【LED Terminology Explanation】Analysis of Color Rendering Index CRI Parameters

For people working in the lighting industry, the color rendering index (CRI) is a common term. People often see CRI values ​​in the data of light sources and know that it reflects the quality of the light source in terms of color rendering, but what does it actually mean? The CRI value helps people determine what 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, if a light source has a CRI value of 95, what information does this convey?

● Definition of CRI

The International Commission on Illumination (CIE) defines color rendering as the effect of a light source on the appearance of an object's color compared to a standard reference light source. In other words, CRI is a measure of how well a light source performs in terms of color recognition compared to 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 a light source. The CRI metric was established not long ago. The original purpose of developing 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 in which occasions fluorescent lamps with linear spectral distribution can be used.

Figure 1: 14 standard color CRI samples

The measurement of CRI is closely related to the difference between the appearance of 14 color samples (hereinafter referred to as "color samples") under the measured light source and the appearance under the standard reference light source. Although the derivation of CRI is done by mathematical methods, and an actual color chart cannot be used to determine the CRI value, the colors of these color samples are real and they are all selected from the Munsell color samples. As shown in Figure 1, 14 standard CRI color samples are shown. The first 8 color samples are usually used to determine the general color rendering index (the CRI value usually 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 are special color samples. TCS09-TCS14 are rarely used. In addition to imitating European skin color and leaf green, they also include primary colors with higher saturation.

● Calculation of CRI

Although these color samples are carefully specified and real objects can produce the colors of these color samples, it is very important to understand that the CRI value is derived entirely through calculation. There is no need to illuminate the real color sample with a real light source. All one has to do is compare the measured light source spectrum with the spectrum of the specified color sample and then derive the CRI value through mathematical analysis. Therefore, the measurement of CRI value is quantitative and objective, and it is not a subjective measurement (subjective measurement is based on a trained observer judging which light source has better color rendering).

Comparisons based on color perception also make sense if the color temperature of the illuminant being measured and the reference illuminant are the same. For example, it is a waste of time to try to compare the appearance of two identical color swatches illuminated by a warm white illuminant with a color temperature of 2900K and a cool white illuminant with a color temperature of 5600K (daylight). They will look different. Therefore, the first step is to calculate the correlated color temperature (CCT) of the illuminant being measured from its spectrum. Once this color temperature is known, a reference illuminant of the same color temperature can be mathematically created. For illuminants with a color temperature below 5000K, the reference illuminant is a blackbody (Planckian) radiator; for illuminants with a color temperature above 5000K, the reference illuminant is CIE Standard Illuminant D.

Figure 2: Spectral distribution of the first 8 of the 14 standard CRI color samples

Now we can combine the spectrum of the reference light source with each color sample to produce a set of ideal reference color coordinate points (color points for short). 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 points under the measured light source correspond exactly to the color points under the reference light source, we consider their color rendering to be the same and set their CRI value to 100. In the color diagram, 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 displacement of the 8 pairs of color samples is calculated separately, and then 8 special color rendering indices (the CRI value of the light source for a certain color sample is called the special color rendering index) are calculated, and then their arithmetic average is taken, and the value obtained in this way 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.

Figure 4: Comparison of the appearance of the color sample under reference light source and incandescent light

Explaining this is a lot to describe and can be confusing, so I thought it would be helpful to use some pictures to help explain it, starting with an incandescent lamp with a CRI of 100. In theory, an incandescent lamp is equivalent to a black body radiator, so by definition it has the best color rendering. Figure 3 shows the tested color points and the reference color points, and Figure 4 shows what the color samples look like. The tested color points correspond exactly to the corresponding reference color points, so they cover the 8 reference color points on the diagram.

For comparison purposes, Figures 5 and 6 show the corresponding data for a mercury lamp, which has a CRI value of 43, which is quite poor.

The spectrum of mercury lamps is discontinuous and linear, which can easily increase the saturation of some colors (move the color point outward), such as the saturated yellow-green labeled TCS03, or reduce the saturation of some colors (move the color point inward), such as the light blue-green labeled TCS05, so that they look washed out and almost gray. No color of the color sample can be correctly displayed, and the color shift is quite large.

(From left to right) Figure 3: Reference color point and color point measured under incandescent light

Figure 5: Reference color point and color point tested under mercury lamp irradiation

Figure 7: Reference color point and color point tested under low-pressure sodium lamp

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 lamps are an old-fashioned light source that radiates orange-yellow light. This light source was often used in street lamps in the past. In essence, it is a monochromatic light composed of two closely adjacent yellow spectral lines with wavelengths of 589.0nm and 589.6nm.

Figure 6: Comparison of the appearance of the color sample under reference light source and mercury lamp illumination

Figure 8: Comparison of color sample appearance under reference light source and low-pressure sodium lamp

Low-pressure sodium lamps have no color rendering capability at all, in fact they are calculated to have a CRI of -47. All eight samples looked shrouded in a muddy yellow hue. This explains why it is so difficult to find your car in a parking lot illuminated by these lamps at night. All cars look the same under low-pressure sodium lamps, no matter what color they are. (Although the definition of CRI allows for negative CRI values, they usually converge to zero. This is a very bad situation.) Low-pressure sodium lamps present a dilemma for responsible lighting designers: they are extremely efficient, up to 150 lm/W, but are generally unpopular because of the unsightly appearance of objects illuminated by them and the lack of color rendering. In many lighting installations, they have been replaced by slightly less efficient high-pressure sodium lamps and some other light sources with higher CRI values. Table 1 lists the color temperature and CRI values ​​of some commonly used light sources.

● Limitations of CRI

CRI is not a perfect metric. In fact, it only reflects the color rendering of a light source for a small range of medium-saturated colors. The color point of the light source is preferably close to the blackbody radiation locus, and the color temperature of the light source should not be too high or too low. In addition, CRI is not a universally applicable metric. Although the CRI value of incandescent lamps is an ideal 100, if you try to identify different deep blues, you will know that using incandescent lamps is very undesirable. Because the spectrum of incandescent lamps lacks blue spectrum. For blue, daylight is a better choice, but if you observe deep red, then the same bluish daylight may not be the best choice. In fact, no single color temperature light source is ideal for color recognition across the entire spectrum. People need to try to develop a measurement standard that can make color rendering performance indicators applicable in the most common fields.

Table 1: Color temperature and CRI values ​​of commonly used light sources

The recent introduction of new light sources, especially LEDs , has led to a closer look at key points in the CRI definition. The CRI metric was not designed to measure the white light cast by a combination of RGB LEDs, and if applied to LEDs, it can be misleading. For example, since the final CRI value is the arithmetic mean of 8 specific color rendering indices, a light source can have a high CRI value even if it renders one or two colors poorly.

Since RGB LEDs lack a large amount of yellow spectrum, their color rendering of yellow is very poor, but a good CRI value can still be obtained. In addition, since the saturation of the 8 standard color samples is quite low, CRI cannot reflect the color rendering of the light source for highly saturated colors. The peaks of the RGB LED spectrum are narrow and the intervals between the peaks are large. Such a spectral distribution has poor color rendering for saturated colors outside the peaks, but the current CRI definition does not make up for this deficiency.

It is well known that RGB LEDs have the potential to significantly save energy, but deficiencies in color rendering may inhibit their market acceptance. Slight changes in the wavelength and bandwidth of the LEDs used can have a significant impact on the color rendering of this combination light source, so developers who develop lamps using these light sources need an effective measurement standard to evaluate the color rendering of their products. The CRI values ​​of LED lamps with seemingly similar RGB combination light sources can range from a very poor 40 to a very good 80. Even with these values, whether they really represent the quality of the light source's color rendering needs further discussion.