The color rendering index CRI is a commonly used parameter in the field of lighting technology. It refers to the measurement of the degree of color conformity when an object is illuminated by this light source and a standard light source (usually the sun is used as a standard light source), that is, the degree of color fidelity.
American lighting technology expert Mike Wood has written a series of articles on color rendering index CRI and color quality index CQS. Here I share some compiled content with you for your reference.
1. CRI definition
For lighting professionals, the color rendering index CRI is a commonly used term. We often see the CRI value in the data of the light source and know that it reflects the color rendering of the light source.
But what is its actual meaning? The CRI value helps to 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 needs to be measured and how to measure it? For example, the CRI value of OLIGHT S1MINI is 90. What information does this convey? The light quality of the museum must be above CRI 95. Why is this?
To put it simply: color rendering is an important aspect of evaluating lighting quality, color rendering index is an important method to evaluate the color rendering of light sources, and an important parameter to measure the color characteristics of artificial light sources. The better the color, the stronger the ability to restore the color of the object.
The International Commission on Illumination (CIE) defines color rendering as: compared with a standard reference light source, the effect of a light source on the color appearance of an object.
In other words, CRI is a measurement method for color recognition when a light source is compared with a standard light source (such as daylight). CRI is a universally recognized metric and is currently the only way to evaluate and report the color rendering of light sources. way.
The establishment of the CRI metric is not far away. The original purpose of this standard was to use it to describe the color rendering of fluorescent lamps that began to be used in large quantities in the 1960s, and to help users understand that fluorescent lamps with linear spectral distributions can be applied. Which occasions.
The measurement of CRI is closely related to the difference between the appearance of the 14 specified color samples (hereinafter referred to as "color samples") under the measured light source and the appearance under the standard reference light source.
Although CRI is derived by mathematical methods, and an actual color chart cannot be used to determine the CRI value, these color samples are real, and they are all selected from Munsell color samples.
14 standard color samples
The first 8 color samples are usually used to determine the general color rendering index (usually the CRI value proposed by people refers to the general color rendering index). The selected TCS01~TCS08 have medium saturation and roughly the same brightness, and the color range covers the entire visible spectrum.
The last 6 pieces are special color samples. TCS09~TCS14 are rarely used. In addition to imitating European skin color and leaf green, they also include more saturated primary colors.
2. CRI technology
Although these color samples are carefully specified, and real objects can produce the colors of these color samples, the CRI value is 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 we have to do is to compare the measured light source spectrum with the spectrum of the formulated color sample, and then calculate the CRI value through mathematical analysis.
Therefore, the measurement of CRI value is quantitative and objective. It is by no means a subjective measurement (subjective measurement only relies on a trained observer to judge which light source has better color rendering).
The comparison based on color perception is also meaningful, provided that the color temperature of the measured light source and the reference light source must be the same.
For example, trying to compare the appearance of two pieces of the same color sample illuminated by a warm white light source with a color temperature of 2900K and a cool white light source (daylight) with a color temperature of 5600K is a waste of time.
They must look different, so the first step is to calculate its correlated color temperature (CCT) from the spectrum of the light source under test. Once you have this color temperature, another reference light source of the same color temperature can be created mathematically.
For the measured light source with a color temperature lower than 5000K, the blackbody (Planck) radiator is selected as the reference light source, and for the measured light source with a color temperature higher than 5000K, the CIE standard illuminator D is selected as the reference light source.
The selection can combine the spectrum of the reference light source with each color sample to produce a set of ideal reference color coordinate points (referred to as color points).
The same is true for the measured light source. Combine the spectrum of the measured light source with each color sample to obtain another set of color points. If the color point under the measured light source is exactly the same as the color point under the reference light source, we consider their color rendering to be the same, and position their CRI value at 100.
In the color chart, the farther the color point under the measured light source is from the corresponding ideal position, the worse the color rendering and the lower the CRI value.
The color displacements of 8 pairs of color samples are calculated separately, and then 8 special color rendering indexes are calculated (the CRI value of the light source for a certain color sample is called the special color rendering index), and then their arithmetic average is taken, so that the value obtained is The final CRI value.
A CRI value of 100 means that there is no color difference between any pair of color samples in the 8 pairs of color samples under the measured light source and the reference light source.
Explaining these requires a lot of content, and it is very likely to cause confusion. Therefore, the editor thinks that using some pictures to help explain will be very helpful. Now start with an incandescent lamp with a CRI value of 100 (in theory, an incandescent lamp is equivalent to A black body radiator, therefore, by definition it has the most ideal color rendering.)
Figure 3 shows the tested color points and reference color points, and Figure 4 shows the appearance of the color sample
The tested color points correspond exactly to the corresponding reference color points, so they cover 8 reference points on the graph.
In order to facilitate comparison, Figures 5 and 6 show the corresponding data of a mercury lamp. Its CRI value is 43, which is quite poor.
The spectral distribution of mercury lamps is discontinuous and linear. Such a spectrum can easily increase the saturation of some colors (the color point moves outward), such as the saturated yellow-green labeled TCS03, or reduce the saturation of some colors ( The color point moves inward), such as the light blue-green labeled TCS05, so that they look like they have been washed out, almost all appearing gray.
The color of no color sample can be displayed correctly, and the color shift is quite large.
The other extreme of color rendering is the color rendering of low-pressure sodium lamps, as shown in Figures 7 and 8.
Low-pressure sodium lamp is an old-fashioned light source that radiates orange-yellow light, which is often used in previous street lights. Essentially, it is a monochromatic light composed of two closely spaced yellow spectral lines with a wavelength distribution of 589.0nm and 589.6nm.
Low pressure sodium lamp does not have color rendering ability at all. In fact, its CRI value is -47 through calculation. All 8 tested color samples appeared to be shrouded in turbid yellow. This explains why it is difficult to find your car in a parking lot illuminated by these light sources at night.
No matter what color the car is, all cars look the same under the illumination of low-pressure sodium lamps. (Although CRI values are allowed to be negative in the definition of CRI, they usually converge to zero, which is very bad.)
Low-pressure sodium lamps make responsible lighting designers a little dilemma: their luminous efficiency is extremely high, reaching 150 lm/W, but they are generally unpopular due to the unsightly appearance of objects under their lighting and the lack of color rendering.
In many lighting equipment, they have been replaced by high-pressure sodium lamps with slightly lower luminous efficiency and other light sources with higher CRI values. Table 1 lists the color temperature and CRI value of some commonly used light sources.
Having said so much, this can clearly answer the question at the beginning, why the quality of the museum's lighting must be above CRI 95, because the color rendering above CRI 95 can make the color reproduction more pure, saturated and thorough, for you truly Present the exquisite details and brilliant colors of precious cultural relics and works of art.
