Color is one of human senses, and it is always related to the observer's personal subjective experience. It is difficult for others to know how everyone 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 colorful. Visual arts, image display and transmission, textile printing and dyeing, color printing, etc. are all inseparable from the study of color. Therefore, the study of color and the objective and quantitative description of color have become the object of many scientists' research.
Newton in 1664 used a prism to disperse white sunlight into a spectrum of different hues, laying the physical foundation for the color of light. In 1860, Maxwell used different intensities of red, yellow, and green to mix light from white light to various colors, laying the foundation for trichromatic chromaticity. On this basis, in 1931, the International Commission on Illumination established the CIE chromaticity system, which has been continuously improved. Today the CIE chromaticity system is widely used to quantitatively express the color of light.
Color is inseparable from lighting. Only under the light can objects show color, and the color of light has a great impact on people's psychology. Professor Yang Gongxia of Tongji University has made a very wonderful description in the fifth chapter of his monograph "Vision and Visual Environment".
Under different light sources, the same object will display different colors. For example, green leaves are bright green under green light, and almost black under red light. It can be seen that the light source plays an important role in the appearance of the color of the illuminated object. Whether the light source can fully display the color of the object when it illuminates the object is called the color rendering of the light source.
In 1965, the International Commission on Illumination recommended that the general color rendering index Ra be used to describe the color rendering of light sources in the CIE chromaticity system. The general color rendering index Ra has been successfully applied and has been widely accepted by the lighting industry, but there are still some problems. This article will introduce the evaluation method of the color rendering of light sources and the progress in recent years.
1. General color rendering index Ra
The evaluation method of the color rendering property 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 the product of such a compromise: it is relatively simple, and only a value within 100 is required to 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, leaves under incandescent light do not look very bright. Where is the problem? Let's discuss what is the general color rendering index.
For the sake of brevity, we only discuss the main composition method of the general color rendering index Ra here, but do not discuss its specific calculation method.
In fact, in our daily life, we often test the color rendering of light sources. Many people have such experience. When a careful lady buys clothes in the mall, she often has to go outside to see its color in the sunlight. In doing so, she is actually testing the color rendering of the shopping mall light source: to see the difference in the color of the same piece of clothing under the lighting of the shopping mall light source and under the lighting of sunlight. So to describe the color rendering of a light source, two additional elements are required: sunlight (the reference light source) and clothing (the colored object).
In the CIE color system, in order to determine the color rendering of the light source to be measured, the reference light source must be selected first, and it is believed that the color of the illuminated object can be displayed most perfectly under the illumination of the reference light source. The CIE color system stipulates: when the correlated color temperature of the light source to be measured 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 measured is greater than 5000K, the D light source with the closest color temperature is used as the reference light source. Here a D illuminant is a series of sunlight whose color coordinates can be represented numerically and are related to color temperature.
After selecting a reference light 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 can adequately represent commonly used colors. The CIE color system has selected 8 colors, which have both a variety of shades and medium lightness and chromaticity.
In the u-v color system, the special color rendering index Ri of the color plate can be obtained by measuring the difference between the color coordinates of each standard color plate under the illumination of the light source to be measured and the illumination of the reference light source, that is, the color shift ΔEi. (Ri=100—4.6ΔEi)
The general color rendering index Ra is obtained by taking the arithmetic mean of the special color rendering index Ri measured by 8 standard color plates. 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 of the light source is the best at this time.
2. The limitations of the general color rendering index Ra
Although the general color rendering index Ra is simple and practical, it shows serious shortcomings in many aspects.
First of all, color is a subjective feeling of people, not an inherent property of objects. It is related to lighting conditions, observers, irradiance, illuminance, surrounding objects and viewing angles, etc. There is no so-called "true color".
However, since in the CIE system, Ra has been defined to have a maximum value of 100 at approximately blackbody radiation, bulb manufacturers have consciously designed bulbs so that the color rendering when illuminating an object with it is as close as possible to that of a blackbody or sunlight. This means that when the spectral distribution of the light source deviates from a 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 is sometimes not necessarily very bad.
But in fact, researchers Judd, Thortontou and Jerome have confirmed that people do not necessarily like the color of the reference light source specified by the CIE. For example, illuminating green leaves with incandescent lamps with a very low color temperature, as already mentioned, is not necessarily the best choice. It is stipulated that the color rendering index is the best value Ra=100 when the black body or sunlight is illuminated, and there are doubts.
The reference light source specified by CIE is the black body or sunlight that is closest to the correlated color temperature of the light source to be measured. They are all light sources that radiate continuous spectrum and have spectral components of multiple colors. When the color temperature is 6500K, the spectral power distribution of the long and short wavelengths is relatively balanced, which should be reasonable as a 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 much smaller than the long-wave spectral power of red, and its color is biased toward red, which is problematic as a reference light source.
In the CIE color system, the 8 standard color palettes are all at medium lightness and saturation, and are equally spaced in the u~v system. They are considered sufficient for indoor lighting to represent a wide range of commonly used colors. However, in outdoor lighting, there are often some colors with high color saturation, and these 8 standard color palettes can no longer fully represent the 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 CIE has 6 swatches with high color saturation of No. 9-14, they are not included in the general color rendering index Ra. In lighting practice, well-known colors are skin, leaves, food, etc. Their colors are extremely important, but they are all excluded from the general color rendering index.
Seim had proposed using 20 standard swatches, but was rejected because it would complicate the calculations. At present, computers are widely used, and it seems that this proposal has to be reconsidered.
Since these two problems exist in the evaluation of color rendering of light sources, and many other evaluation methods have attracted wide interest, this paper will give a brief introduction to the author's knowledge.
3. Fletelli index Rf
Research has shown that people tend to remember the color of a more familiar object, and that it remembers its vivid, more saturated color. This memory color is often consistent with the favorite color, and tends to shift in the direction of high saturation. For example, the memory color of people's skin color tends to shift to the red direction, and the leaf color shifts to the green direction. Obviously different from the Ra method in CIE.
Rf is actually a correction to Ra, which includes two aspects:
First, define Rf=90 under the illumination of the reference light source, only under the illumination of an imaginary "perfect light source", there is Rf=100.
Second, choose 10 standard color swatches, that is, in addition to the original 1-8 standard swatches, add two swatches of 13 and 14, corresponding to skin color and leaf color.
At this time, the "perfect light source" refers to the light source that can shift the colors of the 10 standard color palettes to the favorite direction under its illumination.
It can be seen that for each standard color plate, the color coordinates of the corresponding "perfect light source" are different and can be determined by experiments. This also shows that such a "perfect light source" can only be imaginary.
The calculation method of Rf is similar to that of Ra, but there are 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 light source to be measured is illuminated, the color difference of each color plate is compared with its corresponding "perfect light source".
2. When calculating Rf, take the average value of the color difference of 10 swatches, but each swatch has a different weight. The 13th swatch is the skin tone, the weight is 35%, the 2nd is 15%, the 14th is 15%, and the rest is 5% each. The importance of skin tone is particularly emphasized here. Therefore, the Rf of the light source to be tested can be higher than the reference light source Rf=90, but less than 100.
4. Color Preference Index (CPI)
Color preference index CPI (colour preference index) uses the concept of favorite color proposed in the previous section, and is defined under the illumination of D65 light source, and the color preference index CPI = 100.
So the CPI of the light source to be tested can be obtained as follows: under the illumination of the light source to be tested, calculate the difference between the color coordinates of the 8 standard color palettes and the color coordinates of the favorite color, and find the average value of the vector sum ( ): CPI=156 -7.18()
The above calculations are all carried out in the CIE UV chromaticity system.
Although both CPI and Rf utilize the concept of favorite color, there are significant differences between the two:
1. When calculating Rf, use 10 standard swatches of No. 1-8, 13 and 14, while CPI only uses 1-8 standard swatches
2. When the technical Rf is used, the color difference (ΔE) takes 1/5 of the experimental value, and the CPI takes the original experimental value
3. When calculating Rf, the weight of each swatch is different, and the CPI takes the same weight
4. By definition the maximum value of Rf is 100, while the maximum value of CPI is 156
Finally, it should be pointed out that the researchers who proposed the two indices, Rf and CPI, used experiments to determine their favorite colors, and they used daylight-colored lighting in the experiments. There is now evidence that favorite colors are related to the correlated color temperature of the light source. Therefore, when using Rf and CPI for constant color rendering, only light sources with high color temperature are applicable.
5. Color Discrimination Index (CDI)
Use Ra, Rf or CPI to describe the color rendering of a light source. The reference light source must have the same color temperature as the light source to be measured. The colour discrimination index CDI (colourdiscrimination index) overcomes this limitation.
This index is 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 this light source. When a certain light source is illuminated, the area enclosed by 8 standard color plates in the CIE UV chromaticity diagram is: GA =0.5Σ(UiVj-UjVi) i,j=1,2,…8; i≠j .
Under the illumination of C light source, the area GA=0.005, which is defined as CDI=100 at this time, so under the illumination of the light source to be measured, the color resolution index is: CDI=(GA/0.005)×100.
It can be seen from the above discussion that there are many evaluation methods for the color rendering of light sources, and they are constantly developing and improving. Even the general color rendering index Ra, which is widely used today, has many disadvantages.
Its main disadvantage 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 the spectrally discontinuous light source. The color temperature of the reference light source must be close to the correlated color temperature of the light source to be measured. In fact, for a certain lighting operation, the color temperature itself has a great influence on the color rendering. This method can only be used when the color temperature of the light source has been determined. use under conditions.
Its second disadvantage is the choice of standard color swatches: for indoor lighting, it can be considered that 8 standard swatches can adequately represent various commonly used colors. However, in outdoor lighting, some colors with high color saturation cannot fully represent commonly used colors.
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