Is Ra=100 really the most ideal color rendering?
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.
So 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 International Lighting Commission established the CIE Colorimetric System in 1931 and continued to improve. Today, the CIE chromaticity system has been widely used to quantitatively express the color of light.
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.
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.
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).
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 lower 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 u-v color system, each of the standard swatches is measured, and the difference in color coordinates under illumination of the light 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.