Here are excerpts of a 1700-word “thought leader” piece written for Alloy LED. See the full piece with some nice colorful graphics in PDF form here.

In Pursuit of Natural Lighting: How CRI and R Values Measure Light Quality

When we talk about light quality, whether it’s of incandescent, fluorescent or LED lighting, we are really comparing it to daylight. Daylight is the original white light. Our eyes have evolved to interpret our surroundings under sunlight, where objects appear to have the best color and appearance. I’m going to go into a little detail about current metrics (and a few developing ones) that allow us to determine how well a light fixture stands up to natural light, and try to bring them out of the arcane world of lighting engineering into the one we live in, where there is a dizzying array of LED fixtures to navigate. The only reason for you to learn about these metrics is to separate high-end lighting that makes a space look beautiful from mediocre lighting. Knowing what kinds of metrics and values to look for will help you source great lighting.

CRI Is Not Really About Color Temperature

Color Rendering Index (CRI) is the current standard metric of a light source’s ability to reveal the colors of objects faithfully in comparison with natural light. One thing to note first is that a white light’s color temperature (or CCT, measured in Kelvins) is related to but doesn’t solely determine light quality. Although sunlight seems “warm” in quality, it is actually that very cool 6500K color temperature at midday. It’s the perfect CRI (100) of sunlight that makes it the standard for color rendering to which we compare all artificial light. (When we say “warm” we probably mean familiar.) So the holy grail of LED lighting is to create light that renders colors and whites perfectly no matter what the CCT. This lets you have the warm glow of incandescent (2700K for example) while having high CRI/light quality.

Even More Metrics: CQS and GAI

As a testament to how difficult it is to replicate nature, there are even newer additional metrics. The good news is they’ve only been created, again, to help separate the superior from the average. Now that you’ve gotten used to R values, it’s time to talk about efforts to replace this inadequate metric.

One new metric, as early as 2012, is the Color Quality Scale (CQS). CQS is meant to replace CRI (which was developed in the ‘50s and is now outdated), and adds chromatic discrimination (aka contrast between colors) and observer preferences. This means that it gives more weight to how our subjectivity influences our perception of colors. As you can see below, the colors of the CQS scale are more saturated than the CRI color scale (Figure 3, above), and more representative of the actual color spectrum. At the same time, it still has only 15 colors in the scale, making it no more nuanced than the R1-R15 values.

TM-30: The Spectrum Explodes

It only takes a glance at the color scale for the TM-30 lighting metric (Figure 5) to see that it is vastly more sophisticated than any of the others. The range of colors is a more accurate reflection of the real world, showing the kinds of colors you see in skin tones, plants, food, clothing, and furnishings. The team that developed TM-30 hopes it will be approved by the IES to replace CRI and CQS entirely. Though the information offered on its scale of 99 colors will be a lot to digest, it should also include an averaged figure, as CRI does. It’s hard to imagine a more accurate metric. TM-30 has gotten a lot of coverage over the past year, so stay tuned.

See the full piece with images in PDF form here.