Is Ra=100 really the best color rendering?

led sunlight fixtures spectrum

Foreword

Color is one of the feelings of people. It is always related to the individual subjective experience of the observer. The feeling that everyone sees a color is difficult for others to know. So the study of color is always full of mysterious imagination. At the same time, colors have made the world colorful, and visual arts, image display and transmission, textile printing and dyeing, and color printing are all inseparable from the study of color. Therefore, the study of color and the objective quantitative description of color have become the object of study for many scientists.

In 1664 Newton used a prism to disperse white sunlight into a spectrum of different shades, laying the physical foundation for light color. In 1860, Maxwell used different intensity red, yellow and green light to match the light from white light to various colors, which laid the foundation for trichromatic colorimetry. On this basis, the International Commission on Illumination established the CIE colorimetric system in 1931 and continued to improve it. CIE color systems are now widely used to quantitatively express the color of light.

Color can not be separated from lighting, only in the light of the object is possible to show color, and the color of light has a very great influence on people’s psychology. Professor Yang Gongxia from Tongji University has made a wonderful description in the fifth chapter of his book “Visual and Visual Environment.”

Under different light sources, the same object will show different colors. For example, the green leaves are bright green under green light, and nearly black under red light. Thus, the light source plays an important role in the appearance of the illuminated object. The ability of the light source to fully display the color of the object being illuminated is known as the color rendering of the light source.

In 1965, the International Commission on Illumination recommended using the general color rendering index Ra to describe the color rendering of a light source in the CIE colorimetric system. The general color rendering index Ra has been applied successfully, and it has been widely accepted by the lighting industry. However, there are also some problems. This article will give an introduction to the methods of evaluating the color rendering of light sources and the progress made in recent years.

The general color rendering index Ra

The evaluation method of the light source color rendering is hoped to be both simple and practical. However, simplicity and utility are often two conflicting requirements. In the CIE color system, the general color rendering index Ra is a product of such a trade-off: it is relatively simple, only requires a value within 100 to express the color rendering performance of the light source, and Ra=100 is considered as the optimal color rendering. Sex.

However, sometimes people do not feel that way. For example, leaves under incandescent lighting do not look too bright. Where is the problem? Let’s discuss what is the general color rendering index.

For the sake of simplicity, we will only discuss the main constitutional methods of the general color rendering index Ra without discussing its specific calculation method.

In fact, we often examine the color rendering of light sources in our daily lives. Many people have this experience. When an attentive lady buys clothes at a shopping mall, she often has to look outside to see its color. In doing so, she is actually testing the color rendering of the mall’s light source: look at the same piece of clothing, under the lighting of the mall light source and under the illumination of the sun, what is the color of the clothes. So to describe the color rendering of a light source, two additional elements are needed: daylight (reference light source) and clothes (colored objects).

In the CIE color system, in order to determine the color rendering property of the light source to be measured, a reference light source is first selected, and it is considered that under the irradiation of the reference light source, the color of the illuminated object can be displayed most perfectly. The CIE color system provides that: when the correlated color temperature of the light source to be measured is lower than 5000K, the blackbody with the closest color temperature is used as a reference light source; when the correlated color temperature of the light source to be measured is greater than 5000K, the D light source with the closest color temperature is used as a reference light source. The D light source here is a series of sunlight that can be digitally represented by color coordinates and related to the color temperature.

After the reference light source is selected, colored objects also need to be selected. Due to the diversity of colors, it is necessary to select a set of standard colors so that they can fully represent commonly used colors. The CIE color system has selected 8 colors, which have a variety of hues, as well as medium brightness values ​​and chromaticity.

In the u-v color system, each standard swatch is measured, and the special color rendering index Ri of the swatch can be obtained under the illumination of the light source to be measured and the difference in color coordinates under the illumination of the reference light source, that is, the color displacement ΔEi. (Ri=100-4.6ΔEi)

The average color rendering index Ra is obtained by arithmetically averaging the special color rendering index Ri measured for the eight standard swatches. The maximum value of the general color rendering index Ra of the visible light source is 100, and it is considered that the color rendering property of the light source is best at this time.

Limitations of the general color rendering index Ra

Although the general color rendering index Ra is simple and practical, it shows serious shortages in many aspects.

First of all, color is the subjective feeling of people. It is not an inherent property of an object. It is related to lighting conditions, observers, irradiance, illumination, surrounding objects, and observation angles. There is no so-called “real color”.

However, since in the CIE system, Ra is defined to have a maximum value of 100 under irradiation of approximately black body, the bulb manufacturer consciously designs the light bulb so that the color rendering property when irradiating the object with it is as close as possible to the black body or sunlight. This means that when the spectral distribution of the light source deviates from the black body or sunlight, the color rendering index decreases. For example, a white LED composed of three monochromatic LEDs of red, green, and blue, when its general color rendering index Ra is low, its color rendering property is not always very bad.

But in fact, researchers Judd, Thorntou, and Jerome have proven that people don’t necessarily like the color of the reference light source lighting specified by CIE. For example, it is not necessarily the best choice to illuminate green leaves with an incandescent lamp with a low color temperature. It is stipulated that the color rendering index is an optimum value of Ra=100 in black body or sunlight irradiation.

The reference light source specified by CIE is the black body or daylight that is the closest to the correlated color temperature of the light source to be measured. They are all light sources that emit continuous spectrum and have spectral components of various colors. When the color temperature is 6500K, the spectral power distribution of the long-short wave is relatively balanced, and it should be said as a reasonable reference light source. However, when the color temperature is below 4000K, the spectral power distribution is seriously asymmetric, and the blue short-wave spectral power is much smaller than the red long-wave spectral power. Its color is biased toward red, and there is doubt as a reference light source.

In the CIE color system, the eight standard swatches are at medium brightness and color saturation, and are equally spaced in u~v systems. For indoor lighting, they are considered to be able to adequately represent a variety of commonly used colors. However, in outdoor lighting, there are often colors with high color saturation, and these 8 standard color plates cannot fully represent commonly used colors.

Many scholars believe that the number of standard swatches is too small, which is another deficiency of the general color rendering index. Although the CIE still has 6 color plates with higher saturation levels 9-14, they are not included in the general color rendering index Ra. In lighting practice, people are familiar with the colors of the skin, leaves, food, etc., their color is extremely important, but they are excluded from the general color rendering index.

Seim had proposed using 20 standard swatches, but this was rejected because it would make the calculation too complicated. Currently, computers are widely used. It seems that this proposal has to be reconsidered.

Since there are two major problems in the evaluation of the color rendering of the light source and many other evaluation methods have caused widespread interest, this article will briefly introduce the author’s knowledge.

Fleury index Rf

Studies have shown that people tend to remember the colors of more familiar objects and remember their vivid, saturated colors. This memory color is often consistent with the favorite color, and tends to shift to the high saturation. For example, the memory color of people’s skin color tends to shift in the red direction and the color of the leaves shifts in the green direction. Obviously different from the Ra method in CIE.

Rf is actually a correction to Ra. This amendment includes two aspects:
First, Rf=90 is defined under the illumination of the reference light source, and only Rf=100 under the imaginary “perfect light source” illumination.
Second, choose 10 standard swatches, that is, in addition to the original 1-8 standard swatches, plus 13 swatches of 14th, corresponding to skin color and leaf color.

At this time, the “perfect light source” refers to the light source that can displace the colors of the 10 standard swatches toward the preferred direction under its illumination.

Thus, for each standard swatch, the color coordinates of the corresponding “perfect light source” are different and can be determined experimentally. This also shows that such a “perfect light source” can only be hypothetical.

Rf’s method for calculating bids is similar to Ra, but it differs in two ways:
1. For each standard swatch, the color coordinate of the reference light source needs to be adjusted, that is, the “perfect light source” color coordinate determined according to the experiment. Then, when the light source to be measured is illuminated, the color difference of each swatch is compared with its corresponding “perfect light source”.
2. When calculating Rf, take the average of the color difference of 10 swatches, but the weight of each swatch is different. The swatch number 13 is skin color, the weight is 35%, number 2 is 15%, number 14 is 15%, and the rest is 5% each. This emphasizes the importance of skin color. Therefore, the Rf of the light source to be measured may be higher than the reference light source Rf=90 but less than 100.

Color preference index (CPI)

The color preference index (CPI) uses the concept of love color proposed in the previous section. It is defined under the illumination of the D65 light source and the color preference index CPI =100.

So the CPI of the light source to be measured can be obtained by calculating the difference between the color coordinates of the 8 standard swatches and the color coordinates of the favorite color under the light source to be measured, and calculating the average of the vector sums ( ): CPI=156 -7.18()

The above calculations are performed in the CIE’s UV colorimetric system.

Although both CPI and Rf use the concept of favorite color, the two are quite different:
1. When calculating Rf, use 10 standard swatches for numbers 1 to 8 and 13 and 14, while CPI uses only 1 to 8 standard swatches
2. When the technique Rf, the color difference (ΔE) takes 1/5 of the experimental value, and the CPI takes the original experimental value
3. When calculating Rf, the weights of the swatches in each block are different, and the CPI takes the same weight.
4. According to the definition, the maximum value of Rf is 100, and the maximum value of CPI is 156.

Finally, it should be pointed out that researchers who proposed the two indexes of Rf and CPI both determined the favorite color experimentally, and used daylight color illumination in the experiment. There is now evidence that the love color relates to the correlated color temperature of the light source. Therefore, when using Rf and CPI for constant color rendering, only high color temperature light sources are used.

Color separation index (CDI)

Use Ra, Rf or CPI to describe the color rendering of the light source. The reference light source must have the same color temperature as the light source to be measured. The color discrimination index (CDI) overcomes this limitation.

The index proposes an assumption based on the assumption that under the illumination of a certain light source, the stronger the ability to distinguish colors, the better the color rendering of the light source. When lighting a certain light source, the area of ​​the eight standard swatches in the CIE UV chromaticity diagram is: GA = 0.5Σ(UiVj-UjVi) i,j=1,2,…8; i≠j .

Under C light source illumination, this area GA = 0.005, define CDI = 100 at this moment, so under the illumination of the light source to be measured, its chroma exponent index is: CDI = (GA/0.005) * 100.

Conclusion

From the above discussion, we can see that there are many ways to evaluate the color rendering of light sources, and in the course of continuous development and improvement, this article is only a part of them. Each has its own advantages and disadvantages. Even the widely used general color rendering index Ra has many disadvantages.

Its main drawback is the choice of the reference light source: The reference light source is a spectrally continuous light source, and it is not very suitable to use it as a standard to measure light sources with discontinuous spectra. The color temperature of the reference light source must be similar to the correlated color temperature of the light source to be measured, and in fact, for a certain lighting operation, the color temperature itself has a great influence on the color rendering. This method limits the color temperature of the light source that can only be used. Use under conditions.

Its second drawback is the choice of standard swatches: For indoor lighting, it can be assumed that 8 standard swatches can adequately represent a variety of commonly used colors. However, in outdoor lighting, colors with high color saturation do not fully represent commonly used colors.