SERVICES: DAYLIGHTING

The Daylighting Lab has a mirror-box overcast sky and heliodon sun simulators, and digital photographic and light flux metering equipment for the analysis of physical models at all stages of the design process.

If you plan to build a daylighting model for study in the lab, please see our Resources page.

Overcast Sky Box

Model in Overcast Simulator with Photocells

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 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.

Model on the Heliodon

Interior of a model on the Heliodon showing direct sun angles

The Heliodon
The heliodon is used to examine how the direct rays of the sun interact with an architect's building design. It is comprised of a tilting/rotating table (the earth) and a stationary 1000 watt theatrical light source (the sun). The table can be adjusted to represent the latitude, tilted to simulate any month of the year, and rotated to analyze any time of day.

When a physical model is tested on the heliodon it is typically rotated through the sun angles present on June 21st, September 21st, and December 21st. This yields the sun pattern data for the highest, average, and lowest sun angles for the chosen location.

Our primary tool for recording these tests is a digital video camera. When mounted to a model, it will record the movement of direct sun patterns entering the space throughout the day. Typically these studies seek to examine shading devices that eliminate direct sun from areas where visual tasks are critical. Direct sun can cause problems of heat gain and debilitating glare. The heliodon takes the guesswork out of complex sun-angle geometry and often will provide surprising results.