From the results it is clear that low luminance ratios (of no more than 1:20) and a gradual decrease in luminance values can only be obtained when a. the material absorbs a certain amount (but not all) of the incoming light, b. the material reflects some of the incoming light onto the wall. This conclusion, however, is not complete. The conclusion should be written in terms of reduced transmission: low luminance ratios (of no more than 1:20) and a gradual decrease in luminance values can only be obtained when a. the material reduces the amount of the incoming light by absorption or reflection and b. the material reflects some of the incoming light onto the wall.

By substituting absorption with transmission, the origin of the main difference between the anidolic element described in earlier work and the materials described in this paper is clarified. This anidolic element was designed for optimal luminance ratios between the wall and the window. That this is indeed the case, has already been shown by van der Voorden et al. [3]. Besides increasing the luminance ratios between the inner facade and the window, the anidolic element can also influence the size of the luminance ratio by applying a smaller or larger anidolic element, see figure 4. A larger element will receive more light and distribute thus more light onto the inner facade.

However, the anidolic element takes up a lot of space, especially the larger ones which have the lower luminance ratios. The size of the element perpendicular to the window is about half the size of the opening along the wall. And the anidolic element in its optimal position is situated in such a way that the end of the element is in the same position as the beginning of the wall next to the window. This means that the occupant of the building can still see the sky directly when the person’s main view is not perpendicular to the window but under a slight angle. In such a situation the visual comfort is immensely decreased. An overlap of the other materials like net curtain and screen in front of the window, prevent the occupant’s direct view of the sky. An overlap of the anidolic element with the wall can create a dark line on the wall next to the window when the focus line of the anidolic element is shifted due to a horizontal shift along the window of the element. If not the
element is moved sideways, but the length of the element is increased, then the bright focus line is obscured by the longer element, thus increasing the low luminance ratios. An overlap of the other materials like net curtain and screen + 1 cm, achieves a larger amount of light on the wall compared with the material which does not overlap, due to the diffuse property of the material and its possible internal reflections.

6. Conclusion

All above results were obtained in a black office with a white wall next to the window. In a grey or white office the luminance ratios are automatically lower due to the contribution of the internal reflections to the amount of light on the walls. For new buildings, it is therefore clear that the window posts must be painted as light as possible, as was already mentioned by [7]. For luminance ratios that are still too high a transition region in the window by adding a pattern to the edges of the windowpane will already improve the visual comfort. If the luminance ratio between the window and the wall is even higher, a translucent, diffuse material positioned in front of the window and in front of a part of the wall will improve the visual comfort. This overlap will also prevent the occupant of the building from viewing the sky directly when the person’s main view is not perpendicular to the window. A coloured material can be applied, but this has the adverse effect that the view outside becomes coloured as well.

The luminance ratios for the investigated materials are always lower than the ones that can be achieved with an anidolic element, whether it is perforated or not, due to the focussing power of the specular element. However, the anidolic element takes up more space, and does not increase visual comfort for every position in a room.

References

1. Velds, M, Assessment of lighting quality in office rooms with daylighting systems, Ph. D. Thesis, 2000, Technical University Delft

2. Bokel, R. M.J., Heijmans B. N., Pel M., Voorden M. van der, Proceedings of the International Building Physics Conference, 2000, Eindhoven, The Netherlands.

3. Voorden M. van der, M. B.C. Aries, R. M.J. Bokel, Development of a contrast-reducing anidolic shaped element, Proceedings of PLEA conference 2002 (Toulouse).

4. Dutch NEN-norm 3078

5. Moore, F. Concepts and practice of architectural daylighting, Van Nostand Reinhold Company Inc., New York. 1985.

6. Osterhaus, W. K.E., S. E. Selkowitz, Background and conceptual plan for conducting post occupance evaluations of interior luminous environments, Energy and Environment Division, Lawrence Berkeley Laboratory, Berkeley, USA, 1992