Evaluation method of color rendering of LED lighting

In 1965, the International Commission on Illumination recommended in the CIE chromaticity system, with the general color rendering index Ra to describe the light color. General color rendering index Ra has been very successful, has been widely accepted by the lighting industry, but there are some problems, this paper will introduce the light color evaluation methods as well as the progress of recent years.

1, The general color rendering index Ra

It hopes that the evaluation method of color of light source is to be both simple and practical. However, simple and practical is often two contradictory requirements. In the CIE color system, the general color rendering index Ra is such a compromise product: it is relatively simple, only a value of 100 or less, you can express the light color performance, Ra = 100 is considered the ideal color Sex.

However, sometimes people feel that is not the case. For example, in the incandescent light of the leaves, looks and not very bright. Where is the problem? Let's talk about what is the general color rendering index.

For the sake of simplicity, we only discuss the main constituent methods of the general color rendering index Ra, without discussing its specific calculation method.

In fact, in our daily life, we often check the color of the light source. Many people have this experience, careful ladies in the mall to buy clothes, often also to the outdoor sun and then look at its color. She did so, in fact, is to test the color of the light source of shopping malls: take a look at the same piece of clothing, under the lighting in the mall light and under the lighting of the sun, the color of the clothes are different. So describe the color of the light source, the need two additional elements: daylight (reference light source) and clothes (colored objects).

In the CIE color system, in order to determine the color rendering of the light source to be measured, first select the reference light source, and that the reference light source irradiation, the color of the object can be the most complete display. CIE color system provides that: the color temperature of the light source to be measured below the 5000K, the color temperature of the closest black body as a reference light source; when the measured light source relative color temperature is greater than 5000K, with the most similar color temperature D light source as a reference light source. Here D light source is a series of color coordinates can be expressed in digital, and related with the color temperature of the daylight.

After selecting the reference light source, you also need to select colored objects. Due to the diversity of colors, you need to select a set of standard colors that make them fully representative of the commonly used colors. CIE color system selected eight colors, they have a variety of colors, but also has a moderate degree of brightness and saturation.

In the u-v color system, the specific color rendering index Ri of the color palette can be obtained by measuring the difference between the color coordinates of each block and the color coordinates under the irradiation of the light source and the reference light source.

Ri = 100-4.6 

The average color rendering index Ra is obtained by taking the arithmetic average of the special color rendering index Ri measured by eight standard color plates. The maximum color rendering index Ra of the visible light source is 100, and it is considered that the color of the light source is the best.

2, The general color rendering index Ra limitations

Although the general color rendering index Ra is simple and practical, it exhibits a serious shortage in many respects.

First of all, the color is the subjective feeling, not the inherent properties of the object, it is with the lighting conditions, observers, irradiance, illumination, the surrounding objects and observation angle, etc., there is no so-called "real color." However, since the Ra has been defined to be the highest value of 100 in the CIE system, the bulb manufacturer has intentionally designed the bulb so that the color rendering when irradiating the object is as close as possible to the case of blackbody or sunlight. This means that the spectral distribution of the light source deviates from the blackbody or the daylight, causing the color rendering index to fall. For example, white, LED, consisting of red, green and blue monochrome LEDs, when its general color rendering index Ra is low, its color is sometimes not necessarily bad.

But in fact, the researchers Judd, Thorntou and Jerome have confirmed that people do not necessarily like the CIE's specified reference light when illuminating the color. For example, it has been mentioned earlier with very low color temperature incandescent light green leaves, and not necessarily the best choice. It is assumed that the color rendering index is the optimum value Ra = 100 in blackbody or sunlight.

The reference light source specified by the CIE is the bold or daylight closest to the color temperature of the light source to be measured. They are light sources that radiate continuous spectrum and have spectral components of various colors. When the color temperature at 6500K, the length of the short-wave spectral power distribution is more balanced, as the reference light source should be said to be more reasonable. However, when the color temperature below 400K, the spectral power distribution is seriously asymmetric, blue short-wave spectral power is much smaller than the red long-wave spectral power, the color bias red, as a reference light source in doubt.

In the CIE color system, eight standard swatches are in moderate and color saturation, equidistant intervals in the u-v system. They are for indoor lighting, can be considered to be able to fully represent a variety of commonly used colors. But in the outdoor lighting, there are often some color saturation of the higher color, the eight standard color plate has not fully represent the commonly used color.

Many scholars believe that the standard color plate number is too small, and it is the general color rendering index of another deficiency. Although the CIE also has 9 colors with a high saturation of 9-14, they are not included in the general color rendering index Ra. In the practice of lighting, people are familiar with the color of the skin, leaves, food, etc., their color is extremely important, but they are excluded from the general color rendering index.

Seim had proposed using 20 standard swatches, but this would be rejected because it would make the calculations too complicated. At present, the computer is widely used, it seems that this proposal has to reconsider.

Many other evaluation methods have aroused broad interest due to the existence of these two problems in the evaluation of the color of the light source. This article will give a brief introduction to the author's knowledge.

3, the husbandley index Rf

Studies have shown that people tend to remember the colors of objects that are more familiar, and are reminiscent of its vivid colors when the saturation is high. This memory color is often consistent with the favorite color, and tends to shift to a high degree of saturation. Such as people's color of the memory, tend to shift to the red direction, leaf color to the green direction offset. Obviously different from the Ra method in CIE.

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

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

Second, the choice of 10 standard color plate, that is, in addition to the original standard color card 1-8, but also with No. 13 No. 14 two color plates, corresponding to the skin color and leaf color.

At this time, "perfect light" refers to it in the irradiation, can be 10 standard color palette color to the favorite direction of the light source offset.

Thus, for each standard color plate, the corresponding "perfect light source" color coordinates are different, can be determined by the experiment. This also shows that such a "perfect light source" can only be hypothetical.

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

1, for each standard color palette, the reference light source color coordinates need to be adjusted, that is, according to the experiment to determine the "perfect light" color coordinates. Then, when the light source is illuminated, the color difference of each color plate is compared with its corresponding "perfect light source".

2, in the calculation of Rf, take 10 color plates of the average color difference, but the weight of each color plate is different. 13 color plate is skin color, weight is 35%, 2 is 15%, 14 is 15%, the rest is 5% each. This emphasizes the importance of skin color. The Rf of the light source to be measured may 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) using the previous paragraph of the favorite color concept, defined in the D65 light source lighting, color preference index CPI = 100.

So the CPI of the light source to be measured can be:

Calculate the difference between the color coordinates of the 8 standard color plates and the color coordinates of the favorite colors and calculate the mean value of the vector sum (): CPI = 156-7.18 ()

The above calculations are performed in the CIE's UV chromaticity system.

Although CPI and Rf are using the concept of the most favorite color, but the two are very different:

1, in the calculation of Rf, with 1-8 and 13, 14 a total of 10 standard color palette, and CPI only 1-8 standard color plate

2, the technical Rf, the color difference take the experimental value of 1/5, while the CPI to take the original experimental value

3, the calculation of Rf, the color of the various color plates are different, while the CPI to take the same weight

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

Finally, to point out that Rf and CPI two indicators of the researchers, we are used to determine the color of the experiment, and in the experiment is used in daylight lighting. There is now evidence that the color of love is related to the color temperature associated with the light source. So the use of Rf and CPI to constant color rendering, only for high color temperature light source.

5, color resolution index (CDI)

Use Ra, Rf or CPI to describe the color rendering of the light source. The reference light source must have the same color temperature as the light source to be measured. Color discrimination index CDI (color discrimination index) to overcome this limitation.

The index is based on the assumption that the ability to distinguish between colors is greater in the illumination of a light source, the better the color rendering of the light source. In a light source lighting, 8 standard color palette in the CIE UV chromaticity diagram, the enclosed area is:

GA = 0.5? (UiVj-UjVi) i, j = 1,2, ... 8; i ≠ j

In the C light source illumination, the area GA = 0.005, defined at this time CDI = 100, so in the light source to be illuminated, the color index is:

CDI = (GA / 0.005) x 100

6, concluding remarks

From the above discussion, we can see that there are many methods to evaluate the color rendering of light source, and in the development and perfection, this article describes only a part of them, each of which has advantages and disadvantages.

Even now widely used in general color rendering index Ra, there are many shortcomings.

Its main drawback is the reference light source selection: the reference light source is a continuous light source, using it as a standard to measure the spectral discontinuity of the light source, not very appropriate. The color temperature of the reference light source must be close to the color temperature of the light source to be measured, and in fact, for a certain lighting operation, the color temperature itself has a great impact on the color rendering, this method can only be used in the light source color temperature has been determined Under conditions of use.

Its second drawback is the choice of standard swatches: for indoor lighting, can be considered 8 standard color palette has been able to fully represent a variety of commonly used colors. But in outdoor lighting, the color saturation of some higher color, cannot fully represents the commonly used color.