We’re continuing our series on how to pick the right color temperature and color rendering index (CRI) for various applications. If you missed it, you may want to read our post on color temperature: A guide to choosing color temperature for your lighting. In this article, I’m going to unpack the challenges around CRI and provide some practical tips for making the right choice.
Note: The first half of this article goes into some of the detail behind CRI and where the industry is headed. If you’d like to jump to the guide, click here.
What is CRI? Why does it matter?
CRI is a simple and helpful lighting spec to predict how good the visual output of a lighting product will be. Simply put, the metric is a number between 0 and 100 that is used to predict how well a product renders colors. The closer to 100, the better — or
Operative word: Should.
CRI is calculated based on how well a light source renders eight specific pastel colors. The theory is that if a light renders these colors well, it should render all colors well.
Does this seem like a little abstract?
Here’s an example that the National Institute of Standards and Technology (NIST) produced showing the difference between poor color rendering light (left) and high-color-rendering light (right). Which would you rather have?
Image Source: http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/cqs_rationale_06-10.pdf
Some manufacturers test the performance of their products on additional swatches (if you’ve ever heard reference to R9, that’s a gauge of how well a product should render red colors). There is also a new color performance index (Color Quality Scale, or CQS) in the works that’s based on a much broader set of colors.
Here’s the rainbow of different swatches in one place:
CRI was pretty consistent across the board when it came to fluorescent and high-intensity discharge (HID) lighting. There were clear-cut tiers and price points when sticking with a "standard" CRI and moving to a high CRI "premium" lamp. For example, when Philips released their Elite metal halide lamps, they touted a 90+ CRI, which compared favorably to their standard 80+ offering. That's pretty straightforward –– 10 times out of 10, the Elite lamps would render color better.
With LED and all the variables that go into the creation of each chip (from how it is grown to binning to remote phosphors and beyond), you begin to get a little overwhelmed with selecting the right color/ CRI combination. In fact, the emergence and prevalence of LED lighting today has prompted an industry-wide search for a metric other than CRI –– something that better measures the visual performance of newer lighting technology. The fact is that a high-CRI product may not be a magic bullet for you in today’s lighting industry.
Great strides are being made in that search and the introduction of TM-30-15 –– which is a topic a little beyond the scope of this article –– is a great example of that.
A new technology called color pumping has also emerged recently that enhances and saturates the color spectrum and works ideally in retail where the product displayed is using bright and vivid colors. Philips calls theirs "crisp white," Xicato calls theirs "vivid," and
This is where CRI can actually be misleading because the goal of this is to actually oversaturate and accentuate certain colors (mainly blues, blacks, whites, and greens) instead of matching a reference source.
A practical guide to choosing Color Rendering Index
At this point, you’ve spent money carefully designing the surfaces, atmosphere, products, and other particular details of your space. Or maybe you’re on the other end of the spectrum and you simply need light in your space, but color quality isn’t quite as important (think, parking garages). How do you decide what CRI is right for you?
As a rule, a CRI of 90 or more is considered high and anything below 80 is considered mid-range to low. If color quality is important, seriously consider something in the 90s. Otherwise, you may want to consider other factors like light output or wattage first and CRI as a secondary choice. It’s also worth mentioning that improvements in light quality involve adding more phosphors. As a result, you typically make a sacrifice in energy efficiency as you move to LEDs with better light characteristics. In most cases, this sacrifice is worth it.
Here is a simple guide that can help point you in the right direction:
As with color temperature, there is truly no substitute for getting a few samples and putting them in your space to see how things look. You may be surprised by how well an 85 CRI product performs and disappointed with a 95 CRI product. On the other hand, you may see the difference that moving from low-CRI HID lighting to mid-range CRI LED lighting makes and decide to upgrade. You may also be ready to adopt a product that takes advantage of the color pumping technology previously mentioned if you are looking for retail or gallery lighting that stands out.
In either case, we are here to help you navigate to the best product for your needs. Stay tuned for the final article in this series where we give practical tips from a lighting designer’s perspective.