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What is the Color Rendering Index (CRI)?

Posted: 2013/05/28

Color Rendering Index (CRI or Ra) is a measure of how accurately a light source illuminates objects' colors. CRI typically ranges from 0 to 100 in score, where a score of 100 indicates that a light source illuminates objects in the same way as daylight or an incandescent light. A very high (90+) CRI is desired in applications where color appearance is critical, such as in museums displaying artwork or photography, whereas for typical indoor uses, scores of 75 or better are acceptable. The photo below shows a comparison between high and low CRI values, and why it is important to consider the CRI value for LED lighting products.



An object illuminated under a high CRI light (left) and low CRI light (right).

We perceive our natural world with reference to how things are illuminated and appear under natural daylight. As we developed artificial lighting, however, even though the light emitted still appears white to us, things begin to appear differently because artificial lighting and daylight are inherently different (with the exception of incandescent lighting being similar to daylight). An illustrative example is sodium-powered streetlights, which make everything appear orange. While color rendering is not crucial in city streets, differentiating colors can be difficult. These lights have CRI numbers of about 20 at best.


What determines CRI?

Recall that white light (as we humans perceive it) is created as a mixture of various wavelengths. Traditionally, a common method has been mixing red, green and blue light to create white light. This will sound familiar when we remember that TVs, projectors, and some LED lighting systems use this method of generating white light.



White light is a mixture of colors of the spectrum.

Also recall that an object does not have an inherent color, rather, whatever color that is reflected off of objects is what we perceive to be its color. In other words, a red hammer is not really "red," it reflects red wavelength light, and absorbs everything else. Therefore, if a particular object under sunlight typically reflects a color that is not included in the RGB light source, that color will not reflect as we expect. For example, if we shine a white light that is made of only red, green and blue, an object that normally appears orange in daylight may not appear the same under the light because the light contains only red, green and blue wavelength light.

Modern LED technology utilizes blue wavelength emitting LED chips and LED phosphor conversion. This method of LED lighting provides a significant improvement in CRI, particularly when compared to high power outdoor lighting, though there remains room for improvement in this area.

In short, the best way to increase CRI is to create a light source that includes as many colors of the spectrum as possible. We specialize in the research and development of various LED phosphors which convert various wavelengths of light into other wavelengths in order to cover as much of the spectrum as possible. This is an area of continued intense research where our expertise in LED phosphor technology allows us to continuously develop new and improved high CRI (currently up to 95 CRI) LED lighting products.

What CRI is not

CRI does not indicate a particular color of light or correlated color temperature (CCT, typically measured in Kelvins). Rather, it is merely a number that indicates how similar objects illuminated by a light source will appear when compared to normal daylight. Furthermore, it is not a comprehensive measure of color faithfulness as it is simply an average of each Ri score. In other words, two lights, both with CRIs of 80 can render colors very differently, as one may have all Ri scores of 80, while the other may have half 70s and half 90s. The best (and perhaps only surefire) way to determine whether a particular light's color rendering fulfills your needs is to see the light in person.

How is it calculated?

While new scoring systems have been proposed, the most commonly used system is the Ra Color Rendering Index. This system takes 14 color samples and tests to see if a particular light source can illuminate the sample colors to recreate the same light output. For each sample (called Ri), the difference between what is emitted and what is expected is transformed (mathematically via the CIE color spaces) into a single number, then subtracted from 100. For each Ri from R1...R8, the scores are averaged, creating an aggregate Ra Color Rendering Index number for a particular light source. R9 to R14 are used to provide specific color indices. For example, R9 is a commonly discussed index as it is an important color that affects that appearance of human skin tones. The TCS (test color samples) used are shown in the table to the right.

Beijing Yuji specializes in high CRI applications and has even newer high CRI projects underway. Whether you are looking for an LED lighting supplier for your high CRI lighting needs, or would just like to learn more, please contact us!

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Author: Alex




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