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Appliance Science: How the color rendering index measures the quality of light

How do you find the quality of the light a light bulb produces? Using the color rendering index, a measure of how well a light bulb illuminates a range of colors. Appliance Science looks at the science behind testing the quality of light bulbs and the CRI.

Colin McDonald/CNET

How good are your eyes? Chances are, you think they are pretty good. But ask a scientist, and they might give you a different answer. The truth is that the human eye is good at seeing some things, but not so good at others. That doesn't just apply to baseballs flying toward you at high speed, but also to the colors of objects you look at every day. Your eyes are sensitive to different colors in different ways, emphasizing some and completely missing others.

That's the result of millions of years of evolution, where we evolved to be able to better see both good things to eat, and bad things trying to eat us. This gave us a particular view of the world, a particular way that our eyes see color and our brains process this information.

Now, there are new things that our eyes have to cope with. In particular, they have to work under artificial light, rather than the light of the sun that our ancestors lived under. So, it stands to reason that the best artificial light would be one that produces light that makes things look the same as they are intended to, by nature or design. But how do you test that?

One answer is to measure something called the color rendering index, or CRI. It's one of the measures that we use to test light bulbs, and many manufacturers put this score on their products and use it as a way to sell their products. The CRI is a score out of 100 that shows how well a light source reveals the colors of objects, compared with an idealized perfect light source.

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Colin McDonald/CNET

One of the major misconceptions about the CRI is that it is a test of how close to daylight a light source is: it isn't. A high CRI score doesn't mean that the light bulb produces light that is similar to the sun, or that it will make you home look like a bright sunny day in midwinter. Instead, CRI is a measure of how well a light source will reveal colors compared with an ideal light source of the same color temperature. This ideal light source could be the midday sun, with a color temperature of about 6,500K, or that of an incandescent light, which is about 3,000K. These are very different light sources, but it is equally valid to measure the CRI of a light source against either of them.

So, what does CRI actually measure? It measures the ability of the light source to reveal a range of colors, to illuminate them properly so you can see the colors as they are intended. In a previous column, we discussed how different light sources have different mixtures of light in them (called the spectrum) and how you can measure them yourself. Some light sources have a spectrum with big gaps in it, where there is little or no light of those colors present. A light source can't properly reveal a color if that color isn't in the mix of light that it produces.

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So to test the CRI, you set up a number of color patches with precisely known colors on them. You then measure the color of these patches under the light source using a device called a spectrophotometer, which measures the color of the light being reflected from it. The difference between the real color of the patch and the measured one contributes to the score: the larger the difference (and thus the more wrong the color would look to the eye), the lower the score.

The standard test (warning: PDF link) uses eight colors, which represent a fairly typical range of common colors. Like most of these things, it gets a bit more complicated when you get into the details, as the way the distance between the measured and original color is converted into a score is adjusted to try and compensate for the way your eyes are more sensitive to differences between some colors than others. This involves some complex math that you can read all about here.

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An example of our CRI testing: a Cree LED (left) vs. an Ikea Ledare 1000L LED (middle) and a GE incandescent light bulb.

Tyler Lizenby/CNET

Suffice it to say, it is the sort of thing that scientists love to argue about, and the CRI isn't universally loved. It is, some argue, not that useful, as it is an average from the eight different colors that doesn't represent how any one color is actually reproduced. It won't tell you anything about how a particular color will look under that light source.

You could, in theory, have a high CRI light source that is completely unable to reveal a color because it doesn't have enough of that color of light in its spectrum. It could still get a high CRI score, though, if this color wasn't one of the tested ones. We deal with this issue by showing the individual color errors for each of the tested colors: the taller the bar, the more accurate the color is under this light source.

The CRI graph for the Ikea 1000L Ledare LED. Note how the red line is short: most LED lights don't do a great job with red objects, because LED light output typically drops off at the infrared end of the spectrum.

Ry Crist/CNET

The color rendering index is one of a number of ways that exist to try to measure the quality of light sources like LED light bulbs. It isn't perfect, but it does provide a handy way to judge how well a light bulb will illuminate the world around you as long as you understand what it does (and doesn't) mean.