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Sky Component
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Introduction#
The sky component analysis measures the ratio of direct illuminance from a sky between a point on an unobstructed horizontal surface and an analysis point. It can be run with different sky types but most common is the CIE Overcast sky. The analysis is called Vertical Sky Component (VSC) when run with an analysis point sampled on a vertical surface using the CIE Standard Overcast sky.
Warning
This analysis only considers the direct relationship between the analysis point and the sky. If reflections and transparent materials need to be considered then the daylight factor is better suited.
The sky component can be measured on windows in the facade of a building in order to determine the potential of using daylight to light its interior.
Set Up#
The analysis can run either using a temporary scene or a static scene where geometry has been added prior to running the analysis.
Geometry Group#
The sky component analysis requires at least one geometry group in order to run. It either has to be supplied through the scene objects parameter, if the analysis is running on a temporary scene, or added to a static scene prior to running the analysis.
Analysis Points#
The analysis points specify both the location and the orientation of where the sky component should be evaluated. An analysis point is usually created on a surface using the normal of the surface as the analysis point direction.
Sky Type#
The sky type specifies which luminance distribution should be used. There are three different sky types available, two overcast and one uniform. For a detailed breakdown of the equations behind them, Andrew Marsh provides an excellent overview in his article Sky Distribution Equations.
CIE Standard Overcast Sky#
Idealized sky condition with fixed luminance and peak brightness towards zenith. Based on the CIE standard published in 2003.
CIE Standard Uniform Sky#
Idealized sky condition with fixed uniform luminance across the hemisphere.
CIE Traditional Overcast Sky#
Idealized sky condition with fixed luminance and peak brightness towards zenith. Based on the CIE standard published in 1996. This sky type is included for backwards compatibility and is common in other daylighting softwares.
Visibility Mask#
The visibility mask can be used to exclude geometry groups from the analysis. This can be useful when the same scene is used for multiple analyses that require different geometric compositions.
Results#
The analysis will output a sky component ratio for each analysis point. The ratio can span from 0 for a fully obstructed point to 1 for an unobstructed horizontal point. The UK Building Research Establishment (BRE) has specified guidelines for the vertical sky component where a ratio of 0.27 is recommended for good potential to light a space using daylight.
