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Randall Whitehead, IALD, is a professional lighting designer and author (his seven books include “Lighten Up!”) based in San Francisco.
He has worked on projects worldwide and appeared on the Discovery Channel and HGTV.
Visit his Web site for more information on books, upcoming seminars and the latest lighting trends at www.randallwhitehead.com. |
Q: “Randall, help! Footcandles, candelas, lux, lumens, Kelvin—what’s a guy to use regarding light in a room? One measurement says one thing; info regarding a fixture says another. Also, my customers always want “natural light” and as much “daylight” as possible.
A: I agree. There are too many terms out there, making lighting less clear to both homeowners and professionals. Here are the official definitions:
Footcandle: The unit measurement of luminance equal to one lumen per square foot; originally referring to a candle burning 1 foot away from a given surface.
Candela: The unit of luminous intensity emitted in a specific direction by a source equal to one lumen per steradian—the unit of solid angle that, having its vertex in the center of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere. (There are four steradians in a sphere.) I used to take steradians, but though they helped me bulk up, I became too edgy.
Lux: The metric unit for mea-suring the amount of light that falls on an object. Lux is the European equivalent of the British footcandle (or lumen).
Lumen: The unit measurement of luminous flux equal to the light emitted in a solid angle by a uniform point source of one candela intensity. So, basically, lux, lumen, footcandle and candela are all pretty much the same thing. But Kelvin is way different.
Kelvin: The unit of absolute temperature used to designate the color temperature of a light source.
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| A 2,700K fluorescent lamp has all the visual warmth of an incandescent, so you can look good and feel good about saving energy at the same time. |
Now, let’s clarify daylight/natural light. People think those terms refer to the color of the sun—a warm, yellow light. It is really the blue/white color of the sky.
While daylight, natural light or “full spectrum” light is the perfect type for color matching, it is not so great on skin tone. People want an amber glow that has the feel of firelight. This is really in the range of incandescent light (2,800K). Daylight is 5,000K, a very cold light, like in an operating room. What customers may really be asking for is color-corrected light, which means that the quality of the illumination comes close to incandescent but is from an alternative source.
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