What Is “Colour Gamut”

Colour Gamut

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

Various methods are used to express (diagram) the color gamut, but the common method used for display products is the XY chromaticity diagram of the XYZ color system established by the International Commission on Illumination (CIE). In an XY chromaticity diagram, the colors of the visible range are represented using numerical figures and graphed as color coordinates. In the following XY chromaticity diagram, the area shaped like an upside-down “U” surrounded by dotted lines indicates the range of colors 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 color 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 colors. For LCD monitors, this means that a product compatible with a color gamut associated with a larger triangle can reproduce a wider range of colors on screen.

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 color 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 color between input and output.

However, a look at the xy chromaticity diagram shows that the range of colors that can be expressed using sRGB is narrow. In particular, sRGB excludes the range of highly saturated colors. 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 colors more vivid than those allowed under the sRGB standard, the Adobe RGB standard and its wider color 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 color imaging environments and in the print and publishing industries. Growing numbers of LCD monitors can reproduce most of the Adobe RGB color gamut.

NTSC, the color-gamut standard for analog 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 color gamut covers about 72% of the NTSC (BT-601) colour gamut. While monitors capable of reproducing the NTSC (BT-601) 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.

 

Sources:  Epson, NTSC, ITU

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