It is known that two lamps of different types, even with the same color temperature, such as a fluorescent lamp and an incandescent lamp, often render the colors of the objects they illuminate in different ways. A fluorescent lamp, compared to an incandescent lamp, has less energy in the red region of the spectrum, so the red color appears brighter when the body is illuminated by an incandescent lamp than when the same body is illuminated by a fluorescent lamp of the same color temperature. Thus, the color rendering properties of various lamps directly depend on the nature of their emission spectrum.
A parameter that characterizes the degree to which the natural color of an object matches the visible color of this object when illuminated by a given light source, 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 equal to 100. At the same time, 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 equal to 80, at a color temperature of 2680K.
In studies, daylight is used as a guideline, with which the light of electric lamps is compared. In 1948, Peter Baum, in The Physical Aspects of Color: An Introduction to the Scientific Study of Color Stimuli and Color Sensations, wrote: “Daylight contains a wide variety of colors, making it easy to distinguish subtle shades, and the colors of objects around us obviously look naturally".
In the mid-20th century, scientists began to make attempts 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 continuous spectrum light sources with a color rendering index greater than 90. Technically, you can only compare the color rendering index of light sources that have the same color temperature.
To obtain a CRI value for a particular electric lamp, the color shift is determined when 8 standard reference colors (DIN 6169) are illuminated by the lamp under test. The calculation is made according to the method of the International Commission on Illumination (CIE), which gives the numerical value of the color deviation from the standards.
The smaller the deviation, the better the color rendering parameters of the tested lamp, and, accordingly, the higher its color rendering index. As a result of the measurements, the average deviations are subtracted from 100, and the exact numerical value of the color rendering index is obtained. So, with small deviations, Ra will be closer to 100, and with large deviations, it will be much less. If there are no deviations, then the value of Ra equal to 100 is assigned to the source.
For comparison of color temperatures in the range from 2000K to 5000K, the reference source is a "blackbody emitter", and for higher color temperatures, daylight. It is important to remember, however, that neither incandescent nor northern hemisphere skies have perfect color rendering, but their color rendering index is assumed to be 100. Meanwhile, incandescent lamps are weak under blue tones, and sky (7500K) is weak under red tones .
In practice, a color rendering index of 90 to 100 is considered flawless, and the scope of such sources includes those areas where very accurate color estimation is important. Ra from 80 to 90 are considered good indicators, and if in a given situation an accurate assessment of color is not important, but high-quality color rendering is required, such light sources will do.
If color reproduction is not important, then light sources with satisfactory and poor performance are acceptable, that is, with Ra less than 80. In general, differences in these values \u200b\u200bless than 5 units are hardly noticeable, and it is by no means easy to distinguish between 80 and 84 by eye.
The average values of the color rendering coefficients of lamps of various types are known. The light of incandescent lamps, as well as light, is close to solar, and the Ra value for them approaches 100. The color rendering index of high-pressure metal halide discharge lamps also reaches 90 and higher. 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 package.
As well as luminescent ones, depending on the quality, they can vary in color rendering index, however, the best specimens show a value of Ra 80 and higher. Mercury and sodium discharge lamps have the lowest color rendering index, here Ra is less than 40.
With regard to LEDs specifically, there have been studies that have looked at both red-green-blue LEDs and phosphor-coated white LEDs.
An evaluation was made that they had color rendering indexes in the region of 20, but performed well in rendering colors, as the apparent saturation of specific colors increased without shifting the color rendering of shades. At the same time, the 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.
According to the results of the research, the International Commission on Illumination concluded: "the color rendering index developed by the commission is usually not applicable to predict the color rendering parameters of a set of light sources if this set includes white LEDs."
Thus, the color rendering index Ra can serve as one of the information parameters used to evaluate LED lighting systems, however, in order to obtain the most acceptable results, preliminary tests and individual evaluations of the product are required right at the place of intended use.
