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. 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.

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.

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, GAOPIN engineer  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 specifically important for skin tones because it focuses on the rendering of strong red colors. To understand this, let's delve into a spectral analysis.

When light interacts with an object, certain wavelengths are absorbed, and others are reflected. Skin tones, especially in the red spectrum, are highly influenced by the absorption and reflection of light. The presence of hemoglobin in the blood, for instance, absorbs light in the blue and green regions of the spectrum, while reflecting light in the red region.

CRI R9 specifically measures the rendering of strong red colors, typically within the range of 600 to 700 nanometers. In this range, the absorption and reflection characteristics of skin tones are particularly pronounced. If a light source has a high CRI R9 value, it means that it is effective at rendering these strong red colors accurately.

For accurate rendering of skin tones, it's crucial that the light source provides sufficient illumination in the red part of the spectrum. If a light source has a low CRI R9 value, it may not render red colors accurately, leading to an unnatural and distorted appearance of skin tones under that light.

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.

In summary, CRI R9 is important for skin tones because it quantifies how well a light source renders strong red colors, which are crucial for natural and accurate representation of skin tones under that light.

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?

A high CRI R9 value is generally considered important for accurate rendering of strong red colors, which is crucial for natural and lifelike representation of skin tones. In the context of CRI R9:

• Low value (0-50): The light source may poorly render strong red colors, leading to a distortion in the appearance of objects, particularly those with red hues such as skin tones.
  
• Medium value (50-80): The light source provides moderate rendering of strong red colors. While it may be acceptable for many general lighting applications, it might not be ideal for situations where accurate color representation, especially in the red spectrum, is critical.
  
• High value (80-100): The light source effectively renders strong red colors, contributing to accurate and natural-looking representation of objects with red hues, such as skin tones. A high CRI R9 value is generally desirable for applications where color accuracy is crucial, like in photography, cinematography, or retail environments.
  

For situations where accurate rendering of skin tones is a priority, it's often recommended to choose a light source with a high CRI R9 value, in addition to a high overall Color Rendering Index (CRI) value. Keep in mind that CRI R9 is just one part of the broader CRI, which includes multiple color indices, each representing different parts of the color spectrum. The overall CRI value is an average of these indices, and a higher overall CRI is generally associated with better color rendering across a range of colors.

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  GAOPIN, 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.

If you have requirements for the quality of light, this indicator must be paid attention to, of course, the higher the R9, the higher the quality of light, the higher the cost also means.

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.