixel structure and how effectively it can let light through and block light out from the backlight unit. As a rule of thumb, the higher the contrast ratio, the better. The depth of blacks and the brightness of the whites are better with a higher contrast ratio. This is also referred to as the static contrast ratio.
When considering a TFT monitor, a contrast ratio of 700:1 to 1000:1 is pretty standard nowadays, but there are models which boast specs up to over 1000:1. Be wary of quoted specs however, as sometimes they can be exaggerated. VA panel specs are generally the most reliable and accurate to reality when considering contrast ratio. Figures of 3000:1 are now available using modern AMVA and cPVA panels.
Some technologies boast the ability to dynamically control contrast and offer much higher contrast ratios which are incredibly high (millions:1 for instance!). Be wary of these specs as they are dynamic only, and the technology is not always very useful in practice. Traditionally, TFT monitors were said to offer poor black depth, but with the extended use of VA panels, the improvements from IPS and TN Film technology, and new Dynamic Contrast Control technologies, we are seeing good improvements in this area. Black point is also tied in to contrast ratio. The lower the black point, the better, as this will ensure detail is not lost in dark image when trying to distinguish between different shades.
Brightness
Brightness is a measure of the brightest white the TFT can display. Typically TFT’s are far too bright for comfortable use, and the On Screen Display (OSD) is used to turn the brightness setting down. Brightness is measure in cd/m2 (candella per metre squared). Note that the recommended brightness setting for a TFT screen in normal lighting conditions is 120 cd/m2.
Colour Depth
The colour depth of a TFT monitor is related to how many colours it can produce and should not be confused with colour space (gamut). The more colours available, the better the colour range can potentially be. Colour reproduction is also different however as this related to how reliably produced the colours are compared with those desired.
Panel Colour Depth | Total Bits Per Colour | Steps Per Sub pixel | Total Colours |
6-bit | 18-bit | 64 | 262,144 |
6-bit + FRC | - | - | 16.2 million |
8-bit | 24-bit | 256 | 16.7 million |
8-bit + FRC | "30-bit" | 256 | 1.07 billion |
10-bit | 30-bit | 1024 | 1.07 billion |
The colour depth of a panel is determined really by the number of posible orientations of each sub pixel (red, blue and green). These different orientations basically determine the different shade of grey (or colours when filtered in the specific way via RGB sub pixels) and the more "steps" between each shade, the more possible colours the panel can display.
At the lower end, TN Film panels are quite economical, and their sub pixels only have 64 possible orientations each, giving rise to a true colour depth of only 262,144 (i.e. 64 steps on each RGB = 64 x 64 x 64 = 18). This is also referred to commonly s 18-bit colour (i.e. 6 bits per RGB sub pixel = 6 + 6 + 6) This colour depth is pretty limited and so in order to reach 16 million colours and above, panel manufacturers commonly use two technologies: Dithering and Frame Rate Control (FRC). These terms are often interchanged, but strictly can mean different things. These technologies simulate other colours allowing the colour depth to improve to typically 16.2 million colours.
- Spaital Dithering - The dithering method involves assigning appropriate color values from the available color palette to close-by pixels in such a way that it gives the impression of a new color tone which otherwise could not have been created at all. In doing so, there complex mappings according to which the ground colors are mutually assigned, otherwise it could result in color noise / dithering noise. Dithering can be used to allow 6-Bit panels, like TN Film, to show 16.2 million perceived colours. This can however sometimes be detectable to the user, and can result in chessboard like patterns being visible in some cases.
- Frame Rate Control / Temporal Dithering - The other method is Frame-Rate-Control (FRC), also referred to sometimes as temporal dithering. This works by combining four colour frames as a sequence in time, resulting in perceived mixture. In basic terms, it involves flashing between two colour tones rapidly to give the impression of a third tone, not normally available in the palette. This allows a total of 16.2 reproducible million colors. Thanks to Frame-Rate-Control, TN panel monitors have come pretty close to matching the colors and image quality of VA or IPS panel technology, but there are a number of FRC algorithms which vary in their effectiveness. Sometimes, a twinkling artefact can be seen, particularly in darker shades, which is a side affect of such technologies. Some TN Film panels are now quoted as being 16.7 million colours, and this is down to new processes allowing these panels to offer a better colour depth compared with older TN panels. See this articlefor more information.
Other panel technologies however can offer more possible pixel orientation and therefore more steps between each shade. VA and IPS panels are traditionally capable of 256 steps for each RGB sub pixel, allowing for a possible 16.7 million colours (true 8-bit, without FRC). These are referred to as 8-bit panels with 24-bit colour (8-bit per sub pixel = 8 + 8 + 8 = 24). While most IPS and VA panels support 8-bit colour, modern e-IPS and cPVA panels do sometimes use 6-bit + FRC instead. See this news piece for further information.
10-bit colour depth is typically only used for very high end graphics uses and in professional grade monitors. There are three main ways of implementing 10-bit colour depth support. Most screens which are advertised as having 10-bit support are actually using true 8-bit panels. There is an additional FRC stage added to extend the colour palette. This FRC can be applied either on the panel side (8-bit + FRC panels) or on the monitor LUT/electronics side. Either way, the screen simulates a larger colour depth and does not offer a 'true' 10-bit support. You can also only make use of this 10-bit support if you have a full end-to-end 10-bit workflow, including a supporting software, graphics card and operating system. There are a few 'true' 10-bit panels available but these are prohobitively expensive and rarely used at the moment. See our panel parts database for more information about different panels.
Experiments at the beginning of the last century into the human eye eventually led to the creation of a system that encompassed all the range of colors our eyes can perceive. Its graphical representation is called a CIE diagram as shown in the image above. All the colors perceived by the eye are within the colored area. The borderline of this area is made up of pure, monochromatic colors. The interior corresponds to non-monochromic colors, up to white which is marked with a white dot. 'White Colour' is actually a subjective notion for the eye as we can perceive different colors as white depending on the conditions. The white dot in the CIE diagram is the so-called flat spectrum dot with coordinates of x=y=1/3. Under ordinary conditions, this color looks very cold, bluish.
Above: CIE diagram showing total gamut range of the human eye
If we had three sources of different colours the question is which other colours can be made by mixing the sources? If you mark points with the coordinates of the basic colors in the CIE diagram, everything you can get by mixing them up is within the triangle you can draw by connecting the points. This triangle is referred to as a color gamut.
Above: gamut triangle of a laser display
Laser Displays are capable of producing the biggest color gamut for a system with three basic colors, but even a laser display cannot reproduce all the colors the human eye can see, although it is quite close to doing that. However, in today's monitors, both CRT and LCD (except for some models I’ll discuss below), the spectrum of each of the basic colors is far from monochromatic. In the terms of the CIE diagram it means that the vertexes of the triangle are shifted from the border of the diagram towards its center.
Above: sRGB colour space triangle
The colour space produced by any given TFT monitor is defined by its backlighting unit and is not influenced by the panel technology.
Traditionally, LCD monitors were capable of giving approximate coverage of the sRGB reference colour space as shown in the diagram above. This is defined by the backlighting used in these displays - Cold-cathode fluorescent lamps (CCFL) that are employed which emit radiation in the ultraviolet range which is transformed into white color with the phosphors on the lamp’s walls. These backlight lamps shine through the LCD panel, and through the RGB sub-pixels which act as filters for each of the colours. Each filter cuts a portion of spectrum, corresponding to its pass-band, out of the lamp’s light. This portion must be as narrow as possible to achieve the largest color gamut.
Traditional CCFL backlighting offers a gamut pretty much covering the sRGB colour space. However, the sRGB space is a little small to use as a reference in specifications for colour gamuts and so the larger NTSC colour space reference tends to be more commonly referred to nowadays. The sRGB space corresponds to approximately 72% of the NTSC colour space, which is a figure commonly used in modern specifications for standard CCFL backlit monitors. If you read the reviews here, you will see that analysis with colorimeters allows us to measure the colour gamut, and you can easily spot those screens utilising regular CCFL backlighting by the fact their gamut triangle is pretty much mapped to the reference sRGB triangle. The rRGB colour space is lacking most in green hues as compared with the gamut of the human eye.
Above Left: a typical measurement of a standard CCFL backlit monitor, covering pretty much the sRGB colour space, 72% of NTSC colour space
Above Right: a typical measurement of a monitor with enhanced CCFL backlighting, covering more than the sRGB colour space and about 92% of the NTSC space
Above Right: a typical measurement of a monitor with enhanced CCFL backlighting, covering more than the sRGB colour space and about 92% of the NTSC space
More recent displays have started to utilise a newer generation of CCFL backlighting, offering a widened gamut and typically a coverage of the NTSC colour space of 92 - 102%. There is a difference in practice, however. The colour space available is extended mainly in green shades as you can see from the image above. Red coverage is also extended in some cases. This extended colour space sounds appealing on face value since the screens featuring WCG-CCFL backlighting can offer a broader range of colours. Manufacturers will often promote the colour space coverage of their screens with these high figures. In practice you need to consider what impact this would have on your use.
It's important to consider what colour space your content is based around. sRGB has long been the preferred colour space of all monitors, and is in fact the reference for the Windows operating system and the internet. As such, most content an average user would ever use is based on sRGB. If you view sRGB content on a wide gamut screen then this can lead to some colours looking incorrect as they are not mapped correctly to the output device. In practice this can lead to oversaturation, and greens and reds can often appear false or neon. Colour managed applications and a colour managed workflow can prevent this but for the average user the cross-compatibility of widely used sRGB content and a wide gamut screen may present problems and prove troublesome. Some users don't object to the over saturated and cartoony colours for their use, but to many, it is an issue.
Of course the opposite is true if in fact you are working with content which is based on a wider colour space. In photography, the AdobeRGB colour space is often used and is wider than the sRGB reference. If you are working with wide gamut content, with wide gamut supported applications, you would want a screen that can correctly display the full range of colours. This could not be achieved using a traditional CCFL backlit display with only sRGB coverage, and so a wide gamut screen would be needed. Wide gamut displays are often aimed at colour enthusiasts and professional uses as a result.
A compromise is sometimes available in the form of a screen which can support a range of colour spaces accurately. Some higher end screens come with a wide gamut backlight unit. Natively these offer a gamut covering 92 - 102% of the NTSC colour space. However, they also feature emulation modes which can simulate a smaller colour space. These emulation modes are normaly available through the OSD menu and offer varying options with varying degrees of reliability. In the best cases the screens can emulate the smaller Adobe RGB colour space, and also the sRGB colour space. This allows the user to work in whichever colour space they prefer but gives them compatibility with a wider range of content if they have the need. The success of these colour space emulations will vary from one screen to another however and are not always accurate.
Further reading: X-bit Labs Article
LED backlighting units come in two flavours typically for desktop monitors, those being White-LED and RGB LED. With White-LED (W-LED) The LED's are placed in a line along the edge of the matrix, and the uniform brightness of the screen is ensured by a special design of the diffuser. The colour gamut is limited to around 68 - 72% NTSC but the units are cheaper to manufacturer and so are being utilised in more and more screens, even in the more budget range. They do have their environmental benefits as they can be recylced, and they have a thinner profile making them popular in super-slim range models and notebook PC's.
Above: colour gamut of a typical LED backlit display, covering 114% of the NTSC colour space
RGB LED backlighting consists of an LED backlight based on RGB triads, each triad including one red, one green and one blue LED. With RGB LED backlighting the spectrum of each LED is rather wide, so their radiation can’t be called strictly monochromatic and they can’t match a laser display, yet they are much better than the spectrum of CCFL and WCG-CCFL backlighting. RGB LED backlighting is not common yet in desktop monitors, and their price tends to put them way above the budget of all but professional colour enthusiast and business users. We will probably see more monitors featuring RGB LED backlighting over the coming years, and these models are currently capable of offering a gamut covering > 114% of the NTSC colour space.
Further reading: LED Backlighting Article
Panel manufacturer AU Optronics has successfully developed and commercialized HiColor technology utilizing CCFL backlight to reach a 33% color saturation increase compared with conventional LCDs. Targeting the development trend of LEDs, AUO has further developed the new HiColor Technology with RGB LED backlight. HiColor Technology with LED backlight can reach 105% NTSC, a 45% increase from 72% NTSC. It also provides the true natural performance of Red, Green and Blue and enables bright, rich, and vivid display colors.
In addition, AUO has adopts several specific techniques to enhance the image performance of LED backlights. The Color Management function can eliminate the artificial colors caused by inconsistent chromaticity between the light source and the signal. The Flexible Color Temperature Setting can change the intensity of RGB LEDs to adjust the white point of backlights and meet different application requirements without much luminance loss.
Other advantages of LED backlights include mercury and lead free, instant light, low DC voltage, shock and vibration safe, fast response time, and low temperature start.
You will commonly see a monitor's gamut listed as a percentage compared with a reference colour space. This will vary depending on which reference a manufacturer uses, but commonly you will see a % against the NTSC or Adobe RGB colour spaces. Bear in mind also that the gamut / colour space of the sRGB standard equates to about 72 - 75% of the NTSC reference. This is the standard colour space for the Windows operating system and the internet, and so where extended colour spaces are produced from a monitor, considerations need to be made as to the colour space of the content you are viewing.
Here is how several of the colour spaces are linked:
NTSC (%) | Adobe RGB (%) |
72 | |
92 | 95 |
102 | 97.8 |
116 | 114 |
125 | 123 |
Viewing Angles
Viewing angles are quoted in horizontal and vertical fields and often look like this in listed specifications: 170/160 (170° in horizontal viewing field, 160° in vertical). The angles are related to how the image looks as you move away from the central point of view, as it can become darker or lighter, and colours can become distorted as you move away from your central field of view. Because of the pixel orientation, the screen may not be viewable as clearly when looking at the screen from an angle, but viewing angles of TFT’s vary depending on the panel technology used.
As a general rule, the viewing angles are IPS > VA > TN Film. The viewing angles are often over exaggerated in manufacturers specs, especially with TN Film panels where quoted specs of 160 / 160 and even 170 / 170 are based on overly loose measuring techniques. Be wary of 176/176 figures as these are often over exagerated specs for a TN Film panel and are based on more lapse measurement techniques.
In reality, IPS and VA panels are the only technologies which can truly offer wide viewing fields and are commonly quoted as 178/178. VA panels can show a colour / contrast distortion as you move slightly away from a central point. While most people do not notice this anomaly, others find it distracting. IPS panels do not suffer from this and are generally considered the superior technology for wide fields of view.
Further reading: Viewsonic's Whitepaper - Why Viewing Angle is a Key Element in Choosing an LCD
Refresh Rate
On a CRT monitor, the refresh rate relates to how often the whole screen is refreshed by a cathode ray gun. This is fired down the screen at a certain speed which is determined by the vertical frequency set in your graphics card. If the refresh rate is too low, this can result in flickering of the screen and is often reported to lead to head aches and eye strain. On a CRT, a refresh rate of 72Hz is deemed to be "flicker free", but generally, the higher the refresh rate the better.
TFT screens do not refresh in the same way as a CRT screen does, where the image is redrawn at a certain rate. A TFT monitor will only support refresh rates coming from your graphics card between 60Hz and 75Hz (ignoring modern 120Hz monitors for a moment). Anything outside this will result in a "signal out of range" message or similar. The “recommended” refresh rate for a TFT is 60hz, a value which would be difficult to use on a CRT. The “maximum” refresh rate of a TFT is 75hz, but sometimes if you are using a DVI connection the refresh is capped at 60hz anyway.
As a TFT is a static image, and each pixel refreshes independently, setting the TFT at 60hz does not cause the same problems as it would on a CRT. There is no cathode ray gun redrawing the image as a whole on a TFT. You will not get flicker, which is the main reason for having a high refresh rate on a CRT in the first place. The reason that 60Hz is recommended by all the manufacturers is that it is related to the vertical frequency that TFT panels run at. Some more detailed data sheets for the panels themselves clearly show that the operating vertical frequency is between about 56 and 64Hz, and that the panels 'typically' run at 60Hz (see the LG.Philips LM230W02 datasheet for instance - page 11). If you decide to run your refresh rate from your graphics card above the recommended 60Hz it will work fine, but the interface chip on the monitor will be in charge of scaling the frequency down to 60Hz anyway. The reason that some DVI connections are capped at 60Hz in Windows is that some DVI interface chips cannot scale the frequency properly and so the option to run above 60Hz is disabled. You may find that the screen looks better at 60Hz as you are avoiding the need for the interface chip to scale the resolution. Try it on both and see which you prefer, the monitor can handle either.
One thing which some people are concerned about is the frames per second (fps) which their games can display. This is related to the refresh rate of your screen and graphics card. There is an option for your graphics card to enable a feature called Vsync which synchronizes the frame rate of your graphics card with the operating frequency of your graphics card (i.e. the refresh rate). Without vsync on, the graphics card is not limited in it's frame rate output and so will just output as many frames as it can. This can often result in graphical anomalies including 'tearing' of the image where the screen and graphics card are out of sync and the picture appears mixed as the monitor tries to keep up with the demanding frame rate from the card. To avoid this annoying symptom, vsync needs to be enabled.
With vsync on, the frame rate that your graphics card is determined by the refresh rate you have set in Windows. Capping the refresh rate at 60hz in your display settings limits your graphics card to only output 60fps. If you set the refresh at 75hz then the card is outputting 75fps. What is actually displayed on the monitor might be a different matter though. You can measure the internal frame rate of your system using programs like 'fraps' and also some games report your frame rate. Remember, the frequency of the monitor is still being scaled down to 60Hz by the interface chip. If you are worried about frame rate in fast games then it is a good idea to try the refresh rate at 75Hz and see if you think it looks better. A lot of it could be based on placebo effect though, and if you have a decent graphics card which can handle a constant 60fps it might look just as good as if it were outputting 75fps. See which one you prefer.
One other thing to note for Overdrive (RTC) enabled monitors is that running a TFT outside of it's recommended refresh rate can lead to a deterioration in the performance of this technology and the panel responsiveness is adversely effected! Read the details here.
120Hz Monitors and LCD TV's
You will see more mention of higher refresh rates from both LCD televisions and now desktop monitors. It's important to understand the different technologies being used though and what constitutes a 'real' 120Hz and what is 'interpolated':
- Interpolated 120Hz and above - These technologies are the ones commonly used in LCD TV's where TV signal input is limited to 50 / 60 Hz anyway (depending on PAL vs NTSC). To help overcome the issues relating to motion blur on such sets, manufacturers began to introduce a technology to artificially boost the frame rate of the screen. This is done by an internal processing within the hadrware which adds an intermediate and interpolated (guessed / calculated) frame between each real frame, boosting from 50 / 60fps to 100 / 120 fps. This technology does actually offer a noticeable improvement in practice and is controlled very well. Some sets even have 240 and 360Hz technologies which operate in the same way, but with further interpolation and inserted frames. See here for further information.
- True 120HZ technology - to have a true 120Hz screen, it must be capable of accepting a full 120Hz signal output from a device (e.g. a graphics card). Because TV's are limited at the moment by their input sources they tend to use the above interpolation technology, but with the advent of 3D TV and higher frequency input sources, this will change. Desktop monitors are a different matter though as graphics cards can obviously output a true 120Hz if you have a decent enough card. Some models can accept a 120Hz signal but need different interfaces to cope (e.g. dual-link DVI). These monitors are also introduced with the development of 3D gaming so will no doubt become more and more mainstream. Again these offer obvious advantages in terms of gaming where a frame rate of >60fps can be properly displayed. It also helps improve any motion blur and produce smoother movement. It can also help reduce RTC related artefacts and overshoot which is an added bonus. As an example, see our review of the 120Hz Samsung SM2233RZ. Also, please see here for further information
Pixel Pitch
Unlike on CRT’s where the dot pitch is related to the sharpness of the image, the pixel pitch of a TFT is related to the distance between pixels. This value is fixed and the same for all TFT’s which are the same size. This is because a 17” TFT for instance will always be the same 17” viewable area, and will always have the same number of pixels (1280 x 1024). Pixel pitch is normally listed in the manufacturers specifcation. Generally you need to consider that the 'tighter' the pixel pitch, the smaller the text will be, and potentially the sharper the image will be. To be honest, monitors are produced with a sensible resolution for their size and so even the largest pixel pitches return a sharp images and a reasonable text size. Some people do still prefer the larger-resolution-crammed-into-smaller-screen option though, giving a smaller pixel pitch and smaller text. It's down to choice and ultimately eye-sight.
To calculate the pixel pitch of a given monitor size and resolution, you can use this useful pixel pitch calculator
This relates to the connection type from the TFT to your PC. Nearly all TFT’s come with an analogue connection, which is commonly referred to as D-sub or VGA. This allows a connection from the VGA port on your graphics card, where the signal being produced from the graphics card is converted from a pure digital to an analogue signal. There are a number of algorithms implemented in TFT’s which have varying effectiveness in improving the image quality over a VGA connection.
Some TFT’s offer a DVI input as well to allow you to make use of the DVI output from your graphics card which you might have. This will allow a pure digital connection which can sometimes offer an improved image quality. Whether a DVI connection will make any difference to the image quality depends on several things including the model of TFT, quality of VGA connection and graphics card used. Please see this section for < more info >
It is possible to get DVI – VGA converters. These will not offer any improvements over a standard analogue connection, as you are still going through a conversion from digital to analogue somewhere along the line. Some screens also offer other interfaces designed for external devices such as games consoles and DVD players. HDMI, DisplayPort, S-Video, Composite and Component are available on some models if this functionality is appealing and are widely implemented to allow connection of other external devices. Some of these interfaces are also capable of carrying sound as well as video (e.g. HDMI and DisplayPort)
TCO Standards
The TCO standard is related to the specifications of the model as a whole and is a classification system used to certify a TFT.
The TCO standard is related to the specifications of the model as a whole and is a classification system used to certify a TFT.
TCO- labelling of media displays guarantees:
- Ergonomics-High visual ergonomic requirement on the picture screen which brings with it high picture quality and good color rendition. Good quality even when the screen displays moving pictures by means of short response time, good black level and expanded requirement of grey levels.
- Emission-Substantial reduction of magnetic and electrical fields.
- Energy-Low energy consumption in stand-by mode
- Ecology-That the manufactures are certified according to ISO 14001 or EMAS
-Reduced dispersion into the environment of brominating and chloridizing flame-resistant material and heavy metals (complying even with RoHS directive from 1st of July 2006).
-That the display unit is pre-prepared for recycling which facilitates recycling of materials.
Please see http://www.tcodevelopment.com/ for more info on which standard a TFT meets.
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