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Opinion:

Bad Neighbors

The unanticipated reflection of light from adjacent buildings can cause serious problems.

By James R. Benya
March 2013
Illustration by Andrew Nilsen

There is no question that structures with expansive, shining surfaces are appealing. Their reflectivity also serves a purpose, dramatically reducing solar heat gain throughout the year. But lacking adequate concern for the reflected light and redirected and/or concentrated solar radiation has caused personal injury, glare, heat damage, and overheating of adjacent buildings.

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In 2010 reports emerged about the "Vdara Death Ray"—concentrated reflections that burned sunbathers on the Las Vegas hotel's pool deck. In a similar case, light reflected from the facade of the Wynn hotel tower burned the roofing materials on the building's podium as they were about to be installed. Both cases involved a concave, curved facade of highly reflective glass facing the sun for part of the day. Similar problems were created by Frank Gehry's Disney Hall in Los Angeles, but not by glass; specular polished stainless-steel panels caused excessive solar glare and heat gain in adjacent buildings.

Redirected solar radiation can cause trouble even without being concentrated. The south facade of Museum Tower, a new 42-story residential building adjacent to the Nasher Sculpture Center in Dallas, is redirecting light that defeats Renzo Piano's "magic carpet" roof on the Nasher. The roof is designed to prevent direct solar light from the south, but reflected sunlight now comes from the north.

Unfortunately, because most of the cases involve whole building facades, the options for corrective measures are limited and usually extraordinarily expensive. All of these, of course, could have been prevented by some working knowledge of reflection and the consequences of highly reflective building surfaces.

Materials have two types of reflectance: specular reflectance, which reflects light like a mirror, and diffuse reflectance, which reflects in all directions. The amount of reflectance is expressed as a percentage of incident light. Although diffuse white buildings can be bright and occasionally glaring, extreme problems are generally caused when specular reflectance approaches 20 percent. With up to 100 watts per square foot of solar power hitting the glass, even the directly reflected rays of the sun pack a real energy punch.

To prevent reflectance problems, local codes could ban the use of facade materials with more than 10 percent specular visible-light reflectance. This will work, but it might unreasonably restrict architectural design. As an alternative, communities could require reflectance studies when a highly reflective facade is proposed.

With current technology, reflectance problems can be thoroughly studied using ray-tracing software. These studies produce exact data and can be automated to provide video of the sun striking the site throughout the year. For complex buildings, the cost and time of such studies are probably warranted.

But for many projects, an inexpensive heliodon study could prove adequate. This can be done with a relatively simple building model, finished in exactly the same reflectance and specularity as the proposed building, in a darkened room. It's helpful to include simple models of surrounding structures and topography. Then, using white paper to help detect reflected beams and a light meter to measure the amount, note the solar angles at which beams of light will be reflected onto the site or adjacent buildings. Simple sun-path diagrams will help determine the times when problems might occur. For any study, it is important to understand the community and its structures.

James R. Benya is the director of the Advanced Lighting Design Project at the California Lighting Technology Center, UC Davis.

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