Volume 1 Number 4
Copyright @1998 Rensselaer Polytechnic Institute

To Capture the Sun and Sky

The sun is a source of free, plentiful light, and daylighting - the method of lighting building interiors with sunlight and diffuse skylight - is an effort to reap this bounty. The use of advanced daylighting technologies, such as light pipes, active skylights, and improved windows, may increase the amount of daylight available inside buildings. These technologies are attractive, but do they work and are they cost effective? The most effective daylighting strategies might still be the simplest: optimized building orientation and form, optimized window size and placement, a light switch, maybe a light shelf.

Bringing the light inside

Daylighting provides light that supplements or replaces electric lighting; the addition of daylight in a space may also bring benefits related to aesthetics, health, and energy savings (see "The potential benefits of daylighting" on p. 5). Perimeter daylighting systems, such as windows, bring daylight about 15 feet into a building. Core daylighting techniques bring daylight deeper into a building. However, the application of traditional core daylighting methods, like atria and courtyards, is limited by building height and design. Many building owners are reluctant to turn what could be rentable space into a daylighting atrium.

Light shelves. Light shelves are reflective horizontal surfaces that extend from the exterior into the interior of a building. They can extend the useful range of perimeter daylighting on a building's south side to about 25 feet on sunny days.

Light shelves can prevent unwanted direct sunlight, which is a source of glare, from entering a space. The shelves reflect sunlight onto the ceiling, minimizing glare and boosting light levels in the space. Light shelves work well at high solar angles, but at lower angles the shelves need to extend deeper into the room to catch the sunlight. Light shelves can also reduce the amount of heat that enters a space. 

In the early 1980s, researchers tried to increase the performance of light shelves by developing a reflective light shelf that responds to the angle of the sun. VALRA (variable area light reflecting assembly) uses a roller containing reflective Mylar. At high solar angles, the roller retracts and forms a short tilted mirror. At low solar angles, the film unrolls to produce a wide, almost horizontal mirror. An innovative idea, VALRA was not cost effective for commercial use.

Prismatic glazing. Refraction is the principle behind prismatic glazing, a material with ridges on one side that bend light to specified angles. Prismatic glazing can be used to exclude sunlight from a space or redirect it, usually to the ceiling. Prismatic glazing works effectively only at certain solar angles. As the sun moves beyond the critical angles, sunlight passes through the glazing and can cause glare. Prismatic glazing obscures the view out, so it is usually placed at the top of a window.

Holographic films. Holographic films are created using laser light, which records a three-dimensional pattern of lines on photographic emulsions. The lines create a diffraction pattern through which light exits at certain angles. The film, which is attached to a window, appears clear except at the diffraction angle. Thus, in theory the film will direct sunlight onto the ceiling to be re?ected further into the room while preserving the view out of the window. However, the diffraction separates the sunlight's spectrum and creates colored patterns on the ceiling.

Because the film is attached to a window, it can't track the sun and has a limited time of exposure. It is not effective at manipulating diffuse daylight, and it reduces the total transmittance through the window. Richard Mistrick, a professor at Penn State University, reports a problem in the film's manufacture. The material shrinks as it is cured, making it difficult to maintain the proper number of holographic lines per inch.

Passive skylights. The common skylight, or roof window, provides a simple way of bringing daylight to the upper floor of a deep building. An example of a passive daylighting system, the common skylight contains no moving parts and is ?xed at an optimal position. Skylights can create thermal problems, admitting heat in the summer and losing heat in cold climates. Skylights also lose light due to reflections in the shaft of the well between the roof and the ceiling. Manufacturers have been working to develop systems that minimize thermal and glare problems and increase the light levels brought into a space.

In 1993, Andersen Corp. installed nine prototype skylights in a Wal-Mart store in Lawrence, Kansas. The interiors of the convex windows are covered with a prismatic film to help enlarge the apparent area of the window. According to Patty Perez, Green Coordinator at the Lawrence store, shoppers respond positively to the skylights. "I've had customers come up to me and mention how much they prefer to shop in the areas with skylights," says Perez. Wal-Mart has since opened two more stores that use skylights extensively.

The SunPipe Company produces the SunPipe, an aluminum pipe that is lined with silver to reduce reflection losses. A clear acrylic dome on the top and a translucent dome on the bottom diffuse light through a space. SunPipe developer Greg Miller claims that a SunPipe mounted 16 feet above the ?oor can deliver 20 to 40 footcandles onto the floor directly underneath.

Elbows give the installer flexibility in placing the SunPipe; installation typically takes only three hours in a residential building. "The installer doesn't need to make significant structural modifications to the building because of the size of the pipe," says Miller. The SunPipe Company markets the SunPipe primarily to residential customers, although the company is looking at promoting it for use in warehouses and small of?ces. For residential customers, the attraction is the quality of the light rather than potential energy savings. The price of a 21-inch diameter, 2-foot long pipe ranges from $234 to $327, depending on the quantity purchased.

Active skylights. Active daylighting systems contain moving parts that track the sun. The tracking mechanism is most effective when the sun is out; they cannot redirect signi?cant amounts of diffuse light from cloudy skies. The Natural Light Co. manufactures both passive and active skylight systems. Their basic passive system uses a 4-foot by 4-foot light well with sides made of foil-lined insulation board that has a reflectivity of 85 to 90%. The top is a prismatic lens that helps to redirect low-angle sunlight into the shaft. The bottom of the shaft can be one of three types of diffusers, two of which hang below the ceiling. "In an industrial space requiring 35 footcandles and a mounting height of 20 feet, each unit can cover 800 square feet," says Eric Lancing, business manager of Natural Light Co.

The company's active system adds a rotating mirror assembly to catch the very low-angle light that the sun produces at dawn and dusk. The assembly follows the sun using a pair of IR photodiodes and has been wind-tested to 90 miles per hour.

The passive system is designed to eliminate the need for electric lights from two hours after sunrise until two hours before sunset. The active system can extend the period of useful daylight by two hours. The company's largest market so far has been classrooms in the southeast because of the high availability of sunlight in the area, according to Lancing. Schools benefit because they operate during daylight hours, with little use after dark.

LightScience makes an active system enclosed in a fiberglass shell that protects it from the elements. Its tilting mirrors track the sun on horizontal and vertical axes. The company claims a reduction in lighting costs of 75 to 95% when combined with lighting controls. According to Lee Webster, company president, the biggest marketing hurdle is convincing building owners to make holes in their roofs. "But our ?berglass housing actually helps to prevent leaks," says Webster.

The performance of the active unit is equivalent to that of a passive skylight during the middle part of the day, but its biggest advantage comes during periods of low-angle sunlight, when it can collect light that a passive skylight can't. Webster admits that cloudy days still pose a problem because the housing and mirrors actually reduce the amount of light that comes in. "We're working on it," he says.

More complex active daylighting systems operate on two planes and precisely direct sunlight to ?xed points. These systems produce a uniform and steady source of light and depend only marginally on building orientation. The more precise the optics, the more maintenance these systems require.

One such system is the Himawari ("sunflower" in Japanese), manufactured by Asahi Glass Co., Ltd. The Himawari, which powers itself with solar cells, tracks the sun using a sensor and software. A honeycomb of Fresnel lenses focuses the sun's light onto the ends of quartz-glass optical ?bers. The manufacturer states that a six-?ber, 130-foot cable emits 1182 lumens under 98,000 lux of direct sunlight, which is comparable to the light output of a 75-watt incandescent lamp.

Sales of the Himawari in North America have been slow, although there are quite a few units installed in Japan. "People have trouble getting past windows [when they think of daylighting]," says Kenneth Eben, sales and marketing manager for Mitsubishi International Corp., the Himawari's North American distributor.

Once the system is installed, it costs nothing to operate. However, the lack of a viable payback period slows sales. System cost ranges from $7000 to $300,000, depending on the number of collectors and the length of fiber required. The Himawari is attractive for installations where initial cost is much less of an issue than operating costs, or where special decorative effects are needed.

What's next for daylighting?

Some researchers are looking at decoupling the view and the lighting function of windows, according to Barbara Erwine of the Seattle Lighting Design Lab. Views tend to be oriented downwards and horizontally, whereas daylight comes from above. Glazing systems could preserve the view while improving daylighting performance.

Nippon Sheet Glass is working on a product that appears clear or diffuse, depending on the angle of view. Angle 21 is a laminated film with chemically induced slits that can be preset for a variety of angles. Angle 21 is targeted at the privacy market, but it could be used to maintain view and diffuse sunlight entering from above the horizontal.

Currently, light from core daylighting systems is usually directed into a space by large diffusers that descend below the ceiling plane. Smaller diffusers and hollow light guides that resemble ordinary luminaires are also available. It is reasonable to expect that these systems will have light distributions and glare control similar to those of standard luminaires. However, most manufacturing efforts have gone into producing systems that efficiently collect light and transmit it through the building. More work is needed to produce effective methods of directing light to the work surface.

New daylighting technologies are not likely to become common until their prices drop to reduce the payback period (see "Daylighting economics" on p. 1). In addition, according to Steve Selkowitz of the Windows and Daylighting Group at Lawrence Berkeley Laboratory, the industry needs to seamlessly integrate component technologies into functional systems for optimal building performance. Daylighting systems, electric lighting systems, and controls need to be coordinated to optimize glare control, light balance, and energy savings. Backup electric lighting needs to be incorporated into the system so the changeover from day to night is seamless. And specifiers want greater reliability for dimming systems.

"The building industry has been slow to adopt new building technologies," says Eben. However, there is no reason why daylighting systems cannot become part of the building industry standard. Selkowitz feels that there is a "confluence of interests" between companies that want to make money, architects who want to build interesting buildings, and occupants who want more daylight.

Maarten Mulder is a research associate at the Lighting Research Center.For more than 100 years, lighting research has pursued the obvious - that with light we can see, without it we cannot. This pursuit has produced a dramatic increase in the understanding of how lighting affects vision and led to the creation of a deluge of lighting equipment. Lighting designers and engineers assemble this equipment into the lighting installations that make modern life possible.