It's just a big box, right?
 Light bank to turn on lights and fan
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The artificial sky constructed at CERES follows the principles of the infinite inter-reflection of the four mirrored surfaces to create the illusion of a larger testing chamber and to diminish the apparent wall brightness from top of wall to horizon line. The brightness ratio of the ceiling to wall surfaces is comparable to the zenith to horizon ratio of the overcast conditions being emulated. As with all mirror box devices, there is some degree of fall off of diffusion of light at the corners of the mirror and at the junction between the mirrors and the luminous ceiling of the chamber. These errors fall well within acceptable limits for purposes of building design testing. The challenge is to assure that the more general, mid-mirror, brightness gradients from the zenith to the horizon track the accepted CIE (Commission Internationale de L'Eclairage) brightness distribution curve for a CIE-defined (typical) overcast sky. Calibration testing has confirmed that the CERES Mirror Box Artificial Sky exhibits conformance to this brightness distribution curve that is consistent with other mirror box artificial skies in use around the United States.
 Inside the mirror box artificial sky
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In the case of the CERES artificial sky, there are two lampgroups used to illuminate the interior space. When both banks of lamps are on, the chamber produces approximately 1600-2000 footcandles of illumination at the testing table surface. This illumination level is higher than for a normal overcast day. This high value is used to assure the accurate readout of the instrumentation package by avoiding the low-end of the operational range of the sensors and measurement equipment. This approach is possible because all measurements within the artificial sky are taken as Daylight Factor rather than actual footcandles. Measuring actual footcandle levels would result in abnormally high results and would also limit the results in representing only a single (unusually bright) overcast sky condition. As has been discussed. Daylight Factor represents a ratio of interior illumination relative to what is available outdoors and is expressed as a percentage. Under uniform overcast sky conditions this relationship holds constant regardless of the overall level of illumination available. This allows the illumination level of the test conditions to be artificially increased to suit the measurement equipment without affecting the percentage relationship between interior lighting and exterior available daylighting. It also accounts for minor variations in the artificial sky's lamp output as temperature and other conditions vary. Due to these issues, direct illumination measurements from the Mirror Box Artificial Sky should NOT be used.