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Model In Overcast Sky Above (Actual Space Below) Courtesy BOORA Architects, Heinz Rudolf; Principal-in-Charge
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the overcast sky simulator Testing for the overcast condition occurs in a mirror-box artificial sky. It conforms to the "International Overcast Sky" standard of being three times brighter at the zenith (directly overhead) than it is at the horizon. The mirror-box overcast sky simulates a dome of light that provides diffuse light equally from all sides. Note that a patch of overcast sky is up to 10 times brighter than a section of clear blue sky. Our primary method of testing design decisions in the overcast sky is through photography. This allows us to examine the perceptual quality of a space, the feeling of brightness (diffuse light on vertical surfaces and ceilings), and to ensure that a balanced luminous environment (from perimeter to deep interior) is created. Photocells (pictured above, left) are used to measure the percentage of available daylight (Daylight Factor) entering a space. A "control cell" is placed on top of the model oriented toward the zenith to measure the amount of available daylight. Inside the model, photocells are placed in desired locations to measure the amount of light reaching the interior. The interior readings are then divided by the value of the exterior reading, giving us to the Daylight Factor (percentage of outdoor illumination indoors). These numbers are used as a rough measure of the daylighting design performance as well as for LEED certification. Overcast sky light is ideal for providing gentle, diffuse daylight to building occupants. A well daylit building will maintain reasonably consistent daylight factor readings throughout the room, while the actual illumination varies with the brightness of the sky outside.
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