What is the color rendering index CRI

What is color rendering index? You can figure it out using the example of two lamps: fluorescent and incandescent, both have the same lamps, but the objects they illuminate look different. Why is this happening? A phosphor lamp has less energy in the red range of the spectrum than, as a result, shades of red appear richer and brighter when light from an incandescent lamp falls on it. Thus, the properties of light transmission for different lamps depend on the spectral radiation.

Comparison of color rendering index for different types of lamps

The value characterizing the degree of correspondence of the natural color of an object to the visible color when illuminated by a specific light source is called the color rendering index (CRI or R a), in other words, color rendering coefficient . Translated into common language, we can say that this characteristic allows you to determine how natural a thing looks under the influence of a particular lighting device.

The standard is 100, which corresponds to natural sunlight. The comfortable value for eye perception falls within a range, the minimum threshold of which is 80, the maximum is 100. Gas discharge devices have a CRI of 90 and higher. The color rendering index of fluorescent lamps ranges from 80 to 90, but it is lower for cheap light bulbs. This coefficient is usually indicated on the box. When purchasing a lamp, you should not neglect this figure.

Everyone knows and loves the incandescent light bulb; until recently, it was the most common lighting device. It has its pros and cons. A positive point is that the color rendering of an incandescent lamp is close to 100. Halogen lamps have approximately the same value.

Well proven led light bulbs. They have a number of advantages: they have good light output and low power consumption, which allows them to rightfully be called energy-saving. U different manufacturers color rendering index LED lamps different. Some samples high quality have a CRI value of 80. In the cheap version, low-quality materials are used to manufacture the LED-based lamp.

The more natural the color of the illuminated object seems, the more better characteristic color rendering of this lighting device.

If you take any light source with a color rendering index R a = 100, then it is easy to see that its radiation optimally displays all colors. This lighting corresponds to natural sunlight.

The table gives comparative characteristics color rendering index for different types of lamps.

Practical Application

Typically, light sources with a high color rendering index illuminate many objects so that they appear more beautiful and better than when illuminated by lamps with a low CRI. This property can be used in some areas of activity, for example, to obtain high-quality images.

Some recommendations can be given for the use of lighting fixtures in various retail outlets. Furniture stores recommend using a warm glow. Lamps with an approximate color temperature of 250 K and a color rendering index of about 85 units will illuminate a sofa or armchair favorably.

Various finishing materials, such as wallpaper or paint, need to be carefully considered. Here you can use light sources with an index from 90 to 100. The color temperature should be around 5,000 K. The same characteristics are needed for fabrics.

To successfully illuminate leather products, such as shoes, you need warm light (2,500 K) with a color rendering of 80 or 90.

For the convenience of consumers, the color rendering index is divided into six levels.

Level A1 is considered the highest. Lighting devices of this level are used in places where high color accuracy is required, for example, in museums, retail outlets selling fabrics, curtains, clothing, and so on.

Color rendering level 1B includes fluorescent lamps intended for installation in buildings where it is necessary to create a working environment and concentration. Such facilities include educational institutions, administrations, and industrial enterprises.

Level 2A lamps have good characteristics color rendition.

Level 3 lamps are used in cases where color accuracy is not important.

It is known that two lamps different types, even with the same color temperature, such as a fluorescent lamp and an incandescent lamp, often convey the colors of the objects they illuminate differently. A fluorescent lamp, compared to an incandescent lamp, has less energy in the red region of the spectrum, so the color red appears brighter when illuminated by an incandescent lamp than when illuminated by a fluorescent lamp of the same color temperature. Thus, color rendering properties various lamps directly depend on the nature of their radiation spectrum.

A parameter characterizing the degree of correspondence between the natural color of an object and the visible color of this object when illuminated by a given light source is called the color rendering index Ra, or color rendering coefficient, in English color rendering index, or CRI for short.

This value is relative, and Ra of the reference light source is taken to be 100. In this case, the range of color rendering indices from 80 to 100 is comfortable for the human eye, for example, a conventional 60 W incandescent lamp has a color rendering index Ra of 80, with a color temperature of 2680K.

In studies, daylight is used as a reference, with which the light of electric lamps is compared. In 1948, Peter Baum, in The Physical Aspects of Color: An Introduction to research Color Stimuli and Color Experience,” wrote, “Daylight contains a wide variety of colors, making it easy to distinguish subtle shades, and the colors of objects around us obviously appear natural.”

In the mid-20th century, scientists began attempting to evaluate the ability of artificial light sources to accurately reproduce natural colors, and around the 1960s or 1970s, the term "color rendering index" began to be used. CRI was used to compare light sources of a continuous spectrum, with a color rendering index above 90. Technically, color rendering index can only be compared between light sources that have the same color temperature.

To get the color rendering index value for a specific electric lamp, determine the color shift when illuminated by 8 standard reference colors (DIN 6169) with the lamp under study. The calculation is made according to the methodology of the International Commission on Illumination (CIE), which gives a numerical value of color deviation from the standards.

The smaller the deviation, the better parameters color rendering of the lamp under test, and, accordingly, its color rendering index is higher. Based on the results of measurements, the average deviation values ​​are subtracted from 100, and the exact numerical value of the color rendering index is obtained. So, for small deviations Ra will be closer to 100, and for large ones it will be much less. If there are no deviations, then the source is assigned a Ra value of 100.

To compare color temperatures in the range from 2000K to 5000K, a blackbody emitter is used as a reference source, and daylight is used for higher color temperatures. It is important to remember, however, that neither incandescent lamps nor the sky of the northern hemisphere have perfect color rendering, but their color rendering index is accepted as 100. Meanwhile, incandescent lamps are weak when illuminating blue tones, and the sky (7500K) is weak when illuminating red tones .

In practice, a color rendering index of 90 to 100 is considered impeccable, and the scope of application of such sources includes those areas where very accurate color assessment is important. Ra from 80 to 90 are considered good indicators, and if in a given situation an accurate color assessment is not important, but high-quality color rendition is required, such light sources are suitable.

If color rendering is not important, then light sources with satisfactory and poor indicators are acceptable, that is, with Ra less than 80. In general, differences in these values ​​of less than 5 units are hardly noticeable, and it is not at all easy to distinguish 80 and 84 by eye.

Average color rendering coefficients of lamps various types known. The light of incandescent lamps, as well as light, is close to sunlight, and the Ra value for them approaches 100. Color rendering index of metal halide discharge lamps high pressure also reaches 90 and above. Fluorescent lamps from many manufacturers have fairly high Ra values ​​from 80 to 90, but some budget models may have Ra less than 75; in any case, you should pay attention to the characteristics indicated on the packaging.

Just like luminescent ones, depending on the quality they can vary in color rendering index, but the best examples show a Ra value of 80 and higher. Mercury and sodium have the lowest color rendering index. gas discharge lamps, here Ra is less than 40.

Regarding LEDs specifically, there have been studies that have looked at both red-green-blue LEDs and phosphorus-coated white LEDs.

An assessment was carried out, from which it followed that they had color rendering coefficients in the region of 20, but they performed well when transmitting colors, since the visible saturation specific colors increased without shifting the color rendition of shades. At the same time white light, obtained by mixing the light of red, green and blue LEDs, is preferable to the light of halogen lamps and incandescent lamps, despite the high color rendering indices of the latter.

Based on the results of the study, the International Commission on Illumination concluded: “The color rendering index developed by the commission is generally not applicable to predicting the color rendering parameters of a set of light sources if the set includes white LEDs.”

Thus, the color rendering coefficient Ra can serve as one of information parameters, used for the evaluation of LED lighting systems, however, to obtain the most acceptable results, preliminary tests and individual evaluations of the product right at the site of intended use are necessary.

This relative value, which determines how naturally the colors of objects are conveyed in the light of a particular lamp.

The color rendering properties of lamps depend on the nature of their emission spectrum. The color rendering index (Ra) of the reference light source (i.e., ideally conveying the color of objects) is taken as 100.

The lower this index is for a lamp, the worse its color rendering properties. The color rendering range comfortable for human vision is 80-100Ra.

For example, a traditional incandescent lamp has a color rendering index of 80Ra, with color temperature at 2700K.

If we talk about LED lamps, they have an exceptionally high color rendering index, which is 85-90 Ra.
Color rendering index is a measure of the correspondence between the visual perception of a colored object illuminated by the object under study and standard sources light under certain observation conditions. The objective characteristic here is the value of the color rendering index Ra, the maximum possible value of which is 100. The higher the index, the more accurate the perception of colors will be. It is better to compare different sources in terms of Ra values ​​at similar color temperatures.
In practice, three categories of color rendering are usually used

Ra between 90 and 100.

Excellent color rendering properties. Area of ​​application: Mainly where accurate color assessment is important.

Ra between 80 and 90.

Good color rendering properties. Applications: Where accurate grading is not a priority, but good color rendering is still important.

Color rendering properties range from fair to poor. Areas of application: where color rendering is not important.
Maximum value The Ra coefficient is 100 (this value is taken for sunlight, as well as for most incandescent lamps).

The color rendering characteristic of a lamp describes how natural the objects around us look in the light of this lamp. The expression for this is the overall color rendering index Ra. To determine the value of Ra, 8 test colors are selected from the environment, which are illuminated by the test lamp, and then by a standard lamp having the same color temperature (from black body temperature to daylight). The smaller the difference in color rendering between the test colors, the better the color rendering of the lamp under test. The maximum Ra value is 100 (as the average of 8 test colors).

Depending on the location of the lamp and the task they perform, artificial light should provide the best possible color perception (as with natural daylight). This opportunity determined by the color rendering characteristics of the light source, which are expressed using general color rendering index Ra.

Color rendering index reflects the level of correspondence between the natural color of a body and the visible color of that body when illuminated by a reference light source

For comparison with the light sources considered, the color shift using the 8 (or 14) standard reference colors specified in DIN 6169, which is observed when the light of the test or reference light source is directed towards these reference colors, is recorded. The smaller the deviation of the color of the light emitted by the lamp under test from the reference colors, the better the color rendering characteristics of this lamp. A light source with a color rendering index of Ra = 100 emits light that optimally reflects all colors, like the light of a reference light source. The lower the Ra values, the worse the colors of the illuminated object are reproduced.

Tested colors:

Additional colors tested with saturated inks:

Before understanding what the color rendering index is, it is worth mentioning the spectral composition of light and the ability of it to be reflected and absorbed by the objects around us.

Spectral composition is a set of frequencies that characterize a particular radiation.

Speaking in simple words, this characteristic reflects the presence or absence of certain color shades in light. All objects surrounding a person have the property of absorbing and reflecting these light shades. For example, a green object reflects the green part of the spectrum of incident light, the rest of the spectrum is absorbed by it. The better the light illuminates objects, the better the human eye distinguishes their colors.

Color rendering index (CRI)

Currently, the most used method for assessing the quality of light sources is the color rendering index. This coefficient has a dimensionless value and international system measurement (SI) is denoted as CRI or Ra. The range of CRI values ​​is from 0 to 100. The color rendering index shows how natural the color of objects is when illuminated. The standard is considered to be sunlight, the CRI of which is 100. Until 1974, the International Commission on Illumination (CIE) this technique involved comparing 8 reference colors with the colors obtained from the light source being tested. In 1974, to the 8 standard unsaturated colors, 6 additional ones were added, but already rich colors.
The CRI measurement technique involves calculating the color shifts of 14 samples relative to sunlight or incandescent black body illumination.

The measurement process occurs in the following way:

• the light source under study is directed to a template sample;
• the color of the sample is measured using special instruments;
• the sample is illuminated with reference light;
• the color of the sample is measured under a reference light;
• The difference under different light sources is calculated.

The above algorithm is repeated with all template samples, after which the arithmetic mean CRI value is calculated.

Disadvantages of color rendering index and ways to solve them

Determination of the color rendering index is complete only in the case of continuous spectrum lamps whose CRI coefficient is above 90. With values ​​below 90 units, you can get several sources that will have the same coefficient, but illuminate objects differently and differ in color temperature. So far, international standardization organizations have not been able to get rid of this deficiency, lamp manufacturers continue to list the CRI value on their products.

Today is the vector of development artificial lighting relies on white LEDs, whose color rendering of the R 9 pattern is not very high. The reason for this is the small amount of red in the spectrum. However, visually the color rendering of white LEDs is more high level than the calculated CRI value indicates.
In 2007, the CIE officially stated that the use of the CRI index to determine the quality of color transmission of luminaires based on white LEDs was insufficient. Scientists also stated the need to introduce a new technique that will allow a more accurate assessment of LED radiation.

In 2010, a new technique appeared - CQS (abbreviation for color quality scale), based on 15 only saturated color patterns. First of all, it is worth noting that the calculation of color shifts using the CQS method is carried out in a completely different way than in the CRI method. Therefore, a high color shift in one of the patterns does not allow the color index to remain high.

The red color in the CQS scale is not as intense as in the CRI scale. This allows the color rendering parameter, when testing LED-based products, to roughly numerically correspond to the light sensations of a person.

The CQS technique, like the CRI, has one significant drawback - the lack of parameter adjustment depending on hue and saturation, which would allow taking into account the peculiarities of human vision. white from a mixture of glow from colored LEDs.

The lack of the CQS methodology led to the emergence of the TM-30-15 standard in mid-2015, which takes into account the concepts of accuracy and saturation. For higher measurement accuracy, the new standard evaluates the quality of light not using 15, but 99 templates, which include not only color samples, but also various objects from life.

Color rendering index in LED lamps

Today, the TM-30-15 standard is not mandatory, so manufacturers of LED-based lighting products continue to use the concept of CRI. It is worth emphasizing that the CRI measurement technique is not capable of giving a qualitative assessment of light. However, in the vast majority of cases, the consumer has to rely only on this coefficient. The color rendering index of LED lamps can be in a fairly wide range of CRI values, so you definitely need to pay attention to this parameter. Experts working in the field of lighting recommend choosing LED bulbs for residential premises with a CRI coefficient close to 90. In this case, interior items will look most natural.

LED lamps with a CRI less than 70 are only suitable for production and street lighting, where accuracy of shade transfer is not paramount.

From all of the above, it follows that the color rendering coefficient in LED lamps, despite the inherent disadvantages of the technique, is of the same importance as others technical specifications(power, color temperature, etc.). It is especially important to understand this when choosing lighting for a children's room. New bright objects constantly appear in front of children, the color of which is accepted as the norm and is stored in memory for the rest of their lives. Low-quality LED lamps contribute to the formation of an incorrect perception of the colors of the objects around them. Therefore, for lighting children's rooms, it is recommended to use lamps and luminaires that have been tested according to the TM-30-15 standard.

Read also

Color rendering index (CRI, or color rendering index) is a parameter that characterizes the correspondence of the natural color of the body to what it appears under lighting.

The fact is that illuminating objects with different lamps allows you to see what is possible. different options result. In some cases, colors look more natural and accurate, in other cases they look far different than in daylight. It turns out that two lamps of different types can have the same color temperature, but transmit colors differently. The emission spectrum of lamps is uneven; color rendering depends on their energy in a certain part of the spectrum.

The color rendering characteristic of a lamp describes how natural the surrounding objects are seen in the light of the lamp. And as a quantitative measure, the color rendering index is used. This is a value from 0 to 100, characterizing the level of correspondence of the color obtained from the tested lamp to the natural color of the body. Result 100 is a complete coincidence - as if sunlight, – that is, the color is transmitted as accurately as possible.

The term appeared in the 60-70s of the last century. CRI was originally developed for the purpose of comparing continuous spectrum light sources whose CRI was above 90, since below 90 there could be two sources with the same CRI but with very different color renderings.

Color Rendering Index Measurement

The smaller the deviation of the apparent color from the natural color (high color rendering lamps), the better the CRI characteristic of the source.

A light source with R a = 100 emits light that optimally displays all shades. At lower values, shades are rendered worse:

There is a system that mathematically compares the change in the location of radiation on a spectral scale compared to the colors illuminated by a reference light source. The mean differences are then subtracted from 100 to produce the CRI.

Table of basic shades, the accuracy of which is determined by the CRI index:

For the human eye, a comfortable CRI value is from 80 to 100 R a Here the color rendering index of LED lamps is optimal.

By definition, if there is no difference in how the colors of illuminated objects appear, the light source is assigned a CRI of 100. Thus, small differences in color rendering will result in a CRI value closer to 100, while larger differences will result in a lower CRI value. When comparing color temperatures in the range 2000 – 5000 K, the reference source of light radiation is considered to be a “black body emitter”, with color temperatures more high range- daylight.

LEDs and color rendering index

Studies are being conducted that find that white light produced by mixing red, blue and green LEDs is preferable to the light produced by incandescent and halogen lamps, even though incandescent lamps have higher CRI ratings. In fact, a technical report entitled "Color Rendering of White LED Light Sources" reports that the panel's CRI is generally not useful for making color rendering predictions about light sources that include white LEDs.

This stems from looking at a variety of analyzes that examined both blue-red-green (RGB) LED clusters and phosphor-coated white LEDs. Reviewers appreciated appearance scenes illuminated using luminaires with different color rendering indices and found that there was no precise relationship between the calculated CRIs and the classifications. In many cases RGB LEDs had color rendering indices of approximately 20, but they performed well in rendering colors. Possible explanation this fact is that, as a rule, they tend to increase the saturation of the perception of most colors without shifting the color rendition of shades.

The US Department of Energy makes the following recommendations: Long-term development and research is underway to create an updated system for accurately assessing the quality of light emissions that could be applied to any light source. In the meantime, the color rendering index of LED lamps can be considered one of the parameters when evaluating them and systems based on them. It should not be used to select a specific lighting product without testing the product and preliminary personal assessments at the intended place of use.

1. Determine the visual tasks that a given light source is expected to perform when illuminated. If color fidelity is critical (for example, in a space where fabrics or colors are compared under both electric and daylight), the CRI values ​​of the available metric system may be useful and suitable for use in the evaluation of LED products.
2. If color appearance is more important than color fidelity, don't rule out white LEDs simply because of their comparatively low CRI ratings. Some products with a CRI as low as 26 can still produce a visually pleasing white light.
3. The CRI can be compared if the light sources have the same color temperature. This thesis applies to all light sources, not just LEDs. Differences in CRI values ​​less than five units are not significant. This means that light sources with color rendering indices, for example 82 and 85, are almost the same.
4. In cases where color appearance or color fidelity are important factors, LED systems should be evaluated in person and, if possible, at the intended location.

It should be noted that modern methods computer data processing and spectrum analysis make it possible to fully automate the measurement of the color rendering index, eliminating the use of plates of a given color. Dependency is determined spectral density light radiation depending on the wavelength. And according to the results of this study, CRI is directly calculated using a special algorithm.