What is Color Temperature, Anyway?


If you’ve purchased an LED or CFL lightbulb recently, you’ve probably heard of color temperature. And likewise, if you’ve talked to an older family member recently about their lightbulbs (and honestly, who hasn’t?), then you’ve probably heard about it as well, albeit in stronger language.

So what is color temperature, anyway? Basically, it describes the color of your white light. That may seem counterintuitive; white light is, after all, white. It shouldn’t really have a color. But within the blanket category of “white”, there’s a lot of variation. Think of the cold, white light you might see in a hospital or 1980s office park. Now think of the warm, inviting light from an old incandescent bulb in your family room.

What’s the difference? Color temperature. That sterile, industrial light probably has a high color temperature. The warm, inviting light has a lower one. Old incandescent bulbs were color temperature savants (at least according to most peoples’ tastes), whereas CFL bulbs (at least the older generation ones) were notorious for being in the ugly, cold part of the spectrum. Thus the rant from Grandpa, if he was forced to switch.

Luckily, newer bulbs give you a lot of control over color temperature, whether they’re LEDs or CFLs. So how do you choose? Color temperature is measured on a scale that you’ve probably learned and then forgotten: absolute temperature, measured in Kelvin.

The scale starts in the lower 2,000Ks, with things like a match flame and the “warmest” of incandescents. On the higher end, you get light sources like the sun, which clocks in at a cold, blue 5,800 K (and here I always thought it was yellowish…live and learn). That pleasant, incandescent light you grew up with? It’s probably around 2,700K. The soul-sucking florescent lighting your cubicle? 4000k.

This graphic, and the one above, shamelessly lifted from Wikipedia.

This graphic, and the one above, shamelessly lifted from Wikipedia.

When you’re buying bulbs, then, think about the kind of lighting you like. If you’re nostalgic for the glory days of old fashioned bulbs, go for LEDs or CFLs in the 2,000K range. If you prefer a bluer, “daylight” kind of light, go for the higher end of the color temperature scale. Make sure to think, too, about the kind of space you’re lighting. Higher color temperature bulbs are great for a foyer or perhaps even the kitchen or dining room, but a stark, unforgiving daylight glow probably isn’t right for the den or bedroom.

Of course, nowadays good CFLs and LED bulbs can last 10-25 years, so a poor color temperature choice could haunt you for a long time. For the chronically indecisive, Phillips Hue offers a neat (but expensive) alternative. Their Hue bulbs allow you to change the color temperature from a smartphone app, going instantly from a nice, warm light for relaxing to a stark, bluish white light for reading. Their app even has cute names for each setting–low color temperature settings have names like “Relax”, and high settings are “Refresh” and “Concentrate.”

Again, the most important thing about color temperature is that it’s a personal choice. Make sure to try a few different options before committing to a particular color temperature. And if you still can’t decide, just go with 2,700K. Grandpa will thank you.

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  1. Tungsten lamps can have their color temperature changed by changing the voltage when the voltage is lowered the color temp goes down, the efficiency goes down, but the life goes way up. LED light sources can be manufactured to almost any color temperature you want.

    You missed an important point Color Rendering index (CRI). Light sources with a non continuous spectrum are deficient for viewing color due to excessive amounts of some colors (fluorescents are loaded with green) or deficient amounts of other colors (mercury vapor has no red) The CRI ratings run from 100 for tungsten lamps or sunlight to zero for light sources like sodium vapor (it has only two spectral lines). Compact fluorescent lamps have a CRI from 70-80 while LED lamps can run as high as 90.
    By the way color temperature is defined as spectrum of a black body radiator (often a carbon block) when it is heated to the temperature in Kelvins. Strictly speaking fluorescent lamps cannot have a color since their spectrum in not continuous.

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