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NHK Explains How Gamut Rings Offer Better Way to Visualize Color Gamut

A prototype system for measuring display color gamut using Gamut Rings. (Image credit: NHK STRL)

TOKYO—NHK Science & Technology Research Laboratories (STRL) has released some important details about a new way to visualize the range of colors in a 2D diagram using the new international standard “Gamut rings,” which was invented by NHK STRL. 

The International Electrotechnical Commission (IEC)1), International Committee for Display Metrology (ICDM)2), and the International Commission on Illumination (CIE)3) adopted the gamut ring framework as an international standard in January, July, and November 2021, respectively. (See Figure 1).

NHK STRL has also developed a system  that automatically measures the display color gamut  boundary and renders gamut rings (Photo 1). 

Figure 1: Gamut Rings. The ring area indicates the range of  reproducible colors.  (Image credit: NHK STRL)
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Photo 1: A prototype system for measuring display color gamut using Gamut Rings. (Image credit: NHK STRL)
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Conventionally, the “color gamut” is represented by the  area of a triangle connecting the points of the red, blue, and green primary colors in a chromaticity diagram (Figure  2), NHK STRL explained. 

Figure 2: Conventional  gamut representation using  a chromaticity diagram. The larger the triangle's area, the wider  the gamut was intended to be.  (Image credit: NHK STRL)
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Although this conventional representation is simple,  expressing the color gamut in a 3D color space that includes a lightness axis (L*) is suitable for an effective evaluation (Figure 3). 

Figure 3: 3D plot of color  gamut solid. The volume indicates the range of  reproducible colors.  (Image credit: NHK STRL)
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However, a single-angle view in a 3D plot can often be misleading because parts of the  gamut are always obscured from the view. It has  therefore been a challenge to develop a method to  convert the 3D color gamut in a 2D format, NHK STRL noted. 

To solve that problem Gamut rings were invented by NHK STRL as a way to accurately represent a 3D color gamut in a 2D diagram. 

The Gamut Rings are by the following procedure: The gamut solid  (Fig. 3) is cut into pieces at regular intervals of lightness (L*). Each piece is then stretched into a slice of unity lightness; the area of the slice corresponds to  the volume of the piece. Gamut rings are formed by placing one slice at the lowest lightness level, followed  by positioning the other slices around the bottom slice consecutively. The angle from the center represents the hue such as red, green, and blue, NHK STRL explained. 

Gamut rings can potentially be applied in the design  and evaluation of color reproduction devices, such as  displays and printers, and can be used to conduct performance comparisons for both professional and consumer uses, NHK STRL concluded. (Figure 4).


Figure 4: Sample performance comparison between two displays.  Although two displays have nearly the same RGB triangle area in a chromaticity diagram, the gamut rings reveal a large difference between them; Display A has a higher saturation of colors at high lightness levels, and thus, a larger color gamut than Display B.  (Image credit: NHK STRL)

George Winslow is the senior content producer for TV Tech. He has written about the television, media and technology industries for nearly 30 years for such publications as Broadcasting & Cable, Multichannel News and TV Tech. Over the years, he has edited a number of magazines, including Multichannel News International and World Screen, and moderated panels at such major industry events as NAB and MIP TV. He has published two books and dozens of encyclopedia articles on such subjects as the media, New York City history and economics.