Walls That Save Energy

Because surfaces and objects in typical spaces reflect light, they can play a part in lighting efficiency as extensions of the lighting system. By controlling room surface reflectances, light levels can be improved, creating opportunities to save energy.

If light is not absorbed by a surface, it is reflected or transmitted to other surfaces. Because surfaces with lighter finishes have higher reflectances, they promote interreflections in the space. These interreflections can soften shadows, reduce contrast and reinforce uniformity. They can also increase light levels and perception of brightness.

Which brings us to efficiency. The coefficient of utilization (CU) goes beyond lighting fixture efficiency by considering the effect of the space dimensions and reflectances. A given fixture may emit some of its light directly toward the workplane and some at a nearby wall. Because the wall reflects some of this light to the workplane, the reflectance of the wall, expressed as the fraction of light reflected instead of absorbed, must be considered. The higher the reflectance, the more light is reflected from it, which translates to higher application efficiency or delivered lumens at the task.

This is all expressed in the CU, which gives designers a tool they can use to compare two different fixtures within a given environment. Specifically, fixture manufacturers publish CU tables for their products in the photometry report.

The more efficient the fixture is in its application, the higher the light level. Therefore, less light is required to be produced by the lighting system, which may translate to fewer fixtures or lamps or more flexibility in selecting the light output of the system based on lamp output and ballast factor. The result can be reduced capital costs and/or energy and maintenance costs. The biggest impact, of course, will be with lighting equipment that relies more heavily on room surfaces to distribute light, such as indirect lighting.

Suppose we will be lighting a room with a 30 percent ceiling reflectance, 50 percent wall reflectance and 20 percent floor reflectance. The room cavity ratio (RCR) is 3. Looking up a RCR of 3, we get a CU of 0.19 for a particular 100 percent indirect suspended linear fluorescent fixture. If required lumens = maintained light level (foot-candles or fc) area/ballast factor CU light loss factors, and if our target maintained light level is 35 fc, the ballast factor is 0.88, and the overall lighting loss factor is 0.75, then 63,000 lumens are required for this space.

Now suppose lighter finishes are used that raise ceiling reflectance to 80 percent—this is, after all, a 100 percent uplight fixture, so 80–90 percent is recommended—with wall reflectance increased to 70 percent. Looking at the CU table for the fixture, CU is now 0.64, which reduces our lumen requirement to 19,000 lumens—70 percent less, which can be leveraged for capital and operating-cost savings.
As shown, controlling surface reflectances can affect overall application efficiency, particularly when indirect fixtures are being considered.

What are ideal surface finishes? The first rule is to avoid dark surfaces, which can create uncomfortable contrasts and absorb light, unless they are required by the interior design. This not only goes for walls and ceilings, but also for furniture, partitions, cabinets and other objects in the field of view. Note, however, that darker colors can be useful for accent bands or for furniture, wainscoting and floors below the workplane. If the ceiling is unavoidably dark, consider increasing the brightness of walls through accent lighting, wallwash fixtures, etc.

Light-colored materials have a higher reflectance. The color does not have to be white; high-reflectance (70-plus percent) pastels are available. When possible, the surface should be matte instead of specular (glossy), which lessens glare by diffusing the reflected light.

Meanwhile, the Illuminating Engineering Society offers suggestions in its recommended practice guides for different building types, as does the ASHRAE Advanced Energy Design Guides, a series of publications providing methods for exceeding the ASHRAE 90.1 1999 energy standard by 30 percent. The Advanced Energy Design Guides, for example, recommended a reflectance of 20 percent for floors and the following:

• 80-plus percent (90 percent if indirect lighting) for ceilings and 70-plus percent for walls and 2.5-plus feet vertical partitions in offices
• 70 percent (80–90 percent preferred) for ceilings and 50 percent for walls in K–12 schools
• 80 percent (80-plus if daylight zone) for ceilings and 50 percent (70-plus percent if daylight zone) for walls in small retail buildings
• 85 percent for direct lighting and at least 90 percent for indirect and/or daylighting for ceilings and 50 percent (70 percent for walls adjacent to daylight apertures) for walls in small hospital and healthcare applications
• 80 percent for ceilings and 30 percent for walls in warehouse and self-storage buildings

Controlling room surface reflectances can play a significant role in improving light levels, lighting efficiency and visual comfort.

DILOUIE, a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at www.zinginc.com.

About the Author

Craig DiLouie

Lighting Columnist
Craig DiLouie, L.C., is a journalist and educator specializing in the lighting industry. Learn more at ZINGinc.com and LightNOWblog.com .​

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