Lighting Quality Metrics: CRI And Temperature
“Lighting quality is a result of addressing a full complement of criteria — a holistic approach,” says Gary Steffy, president, Gary Steffy Lighting Design. There are several common quality-of-light metrics facility executives should consider when installing a sustainable lighting system.
The color temperature of a lamp, measured in degrees Kelvin (K), can largely influence the ambiance of a space. People often associate types of lighting with different settings and moods — very cool fluorescent lighting with a doctor’s office, for example. Using cool fluorescent lamps in other spaces will bring the same mood to the space. For example, residential incandescent lamps have a color temperature of 2,800K, or very warm. (Warm refers to the reddish light from a lamp; cool refers to a bluer tone.) Typical fluorescent lighting ranges between 3,500K and 4,100K, much cooler on the spectrum. That’s why occupants generally perceive fluorescent lighting in offices to be cold, clean or harsh. The light from incandescent lamps is thought to be much more intimate and personal.
Facility executives can use these color temperature perceptions to their advantage. In areas where an intimate setting is desired, such as a small café or lounge area, use warm color temperature lamps. Retail spaces, areas where graphics work is done or any other space that requires perceived sharp lighting can benefit from a cooler temperature lamp.
Another aspect related to lighting color is the color rendering index (CRI). CRI is a measure of a lamp’s ability to render colors accurately. The scale ranges from 1 to 100, with 100 representing the same rendering ability as an incandescent lamp.
Kahn provides an anecdotal explanation of color rendering: “Everyone has a pair of pants or a tie that looks one color when you put it on at home under incandescent lamps or daylight,” he says. “But when you get to the office with fluorescent lamps, it looks like a completely different color.”
The goal for facility executives should be to get the best color rendering lamps possible. “When you select a light source, especially fluorescent, you have two choices: good or better CRI,” says Graf. “The better CRI lamp costs a little more, but studies show that the better the color spectrum, the better the visibility.” Occupants in spaces with higher CRI lamps will perceive the light as crisper, because colors and shades will be rendered more clearly.
Other lighting quality issues can’t be measured or obtained from the manufacturer in a spec sheet. But they are just as important to consider in a sustainable lighting system design.
Glare — both directly from a light source to the eye and indirectly off a computer monitor or other surface — is a major issue, especially in office environments. Taking steps to reduce glare from both electric lighting and daylight sources will reduce strain on occupants’ eyes and garner a more favorable view of the lighting system.
Another simple way to help ensure occupants are happy with the lighting system is illuminance contrast. It is often overlooked because many facility executives have never heard the term. Again, it’s best explained anecdotally: How do you feel outside on a sunny day vs. a cloudy day? “You always get the same answer. People prefer sunny days,” says Graf. Recent research has shown that the contrast between sunny and shaded areas on a sunny day stimulates the pleasure center of the brain through the optic nerve, says Graf. It’s a simple concept that can be used in facilities to give more energy, interest and life to a space. Facility executives can use properly placed accent lighting to add contrast and shadows in some areas.