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