Q:

I want to replace the incandescent lightbulbs in my house with compact fluorescents (CFL), but the bulbs I’ve seen in the past are too green for my taste. In shopping for CFLs, I’ve stumbled across terminology such as colorrendering index (CRI) and correlated color temperature (CCT). What are CRI and CCT? How do they affect the quality and appearance of light emitted by a lightbulb?

Dave Larson, Worcester, MA

A:

Nancy McCoy, a lighting designer in Novato, Calif., replies: Simply put, CRI and CCT (also known as Kelvin temperature) have everything to do with the quality and appearance of light. You’ll find these ratings on the packaging of individual bulbs.

A bulb’s CRI determines how well or how accurately an object will be perceived under its light. For example, have you ever looked at your car at night under an orange-colored street lamp and had difficulty recognizing its color? That’s because the bulbs in street lights typically have a CRI of 20, which is very poor on the CRI scale. Incandescent and halogen bulbs have the best CRI, 100, which means objects under their light appear just as they do in daylight. CFLs have a CRI around 85, which still displays objects pretty accurately.

Kelvin temperature, or CCT, indicates how warm or cool the color of the light appears, but opposite to how you’d expect. Higher Kelvin temperatures have a cooler, more bluish light than bulbs with lower Kelvin temperatures, which appear more yellow. An incandescent light has a Kelvin temperature of around 2700°K. CFLs are available with Kelvin temperatures of 2700, 3000, 3500, and 4100. In my opinion, 2700°K CFLs are too yellow to make skin tones look good, and 4100°K bulbs are too cool. My favorite CFL choices are the 3000°K and 3500°K models.

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