Color is one of the human senses, and it is always related to the subjective experience of the observer. It's hard to know how each person feels when they see a color. So the study of color is always full of mysterious imagination. At the same time, color makes the world become colorful, visual art, image display and transmission, textile printing and dyeing, color printing, etc., are inseparable from the study of color. Therefore, the study of color and the objective and quantitative description of color have become the research object of many scientists.

In 1664, Newton used a prism to disperse white sunlight into a spectrum of different shades, laying the physical foundation of light color. In 1860, Maxwell used red, yellow and green light of different intensities to match the light from white light to all kinds of colors, which laid the foundation of tri-chromology. On this basis, in 1931, the International Illumination Commission established the CIE Colorimetry system, which has been continuously improved. Nowadays CIE chromaticity system has been widely used to quantitatively express the color of light.

Color is inseparable from illumination. Only under illumination can objects show color, and the color of light has a great influence on people's psychology. Professor Yang Gongxia of Tongji University has made a wonderful description in the fifth chapter of his monograph "Vision and visual Environment".

Under different light sources, the same object will show different colors. For example, green leaves under the irradiation of green light, have bright green, under the irradiation of red light nearly black. It can be seen that the light source plays an important role in the appearance of the color of the object illuminated. When the light source irradiates the object, whether it can fully show the color of the object illuminated is called the color rendering of the light source.

In 1965, the International Commission on Illumination recommended the use of the general color rendering index Ra to describe the color rendering of light sources in the CIE chromaticity system. The general color rendering index Ra has been successfully applied and widely accepted in lighting field, but there are still some problems. In this paper, the evaluation methods of color rendering of light source and the recent progress are introduced.

1. General color rendering index Ra

The evaluation method of color rendering of light source is expected to be both simple and practical. However, simplicity and practicality are often two contradictory requirements. In the CIE color system, the general color rendering index Ra is such a compromise: it is relatively simple and requires only a value of 100, Ra=100 is considered to be the optimal color rendering performance.

But sometimes it doesn't feel that way. Leaves under an incandescent lamp, for example, do not look very bright. What's the problem? Let's talk about what the general color rendering index is.

For the sake of simplicity, we only discuss the main composition method of the general color rendering index Ra, but not its specific calculation method.

In fact, in our daily life, we often test the color rendering of the light source. Many people have such experience, careful women in the mall to buy clothes, often have to go outside in the sun to look at the color. In doing so, she is essentially testing the color of the store's light: see how the same clothing looks different in the mall light than it does in daylight. So to describe the color rendering of a light source, you need two additional elements: sunlight (the reference light source) and clothing (colored objects).

In CIE color system, in order to determine the color rendering of the source to be measured, the reference light source should be selected first, and it is believed that the color of the illuminated object can be displayed most perfectly under the irradiation of the reference light source. The CIE color system specifies that when the relative color temperature of the source to be measured is below 5000K, the black body with the closest color temperature is used as the reference light source. When the relative color temperature of the source to be measured is greater than 5000K, D light source with the closest color temperature is used as the reference light source. Here D light source is a series of color coordinates that can be expressed digitally and are related to the color temperature of sunlight.

After selecting the reference light source, you also need to select the colored object. Because of the variety of colors, a standard set of colors needs to be selected so that they fully represent the commonly used colors. The CIE Color System has selected eight colors that offer a range of hues, medium brightness values and chroma.

In the U-V color system, the special color rendering index Ri of each standard color plate can be obtained by measuring the difference of color coordinates between the light source to be measured and the light source to be referenced, namely the color displacement δ Ei. (100-4.6 Δ Ri = Ei)

By taking the arithmetic average of the special color rendering index Ri measured from 8 standard color plates, the general color rendering index Ra is obtained. The maximum value of Ra of visible light source is 100, and the light source is considered to have the best color rendering.

2. Limitations of general color rendering index Ra

Although the general color rendering index Ra is simple and useful, it is severely inadequate in many respects.

First of all, color is a subjective feeling rather than an inherent attribute of objects. It is related to lighting conditions, observers, irradiance, illuminance, surrounding objects and observation Angle, etc. There is no so-called "real color".

However, since Ra is defined in the CIE system to reach a maximum value of 100 at near-blackbody radiation, bulb manufacturers have consciously designed the bulb so that the color rendering of the object illuminated with it is as close as possible to that of the blackbody or sunlight. This means that when the spectral distribution of the light source deviates from the black body or sunlight, the color rendering index will decrease. For example, the white LED composed of red, green and blue monochrome LED, when its general color rendering index Ra is low, its color rendering sometimes is not necessarily bad.

But in fact, researchers Judd, Thorntou, and Jerome have shown that people do not necessarily prefer the color of the reference light when illuminated by CIE. For example, using incandescent lamps with very low color temperatures to illuminate green leaves, as mentioned earlier, is not necessarily the best choice. The optimal color rendering index Ra=100 for black body or sunlight is questionable.

The reference source specified by CIE is the blackbody or sunlight that is closest to the color temperature of the source to be measured. Both of them are sources of radiative continuous spectrum with spectral components of multiple colors. When the color temperature is 6500K, the spectral power distribution of long and short wavelengths is more balanced, so it is reasonable to be used as reference light source. However, when the color temperature is below 4000K, the spectral power distribution is seriously asymmetric. The short-wave spectral power of blue is far less than the long-wave spectral power of red, and its color tends to be red, which is questionable as a reference light source.

In the CIE color system, the eight standard palettes are at medium brightness and saturation, spaced equidistant in the U to V system. They are opposite indoor illume, can think already can represent all sorts of commonly used color adequately. But when outdoor illume, often exist the color with a few color saturation higher, these 8 standard color board already cannot represent commonly used color adequately.

Many scholars believe that the number of standard palettes is too small, is another deficiency of the general color rendering index. CIE also has 6 more saturated palettes in numbers 9-14, but they are not included in the general color rendering index Ra. In lighting practice, the well-known colors are skin, leaves, food, etc., whose colors are extremely important, but they are excluded from the general color rendering index.

Seim proposed using 20 standard palettes, but this was rejected as too complicated to calculate. Nowadays, with the widespread use of computers, it seems that this proposal will have to be reconsidered.

Because these two problems exist in the evaluation of color rendering of light source, and many other evaluation methods have aroused wide interest, this paper will briefly introduce the author's knowledge.

3, Fleutley index Rf

Studies have shown that people tend to remember the color of a familiar object when it was vivid and saturated. The memory color is often consistent with the favorite color and tends to shift toward high saturation direction. For example, the memory color of people's skin color tends to shift to the red direction, and the color of leaves tends to shift to the green direction. Obviously different from the Ra method in CIE.

Rf is actually a modification of Ra, which includes two aspects:
First, Rf=90 is defined under the illumination of the reference light source, and Rf=100 is defined only under the hypothetical "perfect light source" illumination.Second, choose 10 standard color plate, that is, in addition to the original 1-8 standard color plate, but also add 13 14 two color plate, corresponding to the skin color and leaf color.

At this moment, "perfect illuminant" point to below its illuminate namely, the illuminant that can offset the color of 10 standard color board to favorite direction.

Thus, for each standard color plate, the color coordinates of the corresponding "perfect light source" are different and can be determined by experiment. This also shows that such a "perfect light source" can only be imagined.

Rf is measured in a similar way to Ra, but with two differences:
1. For each standard color plate, the color coordinates of the reference light source need to be adjusted, that is, the color coordinates of the "perfect light source" determined by the experiment. Then, when the metering source is illuminated, the chromatic aberration of each panel is compared with its corresponding "perfect light source".1. For each standard color plate, the color coordinates of the reference light source need to be adjusted, that is, the color coordinates of the "perfect light source" determined by the experiment. Then, when the metering source is illuminated, the chromatic aberration of each panel is compared with its corresponding "perfect light source".

4. Color Preference Index (CPI)

Using the concept of colour preference index proposed in the previous section, it is defined that the colour preference index CPI =100 under D65 lighting.

So CPI of the source to be measured can be obtained as follows: Under the irradiation of the source to be measured, calculate the difference between the color coordinates of the 8 standard color plates and the color coordinates of their favorite color, and calculate the average value of their vector sum () : CPI=156-7.18 ()

The above calculations were performed in CIE's UV chromaticity system.

Although both CPI and Rf use the concept of favorite color, they are very different:
1. In the calculation of Rf, use 1-8 and 13, 14 a total of 10 standard color boards, while CPI only 1-8 standard color boards
2. For Rf, the color difference (δ E) is 1/5 of the experimental value, while CPI is the original experimental value
3. When calculating Rf, the weights of each panel are different, but the weights of CPI are the same4. By definition, the maximum value of Rf is 100, and the maximum value of CPI is 156

Finally, it should be pointed out that the researchers who proposed Rf and CPI both used experiments to determine their favorite colors, and daylight color lighting was used in the experiments. There is now evidence that color preference is related to the relative color temperature of the light source. So when Rf and CPI are used to measure color rendering, only high color temperature light sources are suitable.

5. Color discrimination Index (CDI)

Ra, Rf or CPI is used to describe the color rendering of the light source. The reference light source must have the same color temperature as the source to be measured. Color Discrimination Index (CDI) overcomes this limitation

The index is based on the assumption that the greater the ability to distinguish colours under a light source, the better the colour rendering of that light source. When illuminated by a certain light source, eight standard color plates are enclosed in the UV chromaticity diagram of CIE, and the area is GA =0.5 σ (uivj-ujvi) I,j=1,2... 8; I indicates j.

Under the illumination of C source, the area GA=0.005, and CDI=100 by definition, so under the illumination of the source to be measured, the color discrimination index is: CDI= (GA/0.005) ×100.

conclusion

From the above discussion, it can be seen that there are many evaluation methods for color rendering of light source, and in the process of continuous development and improvement, this paper introduces only a part of them, each of which has its own advantages and disadvantages. Even Ra, the general color rendering index widely used today, still has many disadvantages.

Its main disadvantage is the choice of reference light source: reference light source is a spectral continuous light source, it is not appropriate to use it as a standard to measure the spectral discontinuous light source. The color temperature of the reference light source must be close to the relative color temperature of the source to be measured. In fact, for certain lighting operations, the color temperature itself has a great influence on the color rendering. This method can only be used under the condition that the color temperature of the light source is already determined.

Its second drawback is the choice of standard color palette: for indoor lighting, it can be considered that 8 standard color palette can adequately represent all kinds of commonly used colors. But in outdoor lighting, to some color saturation higher color, can not fully represent the commonly used color.