What is called color gamut? It defines the specific range of the spectrum visible to the human eye. Since the colors that imaging devices such as digital cameras, scanners, monitors, and printers can vary, a certain gamut is used to match them.
Additive and subtractive types
There are 2 main types of color gamut - RGB and CMYK.
Additive gamma is formed by mixing light of different frequencies. It is used in displays, televisions and other devices. The name RGB is made up of the initial letters of red, green, and blue light used for such generation.
Subtractive gamma is obtained by mixing dyes that block the reflection of light, resulting in the desired color. It is used to publish photographs, magazines and books. CMYK abbreviation is made up of the names of the pigments (cyan, magenta, yellow and black) used in printing. CMYK color gamut is significantly less than RGB space.
Standards
Color gamut is regulated by a number of standards. Personal computers often use sRGB, Adobe RGB, and NTSC standards. Their color models are shown on the color chart in the form of triangles. They represent the peak RGB coordinates connected by straight lines. The larger the area of the triangle, the more shades the standard can display. For LCD monitors, this means that a product compatible with a larger model can reproduce a wider range of colors on the screen.
sRGB
The color gamut for personal computers is determined by the international standard sRGB, established in 1998 by the International Electrotechnical Commission (IEC). He took a strong position in the Windows environment. In most cases, displays, printers, digital cameras, and various applications are calibrated to most accurately reproduce the sRGB model. If the devices and programs used in the input and output of image data are compatible with this standard, discrepancies between input and output data will be minimal.
Adobe RGB
The chromatic diagram shows that the range of values that can be expressed using the sRGB model is rather narrow. In particular, the standard eliminates highly saturated colors. This, as well as the development of devices such as digital cameras and printers, has led to the widespread use of technology capable of reproducing tones that are not in the sRGB range. In this regard, the general attention was drawn to the Adobe RGB standard. It is characterized by a wider color gamut, especially in the G region, that is, due to the ability to display brighter green tones.
The Adobe RGB standard was established in 1998 by Adobe Systems, which created the famous series of Photoshop photo retouching programs. Although it is not international (like sRGB), due to the high market share of Adobe graphics applications in the professional image processing environment, as in the printing and publishing industries, it has become such a de facto. A growing number of monitors can reproduce most of the Adobe RGB color gamut.
NTSC
This analogue television standard was developed by the US National Television System Committee. Although the NTSC color gamut is close to Adobe RGB, its R and B values are slightly different. sRGB occupies about 72% of the NTSC range. Monitors capable of reproducing the NTSC model are necessary for the production of video, but they are less important for individual users or for applications related to still images. Compatibility with sRGB and the ability to reproduce the color gamut of Adobe RGB are key to the displays used for working with photographs.
Backlight technology
In general, modern monitors used with PCs, thanks to the specifications for their LCD panels (and controls), have a color gamut that includes the entire sRGB space. However, given the growing demand for reproducing a wider gamut, the color space of monitors has been expanded. At the same time, the Adobe RGB standard is used as the target. But how does this expansion happen?
This is largely achieved by improving the backlight. 2 main approaches are applied. One of them consists in expanding the color gamut of cold cathodes, which are the main illumination technology, and the other affects LED backlighting.
In the first case, a quick solution is to strengthen the color filter of the LCD panel, although this reduces the brightness of the screen by reducing light transmission. Increasing the brightness of the cold cathode to counteract this effect tends to shorten the life of the device and often leads to poor lighting. The efforts of engineers to date have largely overcome these shortcomings. In many monitors with fluorescent backlight, the expansion of the range is achieved due to the modification of the phosphor. It also reduces cost, since it allows you to expand the color scheme without significant changes to the existing design.
The use of LED backlighting has begun to increase relatively recently. This allowed to achieve higher levels of brightness and color purity. Despite certain drawbacks, including lower image stability (for example, due to problems with radiant heating) and difficulties in achieving uniformity of white across the screen due to the use of a mixture of RGB LEDs, these problems were eliminated. LED backlight costs more than fluorescent lamps and was used less, but because of its effectiveness in expanding the color gamut of the display, the use of this technology has increased. This is true for LCD TVs.
Value and coverage
Manufacturers often indicate the area of color gamut of the monitor (i.e. triangles on the color chart). Many probably saw in the catalogs data on the ratio of the gamma of a device to the Adobe RGB or NTSC model.
However, these figures speak only about the area. Very few products cover the entire space of Adobe RGB and NTSC. For example, the color gamut of the Lenovo Yoga 530 is 60–70% Adobe RGB. But even if the display shows 120%, it is impossible to determine the difference in values. Since such data lead to misinterpretation, it is important to avoid confusion with product characteristics. But how to check the color gamut of the monitor in this case?
To eliminate specifications problems, some manufacturers use the expression “coverage” instead of the word “area”. Obviously, for example, an LCD monitor with a color gamut of the Adobe RGB color model at 95% can reproduce 95% of the gamut of this standard.
From the point of view of the user, coverage is a more convenient and understandable characteristic than the ratio of areas. Although this creates difficulties, the demonstration on the graphs of the color gamut of monitors, which will be used for color control, will undoubtedly make it easier for users to form their own opinions.
Gamma conversion
When checking the color space of a monitor, it is important to remember that the expanded color gamut does not necessarily entail high image quality. This may cause confusion.
Color gamut is a characteristic used to measure image quality on an LCD monitor, but it alone does not determine it. Crucial is the quality of the controls used to realize the full capabilities of the display. Essentially, the ability to generate accurate tones suitable for specific needs outweighs the availability of extended color gamut.
When evaluating a monitor, you must determine if it has a color space conversion function. It allows you to control the gamut of the display by setting a target model, such as Adobe RGB or sRGB. For example, by choosing sRGB mode from the menu, you can configure the monitor with Adobe RGB coverage so that the colors displayed on the screen fall into the sRGB range.
Displays that offer color conversion features are both compatible with Adobe RGB and sRGB standards. This is necessary for applications that require accurate color generation, such as photo editing and web production.
For purposes requiring accurate color reproduction, in some cases the disadvantage is the lack of a conversion function for a monitor with an extended color gamut. Such displays display every tone of the 8-bit gamut in full color. As a result, the colors generated are often too bright to display images in sRGB (i.e., sRGB cannot be reproduced accurately).
Converting a photograph taken in the Adobe RGB color gamut to sRGB results in loss of data about highly saturated data colors and loss of tonal subtleties. Thus, the pictures become faded and tone jumps appear. The Adobe RGB model can reproduce richer colors than sRGB. However, the actual colors displayed depend on the monitor used to view them and the software environment.
Image Enhancement
In cases where the extended color gamut of the monitor allows you to reproduce a wider range of tones, it gives more options for controlling and adjusting images on screens, such problems as violation of tonal gradations, color variations caused by narrow viewing angles, and display unevenness, less noticeable in gamma sRGB range, have become more pronounced. As mentioned earlier, the mere fact of having an extended color gamut display does not guarantee that it will provide high image quality. It is necessary to take a closer look at various technologies for using the extended RGB color gamut.
Graduation enhancement
The key here is the built-in gamma correction function for multi-level tonal transitions. The 8-bit input signals for each RGB color that come from the PC side are smoothed to 10 or more bits in each pixel of the monitor, and then assigned to each RGB color. This improves tonal transitions and reduces color gaps, improving the gamma curve.
Viewing angles
Large screens usually make it easier to perceive differences, especially in devices with extended color gamut, but they may have color problems. For the most part, the change in color rendering due to the viewing angle is determined by the technology of LCD panels, and the best of them do not show tone changes even when viewed from a large angle.
Without going into the particularities of display production, they can be divided into the following types, listed in order of increasing color change: with flat switching (IPS), vertical alignment (VA) and twisted nematic crystals (TN). Although TN technology has advanced to such an extent that its viewing angle characteristics have improved significantly, a significant gap remains between it and VA and IPS technologies. If color accuracy is important, VA and IPS panels are the best choice.
Uneven color and brightness
The heterogeneity correction function is used to reduce display unevenness that affects screen color and brightness. A good-performance LCD monitor provides a low level of unevenness in brightness or tone. In addition, high-performance displays are equipped with systems for measuring brightness and color at each point on the screen and correct them by their own means.
Calibration
In order to fully realize the capabilities of the LCD monitor with an expanded gamut and display tones in accordance with the needs of the user, it is necessary to consider the possibility of using equipment for tuning. Display calibration is the process of measuring colors on the screen using a special calibrator and reflecting the characteristics in the ICC profile (a file that determines the color characteristics of the device) used by the operating system. This ensures the uniformity of the information processed by graphic and other software and the tones generated by the LCD monitor, as well as a high degree of accuracy.
It should be borne in mind that there are 2 types of display calibration: software and hardware.
Program setting is carried out using specialized software that sets such parameters as brightness, contrast and color temperature (RGB balance) through the monitor menu and brings the image closer to the original tone using manual settings. In some cases, instead of a program, these functions are taken over by graphic drivers. Software calibration is low cost and can be used to configure any monitor.
However, fluctuations in color accuracy are possible, since the human factor is present. RGB gradation may suffer from this, as display balance is achieved by increasing the number of RGB output levels using software processing. However, with software tuning, accurate color reproduction is easier than without it.
In contrast, hardware calibration provides a more accurate result. It requires less effort, although it can only be used with compatible LCD monitors, and entails certain costs.
In general, calibration involves the following steps:
- program launch;
- comparison of color characteristics of the screen with their target values;
- direct adjustment of brightness, contrast and gamma correction of the display at the hardware level.
Another aspect of hardware tuning that cannot be overlooked is its simplicity. All tasks, starting with preparing the ICC profile for the adjustment results and recording them in the OS, are performed automatically.
Finally
If color rendering is important, you need to know how many colors it can actually represent. Manufacturer's specifications listing the number of tones are generally useless and inaccurate when it comes to what the display actually displays, compared to what it is theoretically capable of. Therefore, consumers should be aware of the color gamut of their monitor. This will give a much better idea of its capabilities. You need to find out the percentage of coverage of the monitor gamut and the model on the basis of which it is calculated.
The following is a short list of common ranges for displays of different levels:
- the average LCD covers 70–75% of the NTSC gamut;
- professional LCD monitor with extended coverage - 80–90%;
- LCD with backlight by cold cathode lamps - 92–100%;
- LCD monitor with extended gamut and LED backlight - more than 100%.
Finally, remember that these numbers are correct when the display is fully calibrated. Most monitors go through a basic setup and have slight deviations in some respects. As a result, those who need high-precision color must correct it using appropriate profiles and settings, using a special calibration tool.