Defining Daylighting From Windows in Terms of Candlepower Distribution Curves

This work describes a method for evaluating quantitatively the daylight admittance of windows under any outdoor conditions in terms that make it possible to calculate interior light distribution. The work is based on a new concept in quantitative daylight analysis, the Transmission Function Approach, developed by the author while preparing graduate thesis (1976 and 1982) [2], [3], and [4].

The visible daylight flux introduced through a window (or other daylight-admitting aperture) can be considered, from the point of view of the internal space, as being emitted from a point source or from a finite-area uniform source. The photometric properties of those light sources are defined in terms of the well-known candlepower distribution curves. The ways in which this approach can be applied for different window designs are demonstrated.

This approach to the photometric properties of window systems allows one to translate typical daylighting calculation problems into a format in which they can be resolved using traditional electric lighting calculations or computer codes. Even daylighted-oriented computer codes are limited as to the geometric complexity of the windows they can model--this method eliminates such limitations. It will also contribute to a better understanding and visualization of the photometric properties of various windows and other daylight-admitting elements. This approach, therefore, may also serve as an educational tool.