The World Most Comprehensive LED CRI Guide

Color rendering is an important aspect of evaluating lighting quality, and CRI is an important method for evaluating the color rendering of light sources and an important parameter to measure the color characteristics of artificial light sources.

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  1. What is CRI
  2. What is LED high CRI
  3. What determines the CRIof LED lamp beads?
  4. Differences in color rendering, lumen and color temperature
  5. What are the parameters that describe the light source?
  6. What is the relationship between color temperature and color rendering?
  7. Is the higher the CRI the better?
  8. Application of lamps with different Ra
  9. Three ways to achieve white LED light- improving Ra
  10. Factors affecting the color temperature and color rendering of lamp beads
  11. High color rendering index COB
  12. How to choose the right CRI for lights
  13. What CRI is best for growing plants?

What is CRI

high CRI pictures

Color rendering is an important aspect of evaluating lighting quality, and high CRI is an important method for evaluating the color rendering of light sources and an important parameter to measure the color characteristics of artificial light sources. It is widely used in evaluating artificial lighting sources.

Generally speaking, the higher the color rendering index, the better the color rendering of the light source, and the stronger the color reproduction ability of the object. However, this is only in general terms. Is this the case? Is it reliable to use the color rendering index to evaluate the color reproduction power of a light source? Under what circumstances are there exceptions?

To understand these problems, we need to understand what the color rendering index refers to and how it is derived. Just like the company will set a series of indicators for you to better evaluate your work, establish specifications according to a reasonable model, and then score.

QE also defines a set of methods for evaluating the color rendering of light sources. It uses 15 test color samples and tests with standard light sources to obtain a series of spectral brightness values and specifies that its color rendering index is 100. The color rendering index of the light source to be evaluated is scored against the standard light source according to a set of calculation methods. The 15 experimental color samples are as follows:

different RA values

Among them, No. 1-8 are used for the evaluation of the general color rendering index Ra, and 8 representative hues with medium saturation are selected.

In addition to stipulating 8 standard color samples for calculating the general color rendering index, CIE also specifies 6 standard color samples for calculating the color rendering index of special colors, which are used for the selection of a certain special color rendering performance of the test light source.

The higher degree of red, yellow, green, blue, European and American skin color, and leaf green (9-15). Our country’s light source color rendering index calculation method also adds a color sample representing the skin color of Asian women (15).

The general color rendering index is called Ra, and the special color rendering index symbol is Ri. Ra is the average of R1-R8, not involving R9-R15. R1-R8 are light color reductions.

Why is Ra very high, but the color is still not bright?Because R9-R15 are the bright color!So Re comes out, and Re is the average value that reflects R1-R15.

Here comes the problem. Usually what we call the color rendering index (Ra) is based on the color rendering of the light source to 8 standard color samples. The 8 color samples have medium chroma and lightness and are all unsaturated colors. They are used to measure the spectrum. The color rendering of continuous, wide-band light sources has good results, but problems arise when evaluating light sources with steep waveforms and narrow frequency bands.

If the color rendering index Ra is high, the color rendering must be good?

For example, in the following two pictures, the first row in each picture is the performance of the standard light source for various color samples, and the second row is the performance of the tested LED light source for various color samples.

difference between LED A and B

The color rendering index of these two LED light sources, calculated according to the standard test method, is 67 and 80. Can you tell which one is the Ra=67 LED and which one is the Ra=80 LED?

The result is that the top one is Ra=80 and the bottom one is Ra=67.

So, is there a new and better way to evaluate the color rendering of the light source? The answer from NIST (National Institute of Standards and Technology) is CQS (Colour Quality Index Method). Similar to the Color Rendering Index (CI), CQS also uses a test color method, but CQS selects 15 saturated colors, which are evenly distributed across the entire visible spectrum, as shown in the figure below.

15 saturated colors

What is LED high CRI

High CRI is a general color rendering designation of 100 for daylight and near-daylight artificial standard light sources. Under the illumination of the measured light source, the color of the same object is more consistent with the color of the standard light source. The larger the Ra, the better the color rendering performance, and conversely, the worse the color rendering performance.

Strict material control completely achieves the same color temperature and the same spectrum LED light source (under the premise of the same color temperature, the X and Y values are guaranteed to be the same).

The selection of high-quality brackets and related materials ensures the high heat resistance and high reliability of mobile game lamp beads. High CRI can achieve high luminous flux under the premise of ensuring high CRI RA80. (High CRI, High Lumen)

High color rendering index, uniform light color, high light-emitting efficiency, eliminates violet light spectrum, reduces the use temperature of phosphors, improves the consistency and reliability of the spectrum during long-term use, and can be used for a longer time.

different light spectrum

            halogen lamp                                fluorescent lamp                                    LED lights

In addition

When there is little or no main wave reflected by the object under the reference light source in the light source spectrum, the color will produce obvious chromatic aberration. The greater the degree of chromatic aberration, the worse the color rendering of the light source. The color rendering index coefficient remains the common method for defining the color rendering of a light source.

The color rendering index of sunlight is defined as 100, and the color rendering index of incandescent lamps is very close to sunlight, so it is regarded as an ideal benchmark light source. This system is tested with 8 standard color samples with medium chroma and compares the degree of deviation of these 8 colors under the test light source and the benchmark of the same color temperature.

To measure the color rendering index of the light source, take the average deviation value Ra20-100, with 100 as the highest, the greater the average color difference, the lower the Ra value. Light sources below 20 are generally not suitable for general use.

When there is little or no main wave reflected by the object under the reference light source in the light source spectrum, the color will produce obvious chromatic aberration. The greater the degree of chromatic aberration, the worse the color rendering of the light source. The color rendering index coefficient is still a common way to define the color rendering of a light source.

What determines the CRI of LED lamp beads?

High CRI refers to the color rendering index of the LED lamp bead, which is matched by the phosphor powder and the chip band, and the CRI refers to the degree of color reproduction of the object.

There are different requirements in different environments. Generally, general indoor lighting is conventional. Warm white is 65-70, and if it is white, then it is about 75. The luminous flux is reduced by adding high color rendering powder. The CRI of the lamp bead is not directly related to the quality of the lamp bead.

Of course, in the case of the same brightness, the higher the CRI, the better, but the price will be relatively expensive. The CRI is the degree of fidelity with which the human eye sees the object when the light is reflected back through the object. Generally speaking, the worse the CRI is, the worse the color of the object reflected by a person. The higher the CRI, the more realistic it is.

High CRI LED lamp beads to have better color rendering than LED lamp beads. The higher the CRI, the brighter the color.

The CRI is divided into the following stages:

90-100

Excellent, where accurate color contrast is required

80-89

Places where correct color judgment is required

60-79

Ordinary, where medium color rendering is required

40-59

Places with low requirements for color rendering and small color difference

20-39

Poor, places with no specific requirements for color rendering

Differences in color rendering, lumen and color temperature

When buying lamps, some friends may be confused. There are 5000K; 6000K; 3000K.

Are these Ra values?

Of course not!

Generally, the four-digit band “K” refers to the color temperature of the light source. That is the warm light and cold light we see every day. The lower the value, the yellower and warmer the light. The higher the value, the whiter and cooler the light.

What are the parameters that describe the light source?

The parameters describing the light source include luminous flux, illuminance, light intensity, brightness, color temperature, color rendering, etc. The color rendering and color temperature of the light source are two important color indicators of the lighting source.

Color temperature is an index to measure the color of the light source.

Color rendering is an index to measure the visual quality of the light source.

If the color of the light source is within the color temperature range that people are used to, the color rendering should be a more important indicator of the quality of the light source, because the color rendering directly affects the color of the object observed by people. The definition of each parameter is as follows:

Luminous flux: 

The sum of the visible light emitted by the light source per second. Simply put, it is the amount of light emitted. Unit: Lumen (lm).

Illuminance: 

The incident luminous flux per unit area. That is the value obtained by dividing the luminous flux by the area. Unit: lux (lx). Illuminance is divided into horizontal illuminance and vertical illuminance. The horizontal illuminance is the illuminance of the luminous flux incident on the horizontal surface, and the vertical illuminance is the illuminance of the luminous flux incident on the vertical surface.

1 lux (lx) is equivalent to the illuminance when the luminous flux per square meter is 1 lumen (Im).

The ground illuminance at noon with strong sunlight in summer is about 50001x, the ground illuminance on sunny days in winter is about 20001x, and the ground illuminance on a clear moonlit night is about 0.2lX.

Luminous Intensity:

The luminous flux emitted by a luminous body per unit solid angle in a specific direction. Unit: candela (cd).

Luminance: 

Describes the luminous flux per unit solid angle and unit area of the illuminant in a specific direction. Unit: candela per square meter (cd/m2), also known as nit.

Color temperature:

Expressed in absolute temperature K. That is heating a standard black body. When the temperature rises to a certain level, the color begins to gradually change in the order of dark red, light red, orange, yellow, white, and blue. When a light source has the same color as the black body, The absolute temperature of the black body at that time is called the color temperature of the light source.

Correlated color temperature:

When the black body radiation is close to the light color of the light source, the evaluation value of the light color performance of the light source is not an accurate color comparison, so the two light sources with the same color temperature value may still have a slight difference in the appearance of the light color.

 It is impossible to understand the color rendering ability of the light source on the object only by the color temperature.

Color rendering:

The degree to which the light source presents the color of the object itself is called color rendering, that is, the degree of color fidelity. The color rendering of the light source is indicated by the color rendering index. 

It represents the degree to which the color of the object deviates from the color of the reference light (sunlight) under this light source. It can more comprehensively reflect the color characteristics of the light source. 

Light sources with high color rendering are better for colors, and the colors we see are close to natural colors. Light sources with low color rendering perform poorly on color, and we see large color deviations.

The International Commission on Illumination sets the color rendering index of the sun as 100, and the color rendering index of various light sources is different.

Use light source color rendering index to distinguish:

100-75

Good color rendering

75-50

General color rendering

Below 50

Poor color rendering

What is the relationship between color temperature and color rendering?

The color rendering and color temperature of the light source are two important color indicators of the light source.

Color temperature is an index to measure the color of the light source, and color rendering is an index to measure the visual quality of the light source.

If the color of the light source is within the color temperature range that people are accustomed to, the color rendering should be a more important indicator of the quality of the light source. This is because color rendering directly affects the color of objects that people observe.

For the color detection industry, the level of color rendering of artificial light sources directly affects the color evaluation results of the tested samples. Therefore, the CIE standard light source has strict requirements on color rendering, and the standard light source box must use an artificial light source with qualified technical parameters such as color temperature and color rendering index.

The color temperature and color rendering of a light source is essentially determined by its spectral energy distribution.

Taking sunlight as an example, sunlight is composed of red, orange, yellow, green, cyan, purple, and other colors of light mixed in a certain proportion. When sunlight hits an object of a certain color, the object absorbs light of other colors and reflects light of this color.

For example, when a blue cloth is exposed to sunlight, it reflects blue light and absorbs other light, so the cloth seen by the human eye is blue. Because sunlight itself contains various colors, and various objects have different reflection properties for different light, nature appears colorful under sunlight.

This is not the case with sodium lamps. The light emitted by sodium lamps is mainly yellow. When yellow light shines on blue cloth, the blue cloth absorbs the yellow light. Although blue cloth can reflect blue light, there is no blue light in the light emitted by sodium lamps. Therefore, the blue cloth becomes black cloth under the irradiation of a sodium lamp.

The spectral energy distribution of the tungsten filament lamp is continuous. And it has various colors, so it has better color rendering. But its radiant energy distribution is mainly focused on the long wave, and the overall light color is reddish and yellowish.

However, there is no necessary connection between the color temperature of the light source and the color rendering. The difference in color rendering between light sources with the same color temperature may be pretty large, and the difference in color temperature between light sources with the same color rendering index may also be very large. Light sources of various color temperatures may have good color rendering or may have poor color rendering.

Such as a tungsten lamp, which has excellent color temperature and color rendering.

High-pressure sodium lamps have a low color temperature and poor color rendering.

The high-pressure mercury lamp on the road emits is not only bright but also white (high color temperature). But the face of the person irradiated by it seems to be smeared with a layer of blue-gray (poor color rendering).

The high-pressure cylinder lamp emits bright white light (high color temperature), and the color under the lamp is not distorted (good color rendering).

Is the higher the CRI the better?

Different Ra values will have different effects. As shown in the picture below, using Ra 65 light and Ra 80 light on the same woman, Ra 65 gives a faded visual experience, while Ra 80 has a much ruddy complexion.

different RA values

It can be seen from the picture that the color of the upper part of the face is obviously distorted. The color details are lost, the color saturation is very low, and the light does not restore the color very well, giving the impression that the face is gray.

The effect of the middle part is obviously better than that of the upper part. It has a certain color, but the effect is not good. The effect presented is a sallow complexion and no vitality.

The effect presented in the lower part has a high degree of reduction and is more pleasing to the eye. The color saturation is high, the skin on the face is rosy and shiny, and the lips are rosy, which is very attractive.

Application of lamps with different Ra

Ra 90 — 100

Mostly used in museums, and other places that require accurate color contrast

Ra 80 — 89

Can be used for home decoration, bedrooms, etc., where correct color judgment is required

Ra 60 — 79

Can be used in places with medium color rendering

Ra 40 — 59

Can be used in places with small color difference and low color rendering requirements

Therefore, when paying attention to the color rendering of the light source, we will first look at the Ra value. Then, for most LED light sources (the above example is an uncommon exception), it is usually discontinuous and narrow in frequency, depending on the spectral distribution.

On the whole, in places with rich colors and high requirements, such as museums, supermarkets, shops, and restaurants, we should choose light sources with a relatively continuous spectrum and a wider frequency band to truly restore the true, beautiful, and comfortable colors of items, which is helpful for the display and sale of items and the creation of an atmosphere.

Use phosphors to achieve white LED light

The first method:

Blue LED chip + yellow phosphor

This technology is monopolized by the Japanese company Nichia. A principal disadvantage of this scheme is that the emission spectrum of ce3+ ions in the phosphor does not have continuous spectral characteristics. And the color rendering is poor, which is difficult to meet the requirements of low color temperature lighting, while the luminous efficiency is not high enough and needs to be improved by developing new high-efficiency phosphors.

The second method:

Blue LED+Green Phosphor+Red Phosphor

The blue light emitted by the chip is combined with the green light and red light emitted by the phosphor to obtain white light, and the color rendering is good. However, the effective conversion efficiency of the phosphor used in this method is low, especially the efficiency of the red phosphor needs to be greatly improved.

The third method:

Violet LED + tri-color phosphors (multiple colors of phosphors)

Using the long-wave ultraviolet light (370nm-380nm) or violet light (380nm-410nm) emitted by the chip to excite the phosphor to achieve white light emission. This method has better color rendering but also has similar problems as the second one. At present, the red and green phosphors with high conversion efficiency are mostly sulfide systems, which have poor luminescence stability and large light decay.

At present, the color rendering index of the LED phosphor coating cannot reach more than 95%, and the lighting effect is not good in restoring the color of the object.

At present, the realization method of white light LED is phosphor conversion type (pc-led), such as (yellow + blue) white light LED with yellow phosphor powder coated on the surface of the blue light chip. The blue light generated by this light source is more harmful and harmful to the human eyes damage is large.

Due to this and the processing method of lamp bead phosphors, the light source is too concentrated, resulting in poor luminous efficacy and dull color. Generally, the color rendering index (Ra) and the color rendering value (Re) of the color rendering of LED are inconsistent with the visual evaluation.

Factors affecting the color temperature and color rendering of lamp beads

  1. Phosphor quality and ratio, the better the phosphor quality, the better the color temperature consistency, the smaller the color temperature drift, the more scientific the phosphor ratio, and the better the color rendering.
  2. The larger the driving current, the color temperature will drift to a higher color temperature.
  3. The worse the quality of the phosphor, the easier the color temperature is to drift, and the more prone it to light decay.
  4. The working temperature also affects the color temperature drift.
  5. The worse the chip quality, the worse the color temperature consistency.
  6. For products with the same material and packaging process, the consistency of high color temperature (such as 6500K) is easier to achieve, and low color temperature (such as 3000K) requires small color temperature segments.

As mentioned earlier, lamp beads have a great impact on lighting products. Therefore, the control of product quality should start from the first stage of production, and LED products must not be composed of one lamp bead, which makes it difficult to control the quality. It is difficult to screen out low-quality products from multiple lamp beads, so developers are required to start from the most basic aspects of LED lighting products.

High CRI COB

With the continuous improvement of the performance of LED lighting products and the continuous reduction of costs, the application scope of LED lighting products is becoming wider and wider.

In the field of lighting applications, with the improvement of the quality of life, people pay much more attention to the quality of lighting. And the high-definition index two-color COB has also received attention as an indicator that affects the quality of lighting, especially in the indoor environment, which has higher requirements.

In 2009, the U.S. Department of Energy raised the color rendering index of LED lamps for indoor lighting from >70 to >80. China’s lighting design standards also stipulate that the color rendering index of offices, hotels, and restaurants should be above 80.

The evaluation index of traditional LED package light sources is mainly luminous efficiency. With the improvement of people’s requirements for lighting applications, the evaluation index of LED is not only luminous efficiency, but it is not practical for lighting applications without a color rendering index.

Therefore, it is imminent to improve the color rendering index of LED light sources. High color rendering index COB has been researched and discovered, and related lamps have been manufactured. Due to the needs of the scene, the technology of the COB index has been continuously improved.

How to choose the right CRI for lights

Long-term use of low CRI (low color rendering index) will lead to a decline in people’s ability to distinguish colors. And may also cause achromatopsia, achromatopsia, and other eye diseases for children who are in the developmental period. Therefore, adults need to pay special attention to the color rendering index of children’s school classrooms and desk lighting.

right CRI LED light for child

Selecting a high rendering index (high color rendering index) can prevent myopia in addition to highly restoring the original color of the object. The field of optometry has repeatedly proved that natural light can effectively prevent myopia.

From the point of view of protecting eyesight, it is the right choice to choose high CRI lamps, especially table lamps for children. Items such as children’s books, and toys are colorful. If you choose a table lamp with a low CRI, the color reflected back to the eyes of the child will lose saturation, resulting in color distortion, which is likely to cause visual fatigue, which may lead to myopia. The use of high CRI lamps can avoid such problems.

What CRI is best for growing plants?

Just because a light source has a high CRI score, doesn’t mean it’s better for growing plants. A light source can have a good CRI, but not without the wavelengths of light that plants need for photosynthesis.

High-pressure sodium (HPS) lamps, long used as grow lights, have a very low CRI of only 24. So just because the CRI score is low doesn’t mean the light isn’t a good grow light either. This makes CRI meaningless for assessing how well light affects plant growth.

High CRI LED is used to evaluate how well the light compares to visible sunlight. The maximum CRI rating of 100 corresponds to the natural light from sunlight. Good full-spectrum lights for indoor gardening have a CRI rating above 85, but the closer to 100, the better

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