R9: More Important Than CRI?

R9 is a lighting metric concerning the color red. Photo by Thomas Martinsen on Unsplash.

Like flicker, color is a widely-debated and often misunderstood topic among the lighting industry. In the past few years, we have seen the following changes regarding color:

  • TM-30-15, a new color metric from the Illuminating Engineering Society
  • California Title 24’s minimum 90 CRI requirement for residential buildings
  • A minimum R9 of 50 as an optimization for the WELL Building Standard and a requirement for residential lighting under Title 24

We can debate the merits of CRI all day, but in 2018, most LED and luminaire manufacturers do not provide any other color information on their datasheets. Since we’re a long way from a universal understanding and application of TM-30, I am going to focus on CRI and R9 today.

What is CRI, anyway?

The colloquial term CRI refers to CIE Ra, a comparison of a given light source to a “perfect” light source. A perfect light source would have a CRI Ra of 100 for a given color temperature (CCT). Most indoor LED sources have a CRI Ra around 80.

CRI test color samples. Image: Wikipedia.

The palette above shows the color samples used to compute Ra and related metrics.

Note, however, that only the first eight samples (TCS01 to TCS08) are used to calculate Ra. Do you notice anything about those first eight samples relative to the others?

The reference hues used to calculate CRI/Ra are all desaturated.

Did you say “desaturated” or “pastels?” If so, right on. The reference hues used to calculate CRI Ra are all desaturated.

As you might expect, the lack of strong, saturated colors in the calculation of CRI may not translate well to real-world color rendering.

Enter R9.

Have a look again at the color palette above. TCS09, the sample immediately following the Ra colors, is strong red. This is R9.

Like CRI Ra, R9 is ranked from 0 to 100. The higher, the better.

It turns out that most of today’s LEDs are not particularly good at rendering red. An 80 CRI LED might only have an R9 of 20 or even much less.

Warm white LED spectrum. Image: Wikimedia Commons.

Take the spectrum of a typical warm white LED, such as the one above. It has the characteristic blue peak along with a large hump from green to red. Notice that red falls off quite quickly.

Even though this LED likely has a decent CRI Ra value, R9 is relatively low.

R9’s importance.

Without a high R9 value, skin tones and natural finishes, such as wood, look lifeless. You are not very likely to salivate over a desaturated apple, tomato, or steak.

This may be part of the reason many people prefer halogen and incandescent lighting—these sources are rich with red light.

Halogen spectrum. Image: Wikimedia Commons.

Some very high-end LEDs might have R9 values in the 70s or higher. But even an R9 of 50 is going to be much better than most of today’s LED sources, especially if used during the day (when there is less red outside anyway).

As a rule of thumb, many of today’s 90+ CRI LEDs also have higher R9 values. This should be verified when possible, but without proper instrumentation, you can assume a 90 CRI source will have a slightly higher R9 than an 80 CRI source.

What the standards say about R9.

California Title 24 notoriously requires 90+ CRI for residential light sources. It also calls for a minimum R9 of 50 in homes. Products should be tested in accordance to Joint Appendix 8.

The U.S. General Services Administration calls for a minimum R9 of 50 for Service Tier 2 of its 2014 public facilities standard. Refer to section 58.1.b for more information.

It may be difficult for manufacturers to find a suitable 80 CRI LED with an R9 of 50 or higher.

The WELL Building Standard, feature 58: Color Quality also calls for a minimum R9 of 50. Note that this is an optimization, which means it is not currently required for WELL Silver projects. However, it may help achieve one of the higher certification levels.

Also note that only California Title 24 calls for a minimum CRI of 90 in some applications. Both the GSA and WELL guidelines mentioned above request at least 80 CRI. However, it may be difficult for manufacturers to find a suitable 80 CRI LED with an R9 of 50 or higher.

What to do about R9.

If you’re a lighting specifier working on a WELL, GSA, or residential Title 24 project, be sure to find lighting products that specify the R9 value, or ask the manufacturer for the number directly.

If you’re a lighting manufacturer (OEM or LED components), start testing for and specifying R9 on your datasheets. Since most manufacturers do not supply this information today, it can differentiate you and help get your product into potentially higher-margin human-centric projects.


I help companies create healthy lighting products. If you are considering strategy or product development in the human-centric space, visit my website greg.lighting or get in touch directly.