Color is one of the human feelings, it is always related to the viewer's personal subjective experience. Everyone sees the feeling of a color, which is difficult for others to know. So the study of color is always full of mysterious imagination. At the same time, the color has made the world colorful, and visual art, image display and transmission, textile printing and dyeing, color printing, etc. are all inseparable from color research. Therefore, the study of color and the objective quantitative description of color have become the object of many scientists' research.
In 1664, Newton used prisms to disperse white sunlight into spectra of different tones, laying the physical foundation for light color. In 1860, Maxwell used three different colors of red, yellow and green light to produce light from white light to various colors, which laid the foundation for trichromatic colorimetry. On this basis, the 1931 International Lighting Commission established the CIE colorimetric system and continued to improve. Today, the CIE chromaticity system has been widely used to quantitatively express the color of light.
The color is inseparable from the illumination. Only in the light can the object show the color, and the color of the light has a great influence on people's psychology. Professor Yang Gongxia from Tongji University has made a very wonderful description in the fifth chapter of his monograph "Visual and Visual Environment".
The same object will show different colors when illuminated by different light sources. For example, green leaves are bright green under green light and nearly black under red light. It can be seen that the light source plays an important role in the appearance of the color of the object being illuminated. When the light source illuminates an object, the ability to fully display the color of the object 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 a light source in the CIE chromaticity system. The general color rendering index Ra has been applied successfully, and has been widely accepted by the lighting industry, but there are also some problems. This paper will introduce the evaluation method of light source color rendering and the progress in recent years.
1. General color rendering index Ra
The evaluation method of the color rendering of the light source is expected to be simple and practical. However, simplicity and practicality are often two conflicting requirements. In the CIE color system, the general color rendering index Ra is such a compromise product: it is relatively simple, only needs a value of less than 100, can express the color rendering performance of the light source, Ra = 100 is considered to be the most ideal color rendering Sex.
However, sometimes people don't feel that way. For example, the leaves under the illumination of incandescent lamps do not look too bright. Where is the problem? Let's discuss what is the general color rendering index.
For the sake of brevity, we will only discuss the main components of the general color rendering index Ra, and not discuss its specific calculation method.
In fact, in our daily lives, we often test the color rendering of light sources. Many people have this kind of experience. When a careful lady buys clothes in a mall, she often has to look at the color of the outdoor sun. 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's light source and under the illumination of the daylight, the color of the clothes is different. So describing the color rendering of a light source requires two additional elements: daylight (reference light source) and clothing (colored objects).
Xiaobian cited a chestnut: I am a friend, the home decoration to buy wooden flooring.
The color of the wooden floor she saw in the store was like this:
When it was sent to home for installation, the sun came in and found it like this:
She couldn't help but feel that the light in the exhibition hall was like a woman's make-up. After unloading the makeup, it was not the original face!
In the CIE color system, in order to determine the color rendering of the light source to be tested, the reference light source is first selected, and it is considered that the color of the illuminated object can be most perfectly displayed under the illumination of the reference light source. The CIE color system stipulates that when the correlated color temperature of the light source to be tested is less than 5000K, the black body with the closest color temperature is used as the reference light source; when the correlated color temperature of the light source to be tested is greater than 5000K, the D light source with the closest color temperature is used as the reference light source. Here, the D source is a series of daylight coordinates that can be represented numerically and related to color temperature.
After selecting the reference source, you also need to select a colored object. Due to the variety of colors, a set of standard colors needs to be selected so that they fully represent the commonly used colors. The CIE color system selects 8 colors, which have multiple tones and medium brightness values ​​and chroma.
In the uv color system, each of the standard swatches is measured, and the difference in color coordinates under illumination of the source to be measured and under the illumination of the reference source, that is, the color shift ΔEi, obtains a special color rendering index Ri of the swatch.
Ri=100—4.6ΔEi
The general color rendering index Ra is obtained by taking the arithmetic average of the special color rendering index Ri measured by the eight standard swatches. The maximum color rendering index Ra of the visible light source has a maximum value of 100, and it is considered that the color rendering property of the light source is the best at this time.
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