Method for evaluating color rendering of LED illumination source

  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.

2. Limitations of general color rendering index Ra

Although the general color rendering index Ra is simple and practical, it shows a serious deficiency in many aspects.

First of all, color is the subjective feeling of people, not the inherent property of the object. It is related to lighting conditions, observers, irradiance, illuminance, surrounding objects and observation angles. There is no such thing as "real color". However, since in the CIE system, it has been defined that Ra reaches a maximum value of 100 under approximately blackbody radiation, the bulb manufacturer has consciously designed the bulb so that the color rendering when irradiating the object with it is as close as possible to black or sunlight. This means that when the spectral distribution of the light source deviates from the black body or sunlight, the color rendering index is lowered. 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 bad.

But in fact, researchers Judd, Thorntou, and Jerome have confirmed that people do not necessarily like the color of the reference source illumination specified by the CIE. For example, the use of incandescent lamps with very low color temperatures to illuminate green leaves, as mentioned above, is not necessarily the best option. It is stipulated that the color rendering index is the best value Ra=100 in blackbody or sunlight.

The reference source specified by the CIE is the black body or daylight closest to the correlated color temperature of the source to be tested. They are all sources of radiation continuous spectrum with spectral components of multiple colors. When the color temperature is 6500K, the spectral power distribution of the long and short waves is relatively balanced, which should be considered reasonable as a reference light source. However, when the color temperature is below 400K, the spectral power distribution is seriously asymmetrical, and the blue short-wavelength spectral power is much smaller than the red long-wave spectral power, and its color is biased toward red, which is questionable as a reference light source.

In the CIE color system, eight standard swatches are in medium brightness and color saturation, and are equidistantly spaced in the u~v system. They are considered to be adequately representative of various common colors for indoor lighting. However, in outdoor lighting, there are often some colors with higher color saturation. These 8 standard swatches are not enough to represent the common 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 also has 6 swatches with higher color saturation of 9-14, they are not included in the general color rendering index Ra. In lighting practice, the well-known colors are skin, leaves, food, etc., their color is extremely important, but they are excluded from the general color rendering index.

Seim has proposed using 20 standard swatches, but this is rejected because it makes the calculations too complicated. Currently, computers are commonly used, and it seems that this proposal has to be reconsidered.

Since the color rendering evaluation of the light source has two major problems, and many other evaluation methods have attracted wide interest, this article will give a brief introduction to the author's knowledge.

3, the Fleetley index Rf

Studies have shown that people tend to remember the color of a more familiar object, and remember its vivid, saturated color. This memory color tends to be consistent with the favorite color and tends to shift toward a high saturation. For example, the memory color of people's skin color tends to shift toward the red direction, and the color of the leaves shifts toward the green direction. Obviously different from the Ra method in CIE.

Rf is actually a correction to Ra, which includes two aspects:

First, Rf = 90 is defined under the illumination of the reference source, and Rf = 100 only under the imaginary "perfect source" illumination.

Second, select 10 standard swatches, that is, in addition to the original 1-8 standard swatches, plus the 13th and 14th swatches, corresponding to the skin color and leaf color.

At this time, the "perfect light source" refers to a light source that can shift the color of 10 standard swatches in the preferred direction under the illumination thereof.

It can be seen that for each standard swatch, the color coordinates of the corresponding "perfect light source" are different and can be determined experimentally. This also shows that such a "perfect light source" can only be imaginary.

Rf's method of counting is similar to Ra, but there are two differences:

1. For each standard swatch, the color coordinates of the reference source need to be adjusted, that is, the "perfect light source" color coordinates determined according to the experiment. Then, when the light source to be tested is illuminated, the chromatic aberration of each swatch is obtained by comparing it with its corresponding "perfect light source".

2. When calculating Rf, take the average of the color differences of 10 swatches, but the weight of each swatch is different. The 13th swatch is the skin color, the weight is 35%, the 2nd is 15%, the 14th is 15%, and the rest is 5% per piece. The importance of skin color is especially emphasized here. Therefore, the Rf of the light source to be tested may be higher than the reference light source Rf=90, but less than 100.

4, color preference index (CPI)

The color preference index CPI (colour preference index) uses the concept of the favorite color proposed in the previous section, defined under the D65 light source illumination, the color preference index CPI=100.

Then 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 eight standard swatches and the color coordinates of the favorite color, and find the average of the vector sums (): CPI=156-7.18()

The above calculations are all performed in the CIE UV color system.

Although both CPI and Rf take advantage of the concept of favorite color, there is a big difference between the two:

1. When calculating Rf, use 1 to 8 and 13 and 14 total of 10 standard swatches, while CPI only uses 1 to 8 standard swatches.

2. When the technique Rf, the color difference (ΔE) takes 1/5 of the experimental value, and the CPI takes the original experimental value.

3. When calculating Rf, the weights of the swatches are different, and the CPI takes the same weight.

4. According to the definition, the maximum value of Rf is 100, and the maximum value of CPI is 156.

Finally, it is pointed out that the researchers who proposed the two indices of Rf and CPI use experiments to determine the color of love, while in the experiment, daylight illumination is used. There is now evidence that the preferred color is related to the correlated color temperature of the light source. Therefore, when Rf and CPI are used for constant color rendering, only a light source of high color temperature is applied.

5, color resolution index (CDI)

The color rendering of the light source is described by Ra, Rf or CPI, and the reference light source must have the same color temperature as the light source to be tested. The color discrimination index (CDI) overcomes this limitation.

The introduction of this index is based on the assumption that the stronger the ability to distinguish colors under illumination of a certain source, the better the color rendering of the source. When a light source is illuminated, the area of ​​the eight standard swatches in the CIE's 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 defines CDI=100 at this time, so under the illumination of the light source to be tested, the color resolution index is:

CDI=(GA/0.005)×100

6, the conclusion

It can be seen from the above discussion that there are many evaluation methods for the color rendering of light sources, and in the continuous development and improvement, this paper only introduces some of them, and each has its own advantages and disadvantages.

Even with the general color rendering index Ra currently widely used, there are still many disadvantages.

Its main drawback is the choice of the reference source: the reference source is a spectrally continuous source, and it is not very suitable to use it as a standard to measure the discontinuous source of the spectrum. The color temperature of the reference light source must be close to the correlated color temperature of the light source to be tested. In fact, for a certain lighting operation, the color temperature itself has a great influence on the color rendering. This method limits the color temperature that can only be used in the light source. Use under conditions.

Its second drawback is the choice of standard swatches: for indoor lighting, eight standard swatches can be considered to adequately represent a variety of commonly used colors. However, in outdoor lighting, some colors with higher color saturation cannot fully represent the common colors.