Our proposed model, the Angular Variation Model is described elsewhere [4]. It is a semi­empirical model, where the angular variation of the solar absorptance in many windows and window panes was studied. Nine different window groups were discerned. Within each group, the panes (1,2 or three) in every window more or less followed the same angular variation pattern. These patterns were approximated with polynomial fits.

In this report, three different models for approximating the angular variation fo the absorptance have been studied. Figure 3 shows the angular variation of the three models. The horizontal line shows the approximation that the absorptance of a pane is the same for all incidence angles as for normal incidence. The thick dashed line shows the approximation that all panes have the same angular variation as a single clear 4 mm clear float glass. The vague, dotted lines show the seventeen approximations used in the Angular Variation Model. To know which approximation to use for each pane, one has to know to which out of nine window groups the window pertains.

Figure 3 Normalised solar absorptance versus angle of incidence, the “angular profile" of the 17 different pane categories in the Angular Variation Model (thin curves), of a single 4 mm clear float glass window (thick, dashed curve) and a hypothetical pane that has exactly the same optical properties in all directions (thick, full horizontal line).