Artificial Sky = A Constant Sky. 

domeDomed base sky.
Courtesy of Bartenbach L'chtlabor.

Common to all artificial skies is the ability to create a consistent testing environment. While real sky conditions and illumination levels are constantly changing on a minute-by-minute basis (even under seemingly steady overcast skies), the fixed conditions of  the artificial sky allow apples-to-apples comparisons of design alternatives relative to daylighting performance. When testing under a fixed sky condition (unlike outdoor testing) a designer can be assured that any performance change observed between design alternatives can be attributed to the design's performance rather than changing sky conditions.

skyboxInside the mirror box artificial sky

Artificial sky types vary considerably in their purpose and construction. Many mimic the projected curvature of the sky dome itself by being hemispherical in form. Some also attempt to include the influence of the direct sun component. The newest computer controlled skies attempt to simulate various sky conditions including direct beam sunlight, particular cloud patterns, light color and quality and sky vault brightness distributions. These latter devices are highly calibrated and controlled scientific instruments that can even be used in basic research. Although the mirror box artificial sky is a compromise to that more ideal geometry, complexity and comprehensiveness, it is typically more economical to construct and maintain and is easier to accommodate within an indoor space. The mirror box does an economical and effective job of simulating overcast sky conditions for scale model testing to support building design decisions. This controlled condition testing is considered valuable to building designers as overcast sky conditions are often considered "worst case" conditions for a daylighting design.