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Daylighting façade performance | Daylighting façade building roster

Daylighting façades

sunlight redirection | skylight redirection

Conventional side-lighting concepts distribute flux principally 0-15 feet from the window wall causing glare, high contrast, and excessive brightness, leaving the remainder of the perimeter zone and the core "in the dark." Light-redirecting systems rely on principles of reflection, refraction, diffraction or non-imaging optics to alter or enhance the distribution of incoming daylight within the building's room cavity. The benefit of improved distribution is not only increased potential to offset electric lighting requirements with daylight across a greater depth within the perimeter zone but also to improve lighting quality and visual comfort. Similar technologies can improve skylight performance when ceiling height and/or spacing are not adequate.

Most of the systems described here are detailed in a separate document produced by the International Energy Agency Task 21 Daylight in Buildings. A book and/or CD-ROM is available to those residing in the U.S. or Canada. See http://eetd.lbl.gov/Bookstore.html under Practical Guides & Tools for Energy Users for "Daylight in Buildings: A Source Book on Daylighting Systems and Components" to find out how to obtain a copy. For those residing outside of the U.S. or Canada, please visit http://www.iea-shc.org. 

Sunlight redirection 

We make a distinction between light-redirecting systems designed principally to redirect beam sunlight versus diffuse skylight, although with any system, both sources of daylight are affected. Systems using direct sunlight are most effective on the south façade, and for practical geometric simplicity and efficiency, are designed based on seasonal variations in solar altitude. For moderate to hot climates, such as those of California, daylighting strategies must be integrated with solar gain control.

Light shelves are typically a horizontal exterior projection that uses a high reflectance, diffuse, or semi-specular (shiny) upper surface to reflect incident sunlight to a given interior depth from the window wall. Variations include the use of prismatic aluminized films on the upper surface to increase reflective optical efficiency without mirrored imaging, compound geometries tailored to specific solar altitudes, and moveable systems that can be tuned seasonally or tuned to alter the depth of redirection.

Between-pane light shelves employ many of the same principles of their larger counterparts but can be fabricated in volume and protected from dirt and dust between two panes of glass. The Okasolar system mentioned earlier uses triangular section louvers to block sun and can reflect/redirect sunlight to the interior. Optical efficiency with respect to redirection may be poor since the primary design intent is to diffuse incoming daylight.

Laser-cut panels, developed in Australia, use simple linear horizontal cuts in an acrylic panel to refract light at the juncture of the linear grooves. The angle of refraction is a basic material property, so efficiency is dependent on the frequency and spacing of the grooves and thickness of the panel. For practical purposes, there are limits on panel size and spacing within the insulating glass unit (IGU) due to the high coefficient of expansion of acrylic. View is slightly distorted/impaired and glare is not controlled with this system.

Prismatic acrylic panels (described earlier in Solar Control section) also work on the principle of refraction to redirect incident sunlight. The panels are serrated on one side forming prisms or sawtooth linear grooves across the face of the panel. The angles of two sides of the prism are engineered to block certain angles of sunlight and refract and transmit others. For some designs, one or both surfaces of the prism is coated with a high-reflectance aluminum film. The panels should be applied to the exterior of the building and should be adjusted seasonally to compensate for the variation in solar altitude.

In summer, when the sun is high in the sky, lightshelves block direct sun at both the upper and lower windows. In winter, low sun can penetrate to the back to the space through the clerestory, pre-heating occupied space in the morning, and providing light when needed. Tinted glazing can be used at the lower view window, while clear glazing can be used at the clerestory to increase daylight admission.

Above: Laser-cut acrylic panel

Holographic optical elements (HOE) use the principle of diffraction to redirect sunlight. An interference pattern of any specification can be printed/stamped on a transparent film or glass substrate, then laminated between two panes of glass. Diffractive optical efficiency tends to be poor, but may improve as the technology is developed. The HOE technology is in a demonstration phase in Germany.

Sun-directing glass are long, slightly curved sections of glass that are stacked and placed between panes of glass. The refractive index of glass is again combined with geometry to redirect sunlight to the ceiling plane.

In all of the above systems, view is distorted or impaired so placement of such systems above standing view height is typically recommended. With many of the transparent systems, glare is not controlled since the direct sun increases the luminance of the panels well above acceptable limits for most office tasks.

Sky-light redirection 

The second category of light-redirecting systems designed for diffuse sky-light are effective for climates with predominantly cloudy conditions or for urban or other situations where the windows or skylights only "see" the sky. For such systems, the main design objective is to increase interior daylight levels overall with less emphasis on the depth of light redirection. 

Anidolic systems use the principle of non-imaging optics to gather omni-directional diffuse light and guide the flux with mirrored curved geometries. This "focused" daylight can then be redirected along the ceiling plane and distributed via light ducts into the interior. The collector optics are created using plastic injection moulds then coated with a high-grade aluminum coating.

Holographic optical elements (HOE) can also be applied to the redirection of zenithal sky-light. Tilted glass HOE overhangs can be place over north-facing windows so that diffuse daylight is redirected into the building interior. The luminance level of the zenith region of an overcast sky (directly overhead) is typically much higher than horizon-level sky-light, therefore making this a promising strategy. The HOE glazing is still under development.

Question/Information: eslee@lbl.gov