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For those engaged in the lighting industry, the color rendering index CRI is a common term. Everyone often sees the CRI value on the data of the light source, and knows that it reflects the color rendering quality of the light source, but its actual meaning is what? The CRI value helps people determine which light source should be used in a lighting device. The higher the CRI value, the better, but do people know what it actually measures and how to measure it? For example, a light source has a CRI value of 95. What information does this convey?
â— Definition of CRI
The International Commission on Illumination (CIE) defines color rendering as the effect of a light source on the color appearance of an object compared to a standard reference source. In other words, CRI is a measure of color recognition in comparison to a standard source such as daylight. CRI is a universally accepted metric and the only way to evaluate and report the color rendering of light sources. The establishment of the CRI metric is not far away. The purpose of the standard was to use it to describe the color rendering of fluorescent lamps that were widely used in the 1960s, and to help users understand that spectrally distributed fluorescent lamps can be applied. What occasions.
Figure 1:14 standard color CRI color sample
The CRI measurement is closely related to the difference between the appearance of the specified 14 color samples (hereinafter referred to as "color samples") under the measured light source and the appearance of the standard reference light source. Although the CRI is derived mathematically, and an actual color chart cannot be used to determine CRI values, the color of these color samples is true and they are all selected from Munsell color samples. Figure 1 shows 14 standard CRI color samples. The first 8 color samples are usually used to determine the general color rendering index (usually the CRI value mentioned is the general color rendering index). The selected TCS01-TCS08 has medium saturation. It has roughly the same brightness, and the color range covers the entire visible spectrum. The last 6 blocks are special color samples. TCS09-TCS14 is rarely used. In addition to imitating European skin color and leaf green, it also includes higher saturation primary colors.
â— CRI calculation
Although these color samples are carefully and carefully defined, and real objects can produce the colors of these color samples, the CRI values ​​are completely derived from calculations. It is very important to understand that it is not necessary to illuminate the real color samples with real light sources. What people have to do is The measured source spectrum is compared to the spectrum of the specified color sample, and then the CRI value is calculated by mathematical analysis. Therefore, the measurement of CRI values ​​is quantitative and objective, and it is by no means a subjective measurement (subjective measurement relies on only one trained observer to determine which source has better color rendering).
Comparison based on color perception is also significant, provided that the color temperature of both the measured source and the reference source must be the same. For example, trying to compare the difference in appearance between a warm white light source with a color temperature of 2900K and a cold white light source (daylight) with a color temperature of 5600K is a waste of time. They must look different. Therefore, the first step is to calculate its correlated color temperature (CCT) from the spectrum of the measured source. Once this color temperature is available, another reference source of the same color temperature can be created mathematically. For a light source with a color temperature lower than 5000K, the reference light source selects a black body (Planck) radiator; and for a light source whose color temperature is higher than 5000K, the reference light source selects the CIE standard illuminant D.
Figure 2: Spectral distribution of the first 8 color samples in 14 standard CRI color samples
It is now possible to combine the spectrum of the reference source with each color swatch to produce an ideal set of reference color coordinate points (referred to as color points). The same is true for the light source to be measured, combining the spectrum of the measured light source with each color sample to obtain another set of color points. If the color point under the light source is exactly the same as the color point under the reference light source, we think that their color rendering is the same and set its CRI value to 100. In the color map, the farther the color point under the measured light source is from the corresponding ideal position, the worse the color rendering is, and the lower the CRI value is. 8 The color displacements of the color samples are calculated separately, and then 8 special color rendering indexes (the CRI values ​​of the light source for a certain color sample are called special color rendering indexes), and then the arithmetic mean values ​​thereof are taken, so that the value obtained is The final CRI value. A CRI value of 100 indicates that there is no color difference between any one of the eight pairs of color samples under the measured light source and the reference light source.
Figure 4: Comparison of the appearance of the reference light source and the incandescent lamp
Explain that these need to describe a lot of content, and it is very likely to cause confusion, therefore, I think it would be very helpful to use some pictures to help explain, now start with an incandescent lamp with a CRI of 100. In theory, an incandescent lamp is equivalent to a blackbody radiator, so by definition it has the best color rendering. Figure 3 shows the tested color points and reference color points; Figure 4 shows the appearance of the color samples. The tested color points correspond exactly to the corresponding reference color points, so they cover 8 reference color points on the graph.
For ease of comparison, Figures 5 and 6 show the corresponding data for a mercury lamp with a CRI of 43, which is quite poor.
The spectral distribution of mercury lamps is discontinuous and linear, and such a spectrum can easily increase the saturation of some colors (the color point moves outward), such as the saturated yellow-green color of TCS03, or the saturation of some colors ( The color points move inward, such as the light blue-green color labeled TCS05, so that they appear to be washed out, almost all gray. The color without a color swatch can be correctly displayed, and the amount of color displacement is quite large.
(from left to right) Figure 3: Color point tested with reference color point and incandescent lamp
Figure 5: Color point tested with reference color point and mercury lamp illumination
Figure 7: Color point tested with reference color point and low pressure sodium lamp