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Incandescent light might eat a lot of energy, but it has a nice quality shared by natural sunlight: it is full spectrum. Things that give off light because they’re hot – like an incandescent filament or the gas of the sun, emit light across all the colors of visible light. When we’re looking at art or other things illuminated by artificial light at home, we want colors to look natural. If we use incandescent lighting, we get that. But when we switch to florescent or LED lighting, things get complicated, because florescent and LED lights generate light through different physics than does a thermal source.
In a florescent tube, mercury gas in the tube gets jolted by electric current to a higher energy state. When the gas atoms drop back to a lower energy state, they emit light photons at a specific frequency or color – in this case, high energy ultraviolet light we cannot directly see. This ultraviolet light is then absorbed by coatings called phosphors on the inside of the tube, which, like the gas, jump to a higher energy state, and then when their atoms drop back to a lower state, they emit photons at a visible light frequency, the light we see from the tube. Engineers fiddle with the phosphors to create a coating that emits a broad spectrum of light we perceive as white light.
But even with a lot of engineering, florescent bulbs have gaps in their emission spectrum that we perceive as unnatural color rendering. If something is a particular color – reflecting light at a specific wavelength – and the light source with which we illuminate it has a gap at that wavelength, the thing will appear darker and with less saturated color, and its color will shift to something different than we’d see under natural light. Not good!
The physics of LED lights is similar to that of florescent tubes, in that the semiconductor in the LED gets energized to a higher state, then emits light of a specific color – in this case visible blue light. Specially engineered “yellow” phosphors absorb most of this blue light and then glow in a broad range of colors from green through red. The result of blending the blue light with the phosphors’ additional emitted spectrum is white light. Advanced LEDs use multiple different emitters and phosphors carefully combined to more accurately simulate natural light – much better than florescent bulbs. But they’re not perfect – yet. Darn!
So how do we measure how accurately an LED light (or any other light) renders color? Engineers have been thinking about this a long time, and over many years of experimenting have come up with several ways to measure the rendering accuracy of different light sources. The best known of these colorimetric scales is called (surprise!) the Color Rendering Index or CRI. A CRI of 100 is natural rendering, and both sunlight and incandescent light have a CRI of 100.
A bulb with a CRI < 70 such as an old-school florescent tube, will do a poor job of rendering colors. For general purpose lighting – regular lamp bulbs or recessed ceiling lights – LED bulbs with a CRI > 80 are good, with higher CRI > 90 even better. When you really care about the lighting, such as for artwork, your dinner table (you want your food to look appetizing, right?) and your bathroom vanity (you also want to look your best!), choose an LED with a CRI as high as you can, certainly > 90.
CRI is an old measurement but it is popular. It suffers from some big limitations that newer colorimetric scales overcome. The standard CRI metric measures color rendering for eight light pastel swatch colors only, missing more vivid colors. Even so, in practice it does an ok job. Adding a separate “R9” strong red rendering measure helps a lot. But newer metrics such as IES TM-30-15 are more accurate tests of LED color rendering.
TM-30-15 has 3 components. The “fidelity” number Rf is similar to CRI, but uses 99 color samples rather than 8, including important colors from nature, saturated colors, skin tones, and others. The “gamut” number Rg measures how well a light source can produce saturated colors: an Rg of 100 matches the saturation of colors with daylight illumination. The “color vector graphic” display is a more intuitive picture of which hues are more or less saturated when comparing a light source to sunlight. In the sample, the white circle is sunlight, and the black is the tested LED bulb, which is a bit less saturated in reds and purples, and a bit more saturated in greens and yellows, but overall very close to sunlight.
CRI 95, R9 95, Rf 90, Rg 100
So how can you tell what you’re getting? For less demanding applications, look for CRI > 80. Often you won’t even find CRI on the box for regular old LED replacement bulbs, like what goes in a lamp or a ceiling recessed fixture. But if you go to the manufacturer’s website and poke around, you’ll often find spec sheets that give more detail, for example this 3000K PAR20 LED from Feit Electric doesn’t say on the box what the CRI is, but if you go to their website you’ll find the specification sheet, with “CRI > 80”.
For more challenging applications, such as replacing MR16 halogen bulbs in track lighting for illuminating fine art, you’ll want to see more detailed data on color rendering. SORAA is a well-known manufacturer of high-quality, high rendition LED replacement bulbs for this use case. Notice that the box specifies CRI 95. The specification sheet on their site goes into great detail on the bulb’s color rendering and other characteristics, giving you all you need to know about the quality of the light you’ll get with their bulbs.
Once you start doing research for replacing your existing lighting with LEDs, you’ll become familiar with these numbers and know what to look for: correlated color temperature, or CCT 3000K, and color quality, such as CRI or TM-30-15.
Once again California is leading the way with certifications for LED lighting. Remember those nasty looking compact florescent (CFL) spiral shaped bulbs that were energy efficient but terrible to look at? California learned their lesson: don’t regulate efficiency alone! Not only does the state mandate stringent efficiency standards for bulbs sold as retrofit, but they mandate even more stringent standards for new construction. The new Title 24 JA8 requirements include power efficiency, color temperature, color rendering, rated life expectancy, dimmability, and other characteristics that protect consumers and ensure that lighting installed in California meets current good practices, not just for energy use but also for light quality. Look for the JA8 certification to take advantage of California’s stringent quality mandates.
How important is all this stuff for displaying your fine art? See for yourself:
Even with the limitations of digital photography you can see that the higher CRI illumination has more vivid reds and more subtle coloration – check out the color of the woman’s coat in the bottom center of the painting. If you’re going to spend money on excellent art that you love and want to enjoy for years to come, it is a small extra investment to buy a high CRI light source to bring out all of your art’s colorful detail. Your eyes will likely notice an even bigger difference than is visible in these photos – your artwork will “pop” off the walls.
Credits – all photos and images here were created by me except for the spectral output curves image which is CC-BY-SA 4.0 and published on Stack Overflow, the snippet of CRI color swatches, which is CC-BY-SA and published on Wikipedia, and the SORAA VIVID 3000K color vector graphic plot, excerpted from the linked specification sheet and included pursuant to fair use.
Time On My Hands 