The Ability to Display Colour Correctly Is Vital: Understanding the Colour Gamut of an LCD Monitor

This article will address certain points one must know to choose the LCD monitor best-suited to one's needs from the various models available. Part 1 will focus on colour gamut. While wide colour gamuts are the latest trend in LCD monitors, colour gamut is a term that lends itself to misunderstanding. Our hope is that this session will help users better understand the colour gamut of LCD monitors and better select, use, and adjust the products.

Note: Below is the translation from the Japanese of the ITmedia article "IT Media LCD Monitor Course II, Part 1" published on November 11, 2008. Copyright 2011 ITmedia Inc. All Rights Reserved.


Just what is colour gamut?

A colour gamut defines a more specific range of colours from the range of colours identifiable by the human eye (i.e., the visible spectrum). While colour imaging devices include a wide range of devices, such as digital cameras, scanners, monitors, and printers, since the range of colours they can reproduce varies, the colour gamut is established to make these differences clear and to reconcile the colours that can be used in common between devices.

Various methods are used to express (diagram) the colour gamut, but the common method used for display products is the xy chromaticity diagram of the XYZ colour system established by the International Commission on Illumination (CIE). In an xy chromaticity diagram, the colours of the visible range are represented using numerical figures and graphed as colour coordinates. In the following xy chromaticity diagram, the area shaped like an upside-down "U" surrounded by dotted lines indicates the range of colours visible to human beings with the naked eye.

Various standards govern colour gamuts. The three standards frequently cited in relation to personal computers are sRGB, Adobe RGB, and NTSC. The colour gamut defined by each standard is depicted as a triangle on the xy chromaticity diagram. These triangles show the peak RGB coordinates connected by straight lines. A larger area inside a triangle is regarded to represent a standard capable of displaying more colours. For LCD monitors, this means that a product compatible with a colour gamut associated with a larger triangle can reproduce a wider range of colours on screen.

This is a CIE XYZ colour system xy chromaticity diagram. The areas enclosed in dotted lines represent the range of colours human beings can see with the naked eye. The ranges corresponding to the sRGB, Adobe RGB, and NTSC standards defining colour gamuts appear as triangles connecting their RGB peak coordinates. The colour gamut of an LCD monitor's hardware can be indicated using similar triangles. An LCD monitor is not capable of reproduction (display) of colours outside its colour gamut.

Print table
Gamut sRGB Adobe RGB NTSC
R (x/y-coordinate) 0,640 / 0,330 0,640 / 0,330 0,670 / 0,330
G (x/y-coordinate) 0,300 / 0,600 0,210 / 0,710 0,210 / 0,710
B (x/y-coordinate) 0,150 / 0,060 0,150 / 0,060 0,140 / 0,080
sRGB-, AdobeRGB- and NTSC colour coordinates in the CIE XYZ colour system
CIE XYZ color system xy chromaticity diagram

The standard colour gamut for personal computers is the international sRGB standard prepared in 1998 by the International Electrotechnical Commission (IEC). sRGB has established a firm position as the standard in Windows environments. In most cases, products like LCD monitors, printers, digital cameras, and various applications are configured to reproduce the sRGB colour gamut as accurately as possible. By ensuring that the devices and applications used in the input and output of image data are sRGB compatible, we can reduce discrepancies in colour between input and output.

However, a look at the xy chromaticity diagram shows that the range of colours that can be expressed using sRGB is narrow. In particular, sRGB excludes the range of highly saturated colours. For this reason, as well as the fact that advances in devices such as digital cameras and printers have led to widespread use of devices capable of reproducing colours more vivid than those allowed under the sRGB standard, the Adobe RGB standard and its wider colour gamut have recently drawn interest. Adobe RGB is characterized by a broader range than sRGB, particularly in the G domain—that is, by its ability to express more vivid greens.

Adobe RGB was defined in 1998 by Adobe Systems, maker of the well-known Photoshop series of photo-retouching software products. While not an international standard like sRGB, it has become— backed by the high market share of Adobe's graphics applications—the de facto standard in professional colour imaging environments and in the print and publishing industries. Growing numbers of LCD monitors can reproduce most of the Adobe RGB colour gamut.

NTSC, the colour-gamut standard for analogue television, is a colour gamut developed by the National Television Standards Committee of the United States. While the range of colours that can be depicted under the NTSC standard is close to that of Adobe RGB, its R and B values differ slightly. The sRGB colour gamut covers about 72% of the NTSC gamut. While monitors capable of reproducing the NTSC colour gamut are required in places like video production sites, this is less important for individual users or for applications involving still images. sRGB compatibility and the capacity to reproduce the Adobe RGB colour gamut are key points of LCD monitors that handle still images.

sRGB
sRGB
Adobe RGB
Adobe RGB

The visual differences between Adobe RGB (photo at left) and sRGB (photo at right). Converting a photograph in the Adobe RGB colour gamut to the sRGB domain results in the loss of highly saturated colour data and loss of tonal subtleties (i.e., a susceptibility to colour saturation and tone jumping). The Adobe RGB colour gamut can reproduce more highly saturated colours than sRGB colour. (Note that the actual colours displayed will vary with factors such as the monitor used to view them and the software environment. The sample photographs should be used for reference only.)


Similar but different: Adobe RGB ratio and Adobe RGB coverage

In passing, many LCD monitors that extol wide colour gamuts promote the area ratios of specific colour gamuts (i.e., triangles on the xy chromaticity diagram). Many of us have probably have seen indications of attributes such as Adobe RGB rates and NTSC rates in product catalogues.

However, these are only area ratios. Very few products include the entire Adobe RGB and NTSC colour gamuts. Even if a monitor featured a 120% Adobe RGB ratio, it would remain impossible to determine the extent of the difference in RGB values between the LCD monitor's colour gamut and the Adobe RGB colour gamut. Since such statements lend themselves to misinterpretation, it is important to avoid being confused by product specifications.

To eliminate problems involving labeled specifications, some manufacturers use the expression "coverage" in place of "area." Clearly, for example, an LCD monitor labeled as having Adobe RGB coverage of 95% can reproduce 95% of the Adobe RGB colour gamut.

From the user's perspective, coverage is a more user-friendly, easier-to-understand type of labeling than surface ratio. While switching all labeling to coverage presents difficulties, showing in xy chromaticity diagrams the colour gamuts of LCD monitors to be used in colour management will certainly make it easier for users to form their own judgments. 

Adobe RGB and actual monitor color space
AdobeRGB (red) and actual monitor colour space (yellow): 100 % AdobeRGB ratio
AdobeRGB coverage
AdobeRGB coverage(extent of coverage of the AdobeRGB colour space)

With regard to the difference between area labeling and coverage labeling as gauges of an LCD monitor's colour gamut, to use Adobe RGB as an example, in many cases, even a monitor with an Adobe RGB ratio of 100% in terms of area will feature coverage of less than 100 percent. Since coverage impacts practical use, one must avoid the mistake of seeing a higher figure as automatically better.


A misconception: Wide colour gamut means high image quality

When we check the colour gamut of an LCD monitor, it's also important to remember that a wide colour gamut is not necessarily equivalent to high image quality. This point may generate misunderstanding among many people.

Colour gamut is one spec used to measure the image quality of an LCD monitor, but colour gamut alone does not determine image quality. The quality of the controls used to realize the full capabilities of an LCD panel having a wide colour gamut is crucial. In essence, the capacity to generate accurate colours suited to one's own purposes outweighs a wide colour gamut.

When considering an LCD monitor with a wide colour gamut, we need to determine if it has a colour-gamut conversion function. Such functions control the LCD monitor's colour gamut based on the target colour gamut, such as Adobe RGB or sRGB. For example, by selecting sRGB mode from a menu option, we can adjust even an LCD monitor with a wide colour gamut and high Adobe RGB coverage so that the colours displayed on screen fall within the sRGB colour gamut.

Few current LCD monitors offer colour-gamut conversion functions (i.e., feature compatibility with both Adobe RGB and sRGB colour gamuts). However, a colour-gamut conversion function is essential for applications demanding accurate colour generation in the Adobe RGB and sRGB colour gamuts, such as photo retouching and Web production.

For purposes requiring accurate colour generation, an LCD colour monitor lacking any colour-gamut conversion function but having a wide colour gamut can actually be a disadvantage in some cases. These LCD monitors display each RGB colour mapped to the colour gamut inherent to the LCD panel in eight bits at full colour. As a result, the colours generated are often too vivid for displaying images in the sRGB colour gamut (i.e., the sRGB colour gamut cannot be reproduced accurately).

Shown here are examples of an sRGB colour gamut photograph displayed on an sRGB-compatible LCD monitor (photo at left) and on an LCD monitor with a wide colour gamut but incompatible with sRGB and with no colour-gamut conversion function (photo at right). While the photograph at right appears vivid, saturation is unnaturally high in parts of the photo. We also see a significant departure from the colours envisioned by the photographer, as well as so-called memory colours.

Vergleich Bild auf Monitor mit sRGB und nicht sRGB kompatibel

Wide colour gamuts drive demand for technologies that increase image quality

In more than a few cases, as expanding LCD monitor colour gamuts result in the capacity to reproduce a wider range of colours and more opportunities to check colours or adjusting images on monitor screens, problems such as breakdowns in tonal gradations, variations in chromaticity caused by narrow viewing angles, and screen display irregularities, less conspicuous at colour gamuts in the sRGB range, have become more pronounced. As mentioned earlier, the mere fact of incorporating an LCD panel with a wide colour gamut does not ensure that an LCD monitor offers high image quality. On this subject, let's take a close look at various technologies for putting a wide colour gamut to use.

First we look at technologies to increase gradation. Key here is the internal gamma-correction function for multi-level gradation. This function displays eight-bit input signals on screen in each RGB colour from the PC side after first subjecting them to multi-level gradation to 10 or more bits in each RGB colour inside the LCD monitor, then assigning these to each RGB eight-bit colour deemed optimal. This improves tonal gradations and gaps in hue by improving the gamma curve.

On the subject of the viewing angle of an LCD panel, while larger screen sizes generally make it easier to see differences, particularly with products with wide colour gamuts, variations in chromaticity can be an issue. For the most part, chromaticity variation due to viewing angle is determined by the technology of the LCD panel, with superior ones showing no variation in colour even when viewed from a moderate angle. Setting aside the various particulars of LCD panel technologies, these generally include in-plane switching (IPS), vertical alignment (VA), and twisted nematic (TN) panels, listed from smaller to larger chromaticity variation. While TN technology has advanced to the point at which viewing angle characteristics are much improved from several years back, a significant gap remains between this technology and VA and IPS technologies. If colour performance and chromaticity variation are important, VA or IPS technology remains the better choice.

A uniformity-correction function is a technology for reducing display irregularities. The uniformity referred to here refers to colours and brightness (luminance) on screen. An LCD monitor with superior uniformity has low levels of screen luminance irregularities or colour irregularities. High-performance LCD monitors feature systems that measure luminance and chromaticity at each position on screen and correct them internally.

Vergleich von Monitoren mit und ohne Korrektur der Homogenität

This is a comparison of monitors with and without uniformity correction. An LCD monitor with uniformity correction (photo at left) has more uniform luminance and colour on screen than one lacking uniformity correction (photo at right). The two photographs above have been adjusted to equalize levels to emphasize display irregularities. Actual irregularities would be less conspicuous.


Calibrating to increase the value of a wide colour gamut

To make full use of an LCD monitor with a wide colour gamut and to display colours as the user intended, one needs to consider adopting a calibration environment. LCD monitor calibration is a system for measuring colours on screen using a special-purpose calibrator and reflecting the characteristics of the colours in the ICC profile (a file defining device colour characteristics) used by the operating system. Going through an ICC profile ensures uniformity between the colour information handled by graphics software or other software and the colours generated by the LCD monitor to a high degree of precision.
 

Keep in mind that there are two types of LCD monitor calibration: software calibration and hardware calibration.

Software calibration refers to following the instructions of specialised calibration software to adjust parametres such as luminance, contrast, and colour temperature (RGB balance) using the LCD monitor's adjustment menu, approaching the intended colour through manual adjustments. Graphics driver colours are manipulated in some cases in place of the LCD monitor's adjustment menu. Software calibration features low cost and can be used to calibrate any LCD monitor.

However, variations in precision can arise since software calibration involves manual adjustment. Internally, RGB gradation can suffer because display balance is matched by thinning RGB output levels using software processing. Even so, use of software calibration will likely make it easier to reproduce colours as intended than using no calibration at all.

 
In contrast, hardware calibration is clearly more precise than software calibration. It also requires less effort, although it can be used only with compatible LCD monitors and entails certain setup costs. In general, it involves the following steps: calibration software controls the calibrator; matching colour characteristics on screen with target colour characteristics and directly adjusting the LCD monitor's luminance, contrast, and gamma-correction table (look-up table) at the hardware level. Another aspect of hardware calibration that cannot be overlooked is its ease of use. All tasks through the preparation of an ICC profile for the results of adjustment and registering this to the OS are done automatically.
 
The EIZO LCD monitors currently compatible with hardware calibration include models in the ColorEdge series.

By combining a ColorEdge-series monitor with a calibrator and ColorNavigator special-purpose colour-calibration software, one can achieve easy, precise hardware calibration.

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