What is the Gamut Area Index (GAI) for Color Rendition?

The most common metric to measure the color rendition capabilities of a light source is the Color Rendering Index (CRI) R_a value. However, the CRI method was developed before the commercialization of LED lighting and some experts do not consider it the most accurate method to characterize LEDs.

A new metric, the Gamut Area Index (GAI) was developed in the early 1970s as a simple measure of hue saturation under an illumination source. GAI presents an alternative option for determining how a viewer will perceive color under LED lighting. CRI is accurate measure of fidelity (accuracy) of color rendition, thus GAI can be used in combination with CRI to better characterize the color rendition properties of an LED light source.

“Gamut” Terminology

The term “gamut” is used with somewhat different definitions in the LED, architectural lighting, printing, and electronic display industries, which can lead to confusion. In the printing industry, gamut refers to the range (number of) of colors than can be created by inks. For electronic displays such as televisions or computer monitors, gamut refers to the range of colors that can be displayed on the screen. In architectural lighting, gamut may refer to the colors of light that can be created with a color-tunable fixture.

In all these cases, the number of colors created is less than all colors (wavelengths) perceived by the human eye—a gamut is a subset of the CIE color space. For example, various display gamut areas are often shown overlaid on the CIE color space diagram for reference.

Gamut in Color Rendition

In the context of color rendition, gamut area is defined as “the area enclosed by a set of test color samples illuminated by a light source, in a two-dimensional chromaticity diagram or a plane of color space.”¹ Within a defined color space, a “gamut” describes the subset of colors that can be perceived under specific lighting conditions. The TM-30 method uses Rg as a measure of gamut.

The gamut area characterizes color saturation (or chroma): the gamut of a light source designates the overall change in the chroma of object from how it appears under a reference light source. When the Rg value of a light source = 100, it means that, on average, the light source does not alter the chroma appearance of test samples (TCS) when compared to their chroma under a reference light source.

When Rg<100 it means that, on average, the light source renders colors as less saturated than the reference source, and Rg>100 means that, on average, the light source renders colors as more saturated than the reference source. We say “on average” because the light source’s rendering of individual test samples may vary; Rg averages the overall chroma differences.

Gamut Area Index Defined

The Gamut Area Index (GAI) is an extension of this concept. The color coordinates of the reflection spectra of a set of TCS samples illuminated by the test light source define a gamut of a light source. The gamut is represented graphically as a polygon. GAI is calculated of dividing the gamut (polygon area) of the light source by the gamut area of the reference source, where “x” defines the sample set and the type of reference source used. By definition, any light source with CRI of 100 will have a corresponding GAI.

A high GAI value reveals greater color saturation and differentiation. GAI value of 100 means a perfect color match. GAI values can exceed 100, but higher-valued colors may appear oversaturated. Architects, industrial designers, and theatrical designers tend to favor lighting with a GAI value between 85 and 100. Using light sources with a GAI greater than 100 can result in more vivid colors, a strategy employed in our Sensus product line.

Gamut Area Index Refinements

The GAI definition has evolved over time. Originally GAI was a simple model using the equal energy² Illuminant E as a reference and eight color samples (GAI_E8). Since Illuminant E is constant and has a fixed gamut area which is then compared to all test sources the results are distorted as we move away from 5455K (the calculated CCT of Illuminant E).

Later GAI was refined to change the reference to a black body (BB) SPD matching the CCT of the light source and the same eight TCS samples as CRI R_a (GAI_BB8). The GAI has been further refined to use the same BB reference strategy but with 15 TCS samples (GAI_BB15). Each subsequent version of the GAI has been computationally more complex than the previous definition, but all are easily calculated using common software tools.

Plotting the gamut area encompassed by the reference black body along with the gamut area encompassed by the test light source reveals how accurately the two gamut areas match. Calculating the difference in area between the two gamuts provides an indication of saturation range; refer to the polygon outlines in the left image below, which uses GAI_BB8.

Luminus typically prefers the GAI_BB15 measure, which we have found to be more accurate at characterizing the comparative color rendition capability of light sources with different color temperatures (different CCT). Refer to the polygon outlines in the right image below, which uses GAI_BB15. Instead of a smooth “average” shape, individual point variations can be seen.

Experts suggest that GAI, by characterizing a light source’s rendition of saturation/intensity, and CRI, by characterizing chromatic accuracy, complement each other. Used together they can provide a more complete method of evaluating a light source.³ Lights with both high GAI and high CRI will offer perceived color quality (a subjective characteristic) in color rendition. Many psychophysical experiments have demonstrated the breakdown in the ability of Ra—or any measure of average color fidelity alone—to predict perceived color quality, which relies on attributes such as naturalness, vividness, preference, or acceptability.

Color preference studies validate this approach.⁴ For example, the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute (RPI) found that “when given a visual choice, people often prefer ‘white’ or minimally tinted sources that provide good color rendering.”⁵ They proposed a new designation called "Class A" lighting to help simplify these technical concepts for consumers. Class A Color Lighting sources have chromaticity at or near the line of minimum tint,⁶ good color rendering (CRI>80 and 80<GAI<100), and consistent chromaticity.

NOTES & REFERENCES

1. Royer, MP, Houser KW, David A. (2017). “Chroma shift and Gamut Shape: Going Beyond Average Color Fidelity and Gamut Area.” LEUKOS, 14(3):149-65. DOI: 1080/15502724.2017.1372203
2. Equal energy Illuminant E has a constant value, usually 1.0 W/nm, at all wavelengths. It is a useful reference point in colorimetric calculations; however Illuminant E is not used in real room-lighting situations.
3. Royer, Michael P. (2018). “Comparing Measures of Gamut Area.” LEUKOS. DOI: 1080/15502724.2018.1500485
4. Teunissen, C., Van der Heijden, FHFW, Poort, SHM and De Beer, E., “Characterizing user preferences for white LED light sources with CIE colour rendering index combined with a relative gamut area index.” Lighting Research & Technology, Vol 49: 461-480, 2017.
5. Understanding White Light Source Color Rendering and Appearance, Lighting Research Center, Rensselaer Polytechnic Institute. (Accessed October 25, 2021)
6. Freyssinier, J and Rea, M, “Class A Color Designation for Light Sources Used in General Illumination.” Journal of Light & Visual Environment, Vol 37, No.2 & 3, November 7, 2013. DOI: 2150/jlve.IEIJ130000501

To learn more, refer to the Help Center articles:

FAQ -> Color Quality Metrics

White Papers -> Achieving Optimal Color Rendition with LEDs

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