CBS Newsletter
Fall 1994
pg. 10

The Sky Simulator

Werner Osterhaus and Liliana Beltran prepare a scale model for tests in the sky simulator.

Daylight reduces a building's dependency on electrical lighting and improves the beauty of its interior spaces. As architects apply well-established daylighting techniques such as windows and skylights to capture the natural light, scientists are developing innovative ways to bring in the light more efficiently. The sky simulator, operated by the Center's Building Technologies Program, is a 24-foot-diameter dome that researchers use to measure the illuminance levels in building models fitted with various daylighting systems. By testing these models, they can determine how well a building design or daylighting technology permits light to enter under conditions of varying time of day and season, building orientation, or geographic location.

Inside the dome, 108 fluorescent tubes shine onto the dome ceiling. Sky conditions can be varied by dimming the fluorescent tubes and thereby creating different sky luminance distributions. The fluorescent tubes are dimmable individually or in banks of up to 36 at a time. Models are mounted at the level of the dome's horizon on a raised platform in the center of the dome. The platform is actually a rotating, tilting drafting table mounted on a rigid framework. The sun is simulated with a 1,000-watt halogen lamp inside a parabolic mirror dish mounted on a track. It can assume any altitude angle from the horizon to the zenith. Azimuth angles are set by rotating the model platform. Photosensors connected to a data acquisition system are mounted inside the scale model to be evaluated to measure the illuminance in different parts of the proposed building. At least one reference sensor is usually placed on an unobstructed horizontal plane outside the model for daylight factor measurements. Daylight factor is the ratio of illuminance on a horizontal indoor surface to that of an unobstructed horizontal outdoor surface.

In the facility's interior, Osterhaus and Beltran conduct performance tests of a shading device for a hotel atrium using the sun simulator, upper right.

With the sky simulator, scientists in the Windows and Daylighting Group conduct different types of studies, including shading studies that reveal how a building's design blocks or permits light's passage to the interior; solar access studies, using city-scale models; and tests of the reflection and transmittance characteristics of new daylighting technologies. For scale model tests, the minimum suite of measurements of illuminances are taken at conditions simulating 9 a.m., noon and 3 p.m., measured at the solstices and the equiNOxes. Crescent board, the construction material of most models, has known illuminances, and model makers can choose a board with the color and texture that would be closest to the proposed building's appearance.

Windows and Daylighting Group researchers have used the simulator to validate the results of computer models designed to predict the interior illuminance of buildings lit from outside, and from time to time, professional architects send their building models to be evaluated there. The sky simulator's ability to control and reproduce lighting conditions makes it a useful research tool. Located in the College of Environmental Design on the University of California, Berkeley campus, this LBL facility is also an important teaching tool for students.

—Allan Chen

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Werner Osterhaus
Building Technologies Program
(510) 486-6845; (510) 486-4089 fax

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