Everyone must have some experience: the color of the object seen under some light sources is very different from the color in the impression, which is very unreal, such as street lamps; the color matching of the pretty clothes that are clearly seen in the window, placed It is not right to go to the outdoor color; in fact, this is caused by the color rendering characteristics of the light source. Academically, the influence of the light source on the appearance of the object is usually called color development, and the inherent color rendering property of the light source is called Color Rendering Property. The spectral power distribution of the light source determines the color rendering of the light source, and the light source with continuous spectral distribution has good color rendering, such as daylight, incandescent lamp, etc., and there is no necessary relationship between the color temperature and color rendering of the light source. Because light sources with different spectral distributions may have the same color temperature, their color rendering properties may vary widely.
CIE color rendering index
The International Commission on Illumination (CIE) defines color rendering as follows:
The effect of an illuminant on the colour appearance ofobjects by co nscious orsubco nscious comparison with their colour appearanceunder a reference illuminant
The CIE 13.3-1995 standard defines a method for evaluating the color rendering of a light source. The standard stipulates that when the color rendering of a light source with a color temperature of 5000K or less is evaluated, a black body of the same color temperature of 5000K or less is used as a reference light source, and the color rendering index of the corresponding black body is considered to be 100; and the standard illuminant D of the same color temperature is used as an evaluation color temperature of 5000K. The above reference source for color rendering of a high color temperature source. In evaluating the color rendering of the light source, a set of 14 test colors is used for the calculation of the general color rendering index and the special color rendering index of the light source, where R1-R8 represents various common colors with moderate saturation. The brightness values ​​are nearly equal; and the 6 test colors of R9~R14 are some saturated colors and skin colors, which are used for the calculation of special color rendering index. The common R9 is the color rendering of the light source specifically for red.
14 test samples in the CIE color rendering calculation method
Next, the 1960 UCS chromaticity coordinates of the light source to be tested and the test color under the light source to be tested are calculated and corrected by von kries color adaptation. Then, calculate the total color displacement of the test color under the light source to be tested and the reference light source in the uniform color space of CIE1964, which are respectively recorded as ∆Ei (i is the serial number of 14 test colors, ie, i = 1, 2, 3, ..., 14), then calculate the special color rendering index Ri of various test colors:
Ri= 100-4.6∆Ei
The higher the color rendering index of the light source, the better the color rendering. If Ri = 100, it means that the test color sample is consistent with the chromaticity coordinates of the reference light source under the light source to be tested.
The average of the eight special color rendering indexes obtained from the test colors No. 1 to No. 8 is called the general color rendering index CIE Ra.
CIE Ra calculation process
The following table shows the color rendering parameters of some common light sources for reference only. For everyday applications, CIE Ra is generally 80 or above. If it is above 90, it can better restore the color of objects under daylight or blackbody illumination.
Other color rendering index
Due to the vigorous development of LED technology, the evaluation index of CIE color rendering index is often biased for the evaluation of LED color rendering. Some LEDs have higher CIE Ra color rendering index, but the actual color rendering effect is not good; for better description The color rendering index of light sources, international researchers have introduced a variety of calculation methods such as IES TM30-15, CQS, CRI2012, mCRI, FCI, GAI, etc., to improve the CIE color rendering evaluation methods; The calculation steps are consistent with the above, and the selection of the test color, the selection of the reference light source, the selection of the color adaptation formula, the saturation of the test color, etc. have been re-evaluated and optimized; currently the IESTM 30-15 has the highest degree of recognition and is most likely to become International standards.
IES color rendering index
The IESTM-30-15 IES Method for e valuating Light Source Color Rendition was officially released on May 18, 2015 after a two-year work by the North American Institute of Illumination (IES). The new evaluation system was developed by the IES ColorMetrics Task Force, which consists of eight members: Dr. Mike Royer (Northwest Pacific National Laboratory), Dr. Yoshi Ohno (NIST, USA), Kevin Houser Ph.D. (Pennsylvania State University), Dr. Minchen Wei (Hong Kong Polytechnic University), Dr. Paul Fini ( CREE ), Dr. Aurelien David (Soraa), Dr. Kees Teunissen (Philips) and Randy Burkett (Randy Burkett Lighting Design Office). The document was approved after review by the IES Color Council, the Technical Review Board and the Board of Directors.
Compared to the existing CIE CRI, the new evaluation method of IES has several major reforms:
1. Double indicators: Evaluation of light source color development is no longer just using one indicator, but two indicators - Rf and Rg. Rf is used to characterize the degree of similarity of each standard color compared to the reference source under the illumination of the test source (100 represents exactly the same; 0 represents a large difference); Rg represents the saturation of each standard color compared to the reference source under the test source. The change (100 means the same saturation, greater than 100 means that the light source can increase the saturation of the color, and below 100 means the saturation of the color is lower under the test light source).
2. Standard color: The new system uses 99 standard colors compared to CRI's only 8 standard colors. These 99 standard colors are no longer Monser color cards, but are carefully selected from the colors of 105,000 objects. They represent the common colors that can be seen in life (from saturation to unsaturated, from light to dark), and the 99 standard colors are equally sensitive to wavelengths (it is difficult to increase the index by spectral gaming).
3. Reference source: There is a 5000K catastrophe problem due to the reference source used by CRI (ie blackbody radiation below 5000K; natural light model above 5000K). This new system uses blackbody in the range of 4500K-5500K. The spectrum of the radiation mixed with the natural light model serves as a reference source. In addition, both Rf and Rg use the same reference light source color temperature as the source to be tested, thereby overcoming an important drawback of GAI.
4. Colorimetry: The system uses a more uniform chromaticity space, color adaptive equation and color appearance model (CIE Color Appearance Model02).
5. Color distortion icon: Since the two indicators can only comprehensively evaluate the average color rendering ability of the light source for various colors, it is sometimes important for the color rendering ability of certain colors (especially for lighting designers). The new method provides a dual indicator while providing a true color icon that provides more intuitive information to indicate color drift and saturation changes for various colors.
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