Ra=100 is really the best color rendering?

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Color is one of human feelings, which is always related to the subjective experience of the observer. Each person to see the feeling of a color, it is difficult to know others. Therefore, the study of color is always full of mysterious imagination. At the same time, color makes the world 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, the objective quantitative description of color, has become the object of many scientists. In 1664, Newton used a prism to diffuse the white sun's light into different colors of the spectrum, which laid the foundation for the color of light. In 1860, Maxwell used different intensity of red, yellow and green three colors to match the light from white to all kinds of colors, which laid the foundation of trichromatic colorimetry. On this basis, in 1931, the establishment of the CIE Committee of the International Commission on illumination chromaticity system, and constantly improve. Nowadays, CIE color system has been widely used to quantitatively express the color of light.

Color can not be separated from the lighting, only in the light of the object is likely to show color, and the color of light on people's psychology has a very big impact. Yang Gongxia, a professor at Tongji University, has made a very good description of the fifth chapter of his book, the visual and visual environment. Under different light sources, the same object will show different colors. For example, green leaves in the green light, there is a bright green, red light in the near black. It can be seen that the light source plays an important role in the appearance of the object's color. The ability of the light source to display the color of the object when the object is illuminated is called the color rendering of the light source. In 1965, the Commission recommended that the general color rendering index (Ra) was used to describe the color rendering of the light source in the CIE color system. The general color rendering index (Ra) has been applied successfully, and it has been widely accepted in the lighting industry, but there are also some problems. 1, the general color rendering index Ra color rendering evaluation method, hoping to be simple and practical. Simple and practical, however, are often two conflicting requirements. In the CIE color system, the general color rendering index Ra is such a product of compromise: it is relatively simple, only one less than 100 of the value, can express the color performance of the light source, Ra=100 is considered to be the most ideal color. But sometimes people don't feel that way. For example, the leaves under the incandescent light, 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 methods of the general color rendering index (Ra), without discussing the specific calculation methods.

In fact, in our daily life, we often check the color of the light source. A lot of people have such experience, a careful woman in the mall to buy clothes, often have to look at the outside of the sun to see its color. She did so, in fact, is to test the color of the light source of the market: look at the same piece of clothing, in the light of the market and under the illumination of the sun, the color of the clothes are different. Therefore, the description of the color of the light source, the need for two additional elements: sunlight (light source) and clothing (colored objects). In the CIE color system, in order to determine the color of the light source to be measured, the first choice of reference light source, and that in the light source, the color of the object can be the most perfect display. CIE color system: the color temperature of the source to be measured is less than 5000K, the color of the closest blackbody as the reference light; when the color temperature of the source to be measured is greater than 5000K, with D as the light source color temperature close to the reference light source. Here the D light source is a series of color coordinates can be digitally expressed, and the color temperature of the sun. When a reference light source is selected, a colored object is selected. Because of the diversity of colors, it is necessary to select a set of standard colors, so that they can fully represent the commonly used color. CIE color system to choose 8 colors, they have a variety of colors, and has medium lightness and chroma. In the U-V color system, the special color rendering index Ri can be obtained by measuring the difference of the color coordinates of each standard color plate under the illumination of the light source and the illumination of the reference light source, i.e., the color displacement Ei. (Ri=100 - 4.6 Ei) to calculate the special color index Ri of the 8 standard color plates, the average color index Ra. The maximum value of the general color rendering index Ra of visible light source is 100. 2, the limitations of the general color rendering index Ra, although the general color rendering index Ra is simple and practical, but it has a serious shortage in many ways. First of all, the color of people's subjective feeling, the object attribute is not inherent in it, and the lighting conditions of the observer, irradiance and illuminance, surrounding objects and observation angle and so on, what does not exist in the so-called "real color". But in CIE system, defined Ra reached maximum 100 in approximate blackbody radiation, so the lamp manufacturer consciously design lighting, make it in the irradiation when the object color and bold or sunlight as close as possible. This means that the spectral distribution of the light source will deviate from the blackbody or sunlight. For example, red, green, blue three monochrome LED white LED, when the general color rendering index Ra