1, color technical terminology 
Before we get to know the color quality evaluation, let's get familiar with these color technology terminology:
CRI: Color rendering index, color rendering index.
CQS: Color quality scale, color quality.
GAI: Gamut area index, full color gamut index.
CPI: Color preference index, color preference index.
CDI: Color discrimination index, color resolution index.
Color Fidelity: Color fidelity.
Color Vividness: Color vividness.
Color Saturation: Color saturation.
Color Preference: Color preference.
CIE: International Lighting Society.
NIST: (United States) National Institute of Standards and Technology.
LRC: (Lensler Institute of Technology, USA) Lighting Research Center.
ASSIST: American Solid State Lighting Technology and Systems Federation.
2. History of CRI color development characteristics evaluation
Since Bouma proposed in 1948 to use daylight as the ideal reference source to evaluate the color rendering properties of illuminated objects, CIE formed the CRI standard in 1960s and based on the 1960 CIE UVW color space to form the CRI formula, in Daylight. (CCT above 5000K) and the corresponding CCT black body radiation source (CCT below 5000K) as the reference illumination source, CRI is 100.
The CRI specified by CIE has been the most commonly used color quality evaluation index for decades. The value of CRI is based on the color rendering of a standard color sample of 8 unsaturated colors by the light source, for measuring the continuous and wide-band light source. Color rendering is quite good, such as halogen lamps.
Figure*: 8 non-saturated color standard color samples used by CIE.
CRI is also widely used to evaluate fluorescent lamps and HID lamps, and their frequency bands are also wide.
Figure*: Example of spectral distribution of fluorescent lamps and natural light
According to CIE, the value range of Ra(CRI) is 0~100. The higher the Ra value, the better the color rendering property of the light source. It is generally considered that Ra ≥ 80, the color rendering of the light source is excellent, between 50 and 80, the color rendering is general, Ra < 50, and the color rendering of the light source is poor.
Figure*: Example of visual representation of the same object at different Ra values
3. Defects in CRI color development characteristics evaluation
With the emergence of new light sources and deep research, especially the emergence and maturity of LED white light sources as new light sources, people have found that there are certain problems when using CRI to evaluate LED color rendering.
CRI is proposed based on the source of continuous emission spectrum of black body radiation. It may cause problems for spectrally steep waveforms with narrow frequency bands. Therefore, in actual tests, it is found that sometimes LEDs with low color rendering index may even be more color-developing. LEDs with high index have better color rendering.
The standard color plates used in CRI calculations are all unsaturated colors, which is quite good for measuring the color rendering of continuous and wide-band light sources. However, for saturated color light sources such as LEDs, the accuracy of color rendering evaluation will have a certain error. .
Figure*: In addition to the 1-8# unsaturated color block, 9-14# is a saturated color block, and its color rendering index is called the special color rendering index.
Figure*: Select 12 standard color cards. Compared with the standard light source, the tested LED light source A (Ra 80) does not perform as well as the tested LED light source B (Ra 67).
Based on CRI's evaluation of LED light source, there are problems such as color space inhomogeneity, low standard color sample and low saturation. CIE stated in the 2007 technical report that the current CRI index cannot effectively reflect white light including white LED. The color rendering of the light source is good or bad.
4, CQS proposed - using 15 kinds of saturated color samples
Around 2009, NIST Group developed a new color evaluation system CQS (color quality scale) color quality to evaluate the color quality of new white light sources such as LEDs.
To compensate for the CRI method, CQS selected 15 kinds of saturated colors, which were evenly distributed throughout the visible spectrum. 15 test samples changed the color of the sample into a continuous change of partial saturation, which consisted of a nearly saturated red to purple composition. The change.
Figure: CQS uses 15 saturated colors as test samples and is evenly distributed throughout the visible spectrum.
In terms of calculation method, CQS does not take a simple average value of eight sample colors like CRI (Ra), but takes the root mean square finger of 15 sample color data to enhance the weight of the color difference to ensure the color sample. Some large chromatic aberrations have a large effect on the final result.
CQS takes into account the saturation color and the color of the sample color of LED white light, but in the accurate evaluation of the fidelity and preference of color, it still needs to be verified and perfected for visual experiments of different ethnic groups, improving its popularity.
5, CRI + GAI's proposal - supplement the visual experience
In 2010, ASSIST proposed to increase the GAI (Gamut Area Index) full color gamut index based on CRI based on the CLR's leading research to increase the visual experience to evaluate color quality.
CRI is only one of the dimensions that indicate the ability of the light source to develop color, and is not directly related to human visual perception. The GAI indicator can accurately describe the light and color perceived by the human brain according to the pattern of the human brain's perceived light source, reflecting the overall effect of the light source and the subjective feeling of the perceived light source color.
The color quality evaluation indicators of CRI + GAI will reflect the color rendering, fidelity, saturation, and people's visual perception and color preference.
Figure*: AGSIST's GAI full gamut index evaluation model.
Statistical analysis by observer experiments shows that the observers have higher acceptance when the CRI and GAI of the low color temperature and high color temperature sources are higher.
ASSIST recommends a good white light color quality of CRI ≥ 80, 80 ≤ GAI100.
Figure: ASSIST recommends good white light color quality in the gray block of the figure.
6, CRI + GAI - improve color quality evaluation
CRI: “Nature†of object appears, which can measure the natural, natural, fidelity or true color of an object.
GAI: “Vivid†of color appears, which can measure the bright, vivid, vivid, bright, dazzling and lifelike colors of objects.
The use of the CRI index and the GAI index double standard will more accurately and "humanize" the quality of the light source, and will evaluate the quality of the light source in terms of naturalness, fidelity, saturation and brightness, and get a People feel the "most comfortable" light.
The GAI indicator may exceed 100, but the higher the better, the higher the GAI means the color is too saturated and distorted.
However, in some practical applications, a special phosphor formula can be used to obtain a bright light source with a GAI of 110-115, which is properly saturated on some chromatograms to provide brighter, clearer textures and more vivid colors. It does not project uncomfortable light, which is favored in commercial lighting such as high-end retail stores, hotel lobbies, museums and other applications.
Figure: Bright color effect
7, color quality evaluation - pending new standards
Regarding the white light color quality evaluation standard, the 27th CIE Conference in 2011 received 19 research reports from 7 countries and 11 research groups, and recommended 9 new color evaluation systems.
The more representative ones are the color quality (CQS) proposed by NIST and the CRI plus full color gamut index (GAI) proposed by ASSIST. In addition to these two, they also include the Fleetley index Rf, the color preference index CPI. And color resolution index CDI and so on.
NIST is doing a lot of color research and testing (including visual perception experiments) to replace the current CRI evaluation system to form a unified color quality evaluation system.
To fully evaluate the color quality of a light source, there are many factors to consider, such as:
— color reproduction, fidelity
— color vividness, saturation
— color preference
— the distinction of hue
— visual clarity and comfort
— visual perception of people of different colours
Figure: Color Visual Perception Study
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