Evaluation method of color rendering of the hottes

Evaluation method of color rendering of LED lighting source

color is one of human feelings, which is always related to the personal subjective experience of the observer. Everyone's feeling after seeing a color is difficult for others to know. 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 and the objective and quantitative description of color have become the research object of many scientists

color cannot be separated from lighting. Only under light can objects show color, and the color of light has a great impact on people's psychology. Professor yanggongxia of Tongji University has made a wonderful description in the fifth chapter of his monograph vision and visual environment

under the illumination of different light sources, the same object will show different colors. 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 display of the color of the illuminated object. When a light source irradiates an object, whether it can fully display the color of 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 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 methods of color rendering of light sources and the progress in recent years

1. The general color rendering index ra

evaluation method of the color rendering of light sources is expected to be simple and practical. However, simplicity and practicality are often contradictory requirements. In CIE color system, the general color rendering index RA is such a compromise product: it is relatively simple. It only needs a value within 100 to express the color rendering performance of the light source. Ra=100 is considered to be the most ideal color rendering performance

but sometimes people don't feel that way. For example, leaves illuminated by incandescent lamps do not look very bright. What's the problem? Let's discuss what is the general color rendering index

for simplicity, we only discuss the main composition method of the general color rendering index RA, rather than its specific calculation method

in fact, we often test the color rendering of light sources in our daily life. Many people have this experience. When a careful lady buys clothes in a mall, she often has to go outdoors to have a look at its color in the sun. In doing so, she is actually testing the color rendering of the light source in the shopping mall: take a look at the same clothes. Under the illumination of the light source in the shopping mall and the illumination of the sunlight, what is the difference in the color of the clothes. Therefore, two additional elements are needed to describe the color rendering of light sources: sunlight (reference light source) and clothes (colored objects)

take chestnuts for example: a friend of mine wants to buy wooden floors for home decoration

the color of the wooden floor she saw in the store is like this:

when she sent it home for installation, the sun shone in and found it like this:

she couldn't help sighing: the lights in the original exhibition hall were like women's makeup, and it was not the original face after removing makeup

in the CIE color system, in order to determine the color rendering of the light source to be tested, the reference light source must be selected first, and it is considered that the color of the illuminated object can be displayed most perfectly under the illumination of the reference light source. CIE color system stipulates that when the relevant color temperature of the light source to be measured is lower than 5000K, the blackbody with the closest color temperature is used as the reference light source; When the relevant 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, D light source is a series of sunlight whose color coordinates can be expressed digitally and related to color temperature

after selecting the reference light source, you also need to select the colored object. Due to the diversity of colors, it is necessary to select a group of standard colors so that they can fully represent the commonly used colors. The CIE color system selects eight colors, which not only have a variety of hues, but also have medium lightness and chromaticity

in the U-V color system, measure the difference between the color coordinates of each standard color plate under the irradiation of the light source to be measured and the reference light source, that is, the color displacement Δ EI, the special color rendering index RI of the palette can be obtained

Ri=100—4.6 Δ EI

take the arithmetic mean of the special color rendering index RI measured by eight standard color plates to obtain the general color rendering index RA. 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. Limitations of the general color rendering index RA

although the general color rendering index RA is simple and practical, it shows serious deficiencies in many aspects

first of all, color is a subjective feeling of people, not an inherent attribute of objects. It is related to lighting conditions, observers, irradiance, illuminance, surrounding objects and observation angle. There is no so-called "real color". However, in the CIE system, it has been defined that RA reaches a maximum value of 100 in the radiation similar to blackbody, so bulb manufacturers consciously design bulbs to make the color rendering when irradiating objects with it as close as possible to that when irradiating objects with blackbody or sunlight. This means that when the spectral distribution of the light source deviates from that of blackbody or sunlight, the color rendering index will decrease. For example, when the general color rendering index Ra of white LED composed of red, green and blue monochromatic LEDs is low, its color rendering performance is sometimes not necessarily very bad

but in fact, researchers Judd, thorntou and Jerome have confirmed that people do not necessarily like the color of reference light source illumination specified by CIE best. For example, as mentioned earlier, irradiating green leaves with incandescent lamps with low color temperature is not necessarily the best choice. It is specified that the color rendering index is the best value ra=100 when exposed to blackbody or sunlight, which is questionable

the reference light source specified by CIE is blackbody or sunlight that is closest to the relevant 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 long and short wavelength is relatively balanced, which should be said to be more reasonable as a reference light source. However, when the color temperature is below 400k, 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 is biased towards red, which is questionable as a reference light source

in the CIE color system, the eight standard color swatches are at medium lightness and color saturation, and are equidistant intervals in the U-V system. For indoor lighting, they can be considered to be fully representative of various commonly used colors. However, in outdoor lighting, there are often some colors with high color saturation, and these eight standard color palettes can no longer fully represent common colors

many scholars believe that the number of standard color plates is too small, which is another deficiency of the general color rendering index. Although CIE has six color swatches with high color saturation of No. 9-14, they are not included in the general color rendering index RA. In lighting practice, people are familiar with the colors of skin, leaves, food, etc. their colors are extremely important, but they are excluded from the general color rendering index

seim once proposed to use 20 standard color plates, but it was rejected because it would make the calculation too complicated. At present, computers are widely used, and it seems that this proposal needs to be reconsidered

due to these two major problems in the evaluation of color rendering of light sources, many other evaluation methods have aroused widespread interest. This paper will briefly introduce what the author knows

3. The study of freightly index rf

shows that people tend to remember the color of familiar objects, and remember its vivid color with high saturation. This memory color is often consistent with the favorite color, and tends to shift towards 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, it is different from the RA method in CIE

rf is actually a correction of RA, which includes two aspects:

first, rf=90 is defined under the illumination of the reference light source, and rf=100 is only available under the illumination of the imaginary "perfect light source"

second, choose 10 standard color plates, that is, in addition to the original standard color plate, add two color plates 13 and 14, corresponding to skin color and leaf color

the tensile machine board mentioned here does not refer to the computer board

at this time, "perfect light source" refers to the light source that can shift the color of 10 standard color boards to the favorite direction under its irradiation

it can be seen that for each standard color board, the color coordinates of the corresponding "perfect light source" are different, which can be determined by experiments. This also shows that such a "perfect light source" can only be imaginary

rf's calibration method is similar to 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 according to the experiment. Then, when the light source to be tested is illuminated, the color difference of each color plate is obtained by comparing with its corresponding "perfect light source"

2. When calculating RF, take the average value of color difference of 10 color plates, but the weight of each color plate is different. The color palette No. 13 is skin color, and the weight is 35%, No. 2 is 15%, No. 14 is 15%, and the rest is 5% for each piece. The importance of skin color is 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 (color preference index) uses the concept of favorite color proposed in the previous section to define that under the illumination of D65 light source, the color preference index cpi=100

then the CPI of the light source to be tested can be obtained in this way:

under the irradiation of the computer of the light source to be tested, which includes the collection, transmission, reaction, analysis, display and printing of test data, calculate the difference between the color coordinates of the eight standard color plates and the color coordinates of the favorite color, and calculate the average value of the vector sum (): cpi= 18 ()

the above calculations are carried out in CIE's UV colorimetric system

although both CPI and RF use the concept of favorite color, there are great differences between them:

1. When calculating RF, 10 standard color plates of No. 1-8, 13 and 14 are used, while CPI only uses 1-8 standard color plates

2. When technical RF is used, the color difference（ Δ E) Take 1/5 of the experimental value, while CPI takes the original experimental value

3. When calculating RF, the weights of each color board are different, while CPI takes the same weight for these problems of low industry concentration

4. According to the definition, the maximum value of RF is 100, while the maximum value of CPI is 156

finally, it should be pointed out that researchers who proposed the two indexes of RF and CPI use experiments to determine favorite colors, and sunlight color lighting is used in the experiment. Now there is evidence that the favorite color is related to the relevant color temperature of the light source. Therefore, when RF and CPI are used to constant color rendering, it is only suitable for light sources with high color temperature

5, color