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In our blog entry entitled “More for less: A closer look at LEDS and energy use”, we talked about the remarkable energy efficiency of LED lamps—one of the reasons more people are increasingly choosing this lighting technology. Now I’d like to examine one of the perceived weaknesses of LED lamp performance—color rendering.
If you’ve ever put on a green sweater indoors only to discover it was blue when you stepped outside, you’ll know not only that you were hideously mismatched, but also that light sources differ in their abilities to accurately reflect surface color. This ability is determined by the wavelengths of light that make up the source, the range of colors in its spectra, and is measured between zero and 100 on the Color Rendering Index (CRI).
Sunlight has a very broad, even spectrum, so it’s considered ideal and scores 100 CRI. Fluorescents, on the other hand, typically rate around 70, although some speciality fluorescent lamps can achieve CRI ratings in the 90s. Because they are basically brightly glowing heat sources, halogen and incandescent lamps mimic the sun and carry the highest CRI rating among artificial light sources at approximately 97 to 100 (though as mentioned in the post “More for less: A closer look at LEDS and energy use”, the high heat emitted by these bulbs makes them the least energy efficient).
So where do LEDs fall on the CRI? Due to the lack of LED manufacturing standards and a still-young technology, the CRI value of LEDs can range between less than 70 and over 90. This is why it is important to be able to make the distinction between a good quality LED bulb and a bad one. Good color rendering in LED lamps depends on a variety of factors, but by and large, the key is in the phosphors.
How phosphors effect CRI
LED lamps are manufactured with a phosphor-based coating that reacts to, and absorbs, some of the natural blue light emitted by the tiny chip inside the lamp. One of the pesky problems with these phosphors is that they convert less blue light when they get hot. When the lamps get too hot, the phosphors stop working and the light takes on a blue or green hue that’s reminiscent of fluorescent lighting.
Some lamps use temperature sensors that reduce power when the temperature reaches a certain level. While these sensors help maintain a nice white color, they also reduce the light output, which may make the lamp too dim—hardly a solution if you rely on a certain output. Other lamps rely on a heat sink to dissipate the heat and keep the phosphors doing their job. While heat sinks are a necessary part of an LED bulb and essential for reducing a lamp’s temperature, not all heat sinks are optimally designed and therefore not overly effective.
The importance of correlated color temperature (CCT)
Any commercial or retail establishment, such as a grocery store or fine art gallery, want their products displayed under the most optimal lighting. When choosing the right lighting for these applications, high CRI is clearly a factor—but not the only one. Once CRI is higher than 80, the correlated color temperature (CCT) of the lamp plays a significant role too.
We know from high school physics that white light is a combination of red, orange, yellow, green, blue, and violet. CCT is essentially an indication of how red, yellow, or blue the white light is from a given lamp. Different sources—incandescent, CFL, halogen, and LED—have different constituents and proportions of light.
CCT is measured in degrees Kelvin. Red is a relatively cool heat at about 1500 Kelvin, yellow warmer at around 2000K, and blue the hottest at 6000K and higher. Sunlight is considered the true white at around 6500K, which is why you see lamps labelled “Daylight White”. (If all these measures aren’t confusing enough, consider that we tend to refer to blue light as cool and yellow light as hot—an interpretation that is at complete odds with the true temperature of a light.)
After basking for more than a century in incandescent illumination, we seem to have a cultural preference for the yellow-tinted white light it emits—around 3200K. We see it as warm and associate it with the welcome light of a candle even though it’s actually cool compared to an LED.
If you have lamps that have a high CRI, but a color temperature in the yellow end of the spectrum, products, such as food in the case of a grocer or fine art in the case of a gallery owner, may appear less than desirable. High quality LEDs, on the other hand, provide both high CRI and a white light that closely mimics the sun to make colors stand out accurately.
Should I wait for LED CRI performance to improve?
Only if CRI is the sole performance measure you’re concerned about. Lighting is highly subjective, and human perception differs immensely—even based on age. Young people, for example, see blue better than older people.
While most LEDs currently rank below halogen and incandescent in CRI, the marked advances of the past few years suggest all LED lamps will soon equal or outperform older technology. Until then, the relatively high CRI of high-quality contemporary LEDs removes CRI as a significant concern.
What has your experience been with the color rendering of LED bulbs so far? Has a low quality bulb with a poor CRI turned you against the technology or has it simply made you look more closely at the specifications of bulbs?
For more information about the CRI rating of our Leapfrog Lighting PAR30 dimmable high output spotlight—one of the highest among the latest LED lamps—as well as our quality controls, see the Leapfrog Lighting PAR30 dimmable high output spotlight datasheet.