A lamp of a different Color

Store Equipment & Design, August, 2000 by Marilyn R. P. Morgan

A new study on light variation explores when imperfectly matched lighting is OK and when it's not.

Light sources of the same type are not always exactly the same color. That can cause problems for retailers, who suffer when such mismatches give customers an impression of lower quality.

Even though two lamps may produce white light, one might be a yellowish white while the other might be slightly blue. Such ever-so-subtle differences can make a store look less attractive. Upscale retailers must be especially careful about their lighting in order to maintain their images.

What people involved with lighting need to know is how closely individual lamps must match for a given application so the lamps can be obtained as inexpensively as possible while still producing attractive lighting. Producing lamps with no apparent color differences requires a manufacturing process with close tolerances, which can increase the cost of the lamps.

Just how much variation is acceptable? That depends greatly on the application. For example, if the lamps are close together and directly visible to the observer, a small color variation will be noticeable. On the other hand, if the same lamps are placed far apart, the color variation may not be noticeable.

Because color discrimination depends on the application, setting a general criterion for acceptable color variation for a lamp type may not be appropriate: What is excessive for some applications may be inadequate for others. How, then, can manufacturers and users of lighting products know how much variation is acceptable?

AN ENLIGHTENING STUDY

The Lighting Research Center (LRC), based in Troy, N.Y., recently completed a research project in this area. As part of this effort, researchers, including Nadarajah Narendran, Sandra Vasconez, Peter Boyce, and Neil Ecklund, investigated at what point observers perceive a color difference between similar lamps when used in display lighting. "The LRC is working to improve lighting in both homes and businesses," says Mark S. Rea, Ph.D., director of the center. "Part of that improvement is in the area of cost. Neither lamp manufacturers nor retailers should have to spend more money than is truly necessary to meet the needs of their customers."

This study simulated the frozen food aisle in a grocery store using a mock-up refrigerator display case with two side-by-side cabinets divided by an interior wall so each cabinet could be illuminated separately. The experimental setup duplicated the light levels of typical freezer cabinets and was placed in a room that had general lighting similar to that of a supermarket. In the cabinets, the researchers placed common frozen food items such as entrees and ice cream. The researchers systematically varied the color of the lighting in one of the two cabinets while keeping the color in the other cabinet constant. Then they asked test subjects whether they could detect any difference.

The research confirmed that a single standard for consistency in color among lamps is not adequate for all situations. "With visually complex displays, such as those that include objects of many colors and lots of fine detail," Vasconez says, "you find that people have a greater tolerance for illumination from lamps of different colors than they do otherwise." The color of lamps for refrigerated grocery display cases can vary significantly before customers start noticing a color difference because colored objects inside the cases make color discrimination more difficult.

On the other hand, less visually complex displays, especially those that include white objects, require lamps made to closer tolerances. White objects easily reveal the color of any light that shines on them, so they pose a special challenge in lamp color matching. Lamps used as wall washers, for example, should be closely matched because wall and ceiling surfaces are likely to be white and relatively simple, visually.

COMMON MISTAKES

What should retailers know about buying lighting products for their stores? The most common mistake buyers of lighting products make is not realizing that there are different colors of white. White lamps are broadly differentiated into warm white and cool white. Within these categories are different whites distinguished by color temperature. "Someone who doesn't know that a 3000-K lamp will be yellow-white while a 5000-K lamp will be blue-white may order lamps randomly and put them up in the ceiling together, and that's going to look pretty bad," Vasconez says.

Retailers should stick with a single lamp manufacturer whenever possible rather than buying lamps from many different manufacturers. Two 3000-K lamps from the same manufacturer will probably look more alike than two 3000-K lamps from two different manufacturers. If it's necessary to buy lamps from different manufacturers, it's best to use them in applications where there will be visual complexity so customers are less likely to see any difference in color.

Lighting experts usually advise those in charge of maintaining facilities to replace all the lamps in a system together at fixed intervals rather than replacing lamps piecemeal as they fail. This "group relamping," as the practice is called, can reduce the cost of operating the lighting system by saving on labor costs. It also keeps illuminance levels close to the design value for the facility. Another reason for group relamping is that it avoids mixing old lamps with new. Some lamps, especially metal halide lamps, shift in color as they age, so mixing old lamps with new ones may introduce color differences even though all the lamps have the same color temperature rating.