Traditional computational models predict daylight illuminance in a space by dividing window surfaces into discrete areas and then calculating the apparent luminance of each window element by multiplying the luminance of the natural light source in a given viewing direction by the window transmittance in that direction. This approach works well for conventional glazing materials but is incapable of modeling commonly used, but complex, window systems such, as those with specular reflective venetian blinds. We describe a new approach that combines measured luminance distributions for complex window systems with a flux transfer calculation within the space. This method resembles the calculation of illuminance from electric light fixtures where the candlepower distribution of the fixtures is measured and used as an input to the calculation. Based on the variable luminance characteristics of the window system, the SUPERLITE program calculates illuminance at the workplane over the entire space. The measurement techniques and mathematical implementation in the SUPERLITE program are described. This approach allows a wide range of complex window and shading systems to be evaluated without continuous changes in the computational program. A special apparatus for measuring the bidirectional transmittance of window systems has been built in conjunction with this approach. Sample results from the program are compared to measurements made in scale models in a sky simulator.

1 aKim, Jong-Jin1 aPapamichael, Konstantinos, M.1 aSelkowitz, Stephen, E.1 aSpitzglas, Mark1 aModest, Michael, F. uhttps://windows.lbl.gov/publications/determining-daylight-illuminance-rooms-having-complex-fenestration-systems01662nas a2200133 4500008004100000050001400041245008500055210006900140300001000209490000600219520116600225100002401391856011301415 1981 eng d aLBL-1259900aDaylighting Calculation for Non-Rectangular Interior Spaces with Shading Devices0 aDaylighting Calculation for NonRectangular Interior Spaces with a69-790 v53 aAn analytical model and a computer code have been developed which calculate the amount of daylight illumination on a working surface inside an arbitrary room, for overcast as well as for clear sky conditions. The room may have windows as well as skylights, with clear glass, diffusing glass, or glass fitted with thin shading devices (such as sheer curtains or shades), as well as overhangs. The shape of the room is not limited to simple, rectangular enclosures, allowing the treatment of L-shaped rooms, A -frame buildings, etc.

The illumination generally consists of three parts: direct sky illumination, illumination from external reflection, and illumination from internal reflection. First, the luminances eminating from surrounding obstructions are determined. Next, illumination traveling through the windows directly to inside walls and working surface is calculated. Finally, interreflection inside the room is taken into account to establish the luminance distributions of inside walls. After determination of all inside and outside luminances, it is a simple matter to calculate illumination and daylight factor for the working surface.

1 aModest, Michael, F. uhttps://windows.lbl.gov/publications/daylighting-calculation-non-rectangular-interior-spaces-shading-devices