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What is CRI R9 and Why is it Important?

Home /  Blog /  Tech & Color Science /  What is CRI R9 and Why is it Important?
CRI R9 is one of the test color samples (TCS) used in the calculation of extended CRI. Many manufacturers will only report general CRI, however, which does not include the CRI R9 score. (See here for CRI extended vs CRI general). CRI R9 is therefore oftentimes a useful supplemental score to judge a light source's color rendering ability, specifically as it concerns objects whose reflectance spectra contain red wavelengths.

A closer look at how R9 is calculated, along with  its corresponding test color sample (TCS9) is a common recommendation for anyone who needs to know about a light source's color quality.

 

What is CRI R9?


R9 is the score that represents how accurately a light source will reproduce strong red colors.

"Accurate" is defined as similarity to daylight or incandescent bulbs, depending on the color temperature.

 

 

Just like each of the CRI R value calculations, R9 is calculated by calculating the reflected color from a theoretical object with the reflectance profile defined as TCS9. The reflectance spectra is provided below:


What is notable is that the TCS9 spectrum is almost entirely composed of red light. Spectrum-wise, we see this as wavelengths longer than 600 nm.


This means that if there is not enough red light in the light source, it will make red colors appear "off" or different.


Below is a typical LED spectrum compared to the reference source (daylight). There is a very notable lack of red light emitted by the LED at wavelengths past 600 nm.


As a result, the CRI R9 value for this LED is at -1.4. (That's right, a negative number!) This is in spite of the fact that the general CRI (Ra) comes in at 79.




Why is CRI R9 important?


CRI R9 is a very important metric because many light sources will be lacking in red content, but this fact will be hidden due to the averaging out of CRI calculations which do not include R9.


As the chart below shows, an light source can actually perform quite well with the first 8 test color samples, scoring quite well for R1-R8. For the general CRI Ra metric, this means that an LED with poor red color rendering can still get by with an 80 CRI (Ra) rating.






A closer look at the R9 value, however, reveals that the light will perform very poorly for red colors in particular.


What is a good CRI R9 value?



Although the maximum possible value of R9 is also 100, unlike average CRI numbers, R9 should be judged a bit differently.


Mathematically, R9 is far more difficult to achieve a high score compared to the other R values that comprise the CRI calculations, and is far more sensitive to spectral variations. Therefore, an R9 score of 50 or above would be considered "good" while an R9 score of 90 or above would be considered "excellent."


You will therefore find that most lighting products available in the market will rarely specify the R9 value, and when they do, rarely will they guarantee anything higher than 50. Even at Waveform Lighting, we specify R9 > 80 or R9 > 90, and are unable to guarantee anything higher than R9 > 95 due to this sensitivity.


This is due to the fact that CRI utilizes the CIE 1960 uv color space, which is skewed in a way that exaggerates color differences in the red region of the chromaticity diagram. Since CRI is a calculation that quantifies color differences between a light source and a reference source, a larger calculated color difference will result in a larger decrease in the R score.



Why is red such an important color?



Red is a crucial color for many applications including photography, textiles and reproduction of human skin tones.


Many objects that do not appear red actually are a combination of colors, including red. Skin tones, for example, are very much influenced by the redness of the blood that flows right beneath our skin.


Therefore, a light that lacks red will make a person look pale, or even green. This can be problematic for medical applications where color appearance is critical for accurate diagnoses. In other applications such as photography, aesthetic appearance is crucial and many times cannot be corrected for even in post production and digital editing.


When searching for a high color quality LED, be sure to inquire about the CRI as well as its R9 value.