In figure 3 the results of the luminance ratios between the window and the transition zone for the different variants is shown. The luminance value of the artificial sky luminance was not constant for the various measurements; therefore the average value of the artificial sky luminance measurements is plotted in figure 3. An other artefact that has been found is that the reference variant without a transition zone is shown to have a larger luminance in the empty transition zone than for the unobstructed artificial sky. The effect is most likely caused by a non-linearity of the simulated overcast sky. This effect can also be seen in some other variants where the second point on the transition zone has a higher luminance than expected, such as the variant with the dots on the glass or the variant with the coloured glass. Ignoring this effect, all non-empty variants have a gradual decrease in luminance values from the artificial sky to the wall.

Figure 3. Luminance distribution on the wall for the various variants given in Table 2.

Combining the results of figure 2 and figure 3, the luminance ratios between the artificial sky and the transition region, the luminance ratios between the transition region and the wall and the luminance ratios between the artificial sky and wall can be calculated. This has been done and the results are shown in table 3. Considering the required luminance contrast ratio of 1:20 between the artificial sky and the transition region and between the transition region and the wall, only the net curtain can fulfil this requirement.

In retrospect this should not come as a surprise. Curtains have been used extensively in the home environment and not only as a decorative addition to the window, but as a
luminance regulation, as we have shown, as well. And as a third positive point, the curtain can also be used as a privacy screen by applying the curtain over the whole window. However, for other functions of a space, such as offices, schools etc., a curtain is not thought to be suitable.