Combining daylight redirection with control

Sale and installation of light shelves and sun-control products can be a valuable niche
By John Carmody
June 1, 2005
COMMERCIAL, FABRICATION : GREEN

Shading devices control light and heat gain; they also can reduce the amount of light in a space, increasing electric lighting loads. Cooling loads are often highest on summer afternoons when there is abundant sunlight. Shades can be drawn to block the sun, but if electric lights need to be turned on, cooling and lighting energy use rise. It is possible, however, for sun control and daylighting to be addressed in tandem. For instance, an element such as a light shelf can both shade and reduce glare, while allowing daylight to better penetrate the space.

 

Sun-control solutions should offer shading of direct sun and reduce glare from sky brightness. The most innovative approaches also improve daylight illuminance distribution and increase illuminance levels deeper into the space. Daylight typically penetrates a room by about 1.5 to two times the window head height. Courtyards and notched floor plans are direct consequences of this rule of thumb that has guided architectural development through the centuries. Daylight and ventilation requirements, in eras without air-conditioning or electric light, meant that window design governed building depth until well into the 1900s.


A problem with providing daylight is that there is often too much light and glare near the window and not enough light further back. Light shelves and prismatic glazing, as well as holographic films and innovative blind systems, address glare while facilitating daylighting. There is widespread interest in increasing the floor area that can be effectively daylit from a window.


Advanced light-control elements and systems are based on physical phenomena such as reflection with light shelves, refraction with prisms and diffraction with holographic elements. These alternatives, connected with the window glazing or immediately adjacent to it, are not all in widespread use, but are worth consideration, both as expressive opportunities and effective light-control measures.


Reflectivity and light shelves

Architects have long harnessed the principle behind light shelves: reflectivity. Reflecting pools have the capacity to capture the play of light and bounce it into buildings. In Scandinavia, the reflectivity of white snowfields in winter plays a similar role. Water and snow can be considered light shelves that extend well beyond the immediate building envelope. However, they also can be sources of unwanted glare and light.


At the building scale, light shelves are typically flat or curved elements in the window façade that reflect incoming light. The elements typically divide the window aperture into a view window below the shelf with a clerestory above. The lower window provides view for the occupants while the upper transmits daylight. Light shelves improve the quantity and quality of light in a space and should be designed specifically for each window orientation, room configuration, building latitude and climate. They are most appropriate for clear sky climates, particularly on south building elevations. Light shelves are less effective on eastern and western orientations and in overcast climates. There are significant light-shelf nuances and variations, be they interior, exterior or composite, as shown in the illustrations above.


Exterior light shelves are most effective with mirrored finishes, with their angle impacting performance. As the light shelf reflects light up and back in a space, ceiling reflectivity is critical; white ceilings are most effective. Higher ceilings also will positively impact light-shelf effectiveness. Exterior light shelves or exterior components of a composite light shelf mainly contribute to higher light level in the space; their interior counterparts mainly address the glare issue.


Exterior light shelves, as with other shading and light control elements located outside the glazing, have the advantage of reducing thermal gains as well as helping control light. By contrast, interior light shelves and shading devices absorb heat retained in the space. This type of issue must be balanced against factors such as visual and thermal comfort, energy costs and initial and long-term costs in developing a building envelope. Elements outside the building envelope are generally more costly and require more to maintain.


There is wide variety of light-shelf designs. One notable example is Lockheed Building 157 in Sunnyvale, Calif., that was completed in 1983 and has light shelves more than 10 feet deep. The facility illustrates the full impact light shelves can have on building development and final form. Composite light shelves on the facility’s north and south façades, along with other daylighting strategies, such as 18-foot floor-to-floor heights and internal atriums, called litriums, shown in the diagram at right, all contribute to Building 157’s celebrated  lighting design.


A number of innovative blind and louver systems have been developed employing the light shelf concept. The mini-optical light-shelf system in the Xilinx Development Center in Longmont, Colo., or the systems of reflective blinds within several brands of insulating glass units function as light shelves by helping to redirect light and temper sunlight.


The Xilinx building features glass with high visible light transmission. It provides adequate daylight onto the Boulder, Colo.-based Architectural Energy Corp.’s innovative, patented optical light-shelf daylighting system, shading occupants from direct sunlight while redirecting daylight deep into the open office space.


Each MLS unit consists of a frame supporting a series of fixed horizontal reflective louvers of a special compound geometry that redirects daylight uniformly across the ceiling surface. The illuminated ceiling surface provides ambient light to the space below. Photosensors determine the ambient light from daylight and raise or lower the indirect electric lighting to maintain the minimum ambient lighting level.


All of these products enhance the comfort of people who work in commercial buildings, contribute to energy conservation and create potentially profitable opportunities for contract glaziers and glass shop owners.


 

The author is director of the Center of Sustainable Building Research at the University of Minnesota in Minneapolis. This article is an excerpt from his book Window Systems for High Performance Buildings, pp. 118-119, W.W. Norton & Co., New York, 2004.