CRI R9 is one of the test color samples used in the calculation of extended CRI. Many manufacturers will only report general CRI, however, which does not include the CRI R9 score. 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 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.

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. 

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.