High-Performance Façades: It’s What’s Outside that Counts (2 of 4)

April 3, 2006

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Sunlight Redirection
Sunlight-redirection systems rely on reflection, refraction, diffraction, or non-imaging optics to modify or enhance the distribution of incoming daylight. Benefits include not only the potential to offset lighting requirements, but to improve lighting quality and visual comfort.

Light shelves, one example of a sunlight-redirection system, are horizontal, exterior projections that use a high-reflectance, diffuse, or semi-specular (shiny) upper surface to reflect sunlight to a certain interior depth from the window wall. Between-pane light shelves employ many of the same principles, but are protected between two panes of glass. Laser-cut panels use simple linear horizontal cuts in an acrylic panel to refract light at the juncture of the linear grooves. Prismatic acrylic panels (which increase reflective optical efficiency without mirrored imaging) also work on the principle of refraction to redirect incident sunlight.

Systems using direct sunlight are most effective on the south façade and are designed based on seasonal variations in solar altitude. For moderate and hot climates, daylighting strategies must be integrated with solar-gain control. In the summer, light shelves block direct sun at both the upper and lower windows. In the winter, low sun can penetrate to the back through the clerestory, preheating occupied space in the morning and providing light. Tinted glazing can be used on lower-view windows, while clear glazing can be used at the clerestory level to increase daylight admission.

In all of the above systems, view is distorted or impaired; placement above standing-view height is typically recommended.

Skylight Redirection
These systems, designed for diffuse skylight, are intended to increase overall interior daylight levels with less emphasis on the depth of light redirection. Typical characteristics of anidolic systems include sharp cut-off angles for admitting and delivering light, and very few reflections of light rays when passing light through the system. Holographic optical elements (HOEs) can also be applied to redirect skylight. Tilted-glass HOE overhangs can be placed over north-facing windows so that diffuse daylight is redirected into the building interior. (HOE glazing is still under development.)

Skylight-redirection systems are efficient in climates with predominantly cloudy conditions, urban environments, or other situations where windows or skylights only “see” the sky.

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