Radiance is a lighting simulation program developed by the Lawrence Berkeley National Laboratory [1]. It is a backwards ray-tracing program which can give calculations and a visualisation of illuminance and luminance values. The desktop version of Radiance is a plug-in module that works with computer aided design (CAD) tools.

Calculations in Radiance can be done with a clear sky and with a CIE-overcast sky.

An exposition space is modelled in Radiance. The dimensions of the exposition space are 10 x 10 x 6 m3and of the tube 2 x 2 x 6 m3(fig.5). The materials of the wall, the floor and the ceiling are chosen from the library of Radiance. Calculations are done with a CIE overcast — sky and with a clear sky. The location on earth for the calculations is adjusted to Delft in the Netherlands. Model calculations are done for December, March and June. Figure 6 shows the illuminance values for simulations with a CIE-overcast sky for the 21st of March at 14:00 h. At that moment the illuminance in the free-field situation is 10300 lux, a value which is present during 70 % of the year in the Netherlands. The camera position of figure 6 is placed in a corner of the space, so the picture shows a part of the ceiling, the floor and the four walls. A contour of 150 lux is shown on the wall, which corresponds to a daylight factor of

1.5 %. The contours shown on the floor are of 250 lux, 350 lux and 450 lux, or daylight factors 2.5 %, 3,5 % and 4.5 %. These values are well suited for an exposition space. In summer the illuminance values are most of the time so high that a sunshade will be needed. Figure 7 and 8 show model calculations with a clear sky for 21 March and 21 June. These simulations show the problems of the direct sun: At some places there are lightspots of high illuminance levels, which are in general visually uncomfortable and in any case unsuitable for an exposition space. To avoid the negative effects of direct sunlight we must protect the exposition space against it. Only in winter the direct sunlight is not strong enough to damage the art objects (fig. 9).

To protect against the direct sunlight three possible solutions are investigated: A tube with a shed roof construction (fig. 10), a tube with a light shelf underneath (fig. 11) and a tube with a special ratio of specular and diffuse reflecting materials (fig.12).

Figure 13 shows one of the simulations with a clear sky for a shed roof construction. The window in the shed roof is 2 x 2 m2 and 1 m above ground level. It is possible to exclude the direct sun, but in reallity the tube must be 2 m higher, otherwise people will walk in front of the window. In that case the illuminance levels are too low most of the year.

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fig.5. Model of an underground exposition space.

Materials from Material of the Material of the Material of the Material of the

the Radiance Library: ceiling: White

reflectance 85.77 % specularity 0.00 % walls: Beige Paint 2k216

reflectance 71.00 % specularity 0.00 % floor: Gray

reflectance 21.70 % specularity 0.00 % tube: Luminaire Reflector reflectance 95.00 % specularity 95.00 %

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fig. 6. Model calculations with a CIE sky of the illuminance values for 21 March 14:00 h.

fig. 7. Model calculations with a clear sky of the illuminance values for 21 March 12:00 h.

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fig. 8. Model calculations with a clear sky of the illuminance values for 21 June 12:00 h. fig.9. Model calculations with a clear sky of the illuminance values for 22 December 12:00 h. Figure 14 shows the simulation of the situation with the light shelf. The illuminance values are largely below 150 lux, only the upper parts of the walls show more than 150 lux. This situation is not suitable, the underground space is much too dark. Figure 15 shows a simulation with a complete diffuse reflecting tube and figure 16 with a partly diffuse and a partly specular inner tube material. Different possibilities in the ratio specular/diffuse and the ratio diffuse/specular materials in the tube are varied. The best situation to avoid lightspots with high illuminance values is with a diffuse reflecting upper part in the tube and a specular reflecting lower part. After these model calculations a design concept is developed to vary the ratio diffuse reflecting and specular reflecting material in the tube in order to adapt to the sun situation during the year and to different weather conditions.

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fig.13. Simulaton of the shed roof construction with a clear sky for 21 June at 14:00 h.

fig. 14. Simulation of the underground space with light shelf, clear sky,21 June at 14:00 h.

— 50 ¥ 250 150 fig. 15. Simulation of the space with a tube with complete diffuse reflecting material, with a clear sky for 21 June at 14:00 h.

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fig. 16. Simulation in the case the tube has 2/3 diffuse and 1/3 specular reflecting material, with a clear sky for 21 June at 14:00 h.

2. A DESIGN CHOICE

Different design choices are possible to vary the ratio diffuse reflecting and specular reflecting material in the tube. It is possible to do that, for example, by shifting or rotating mirror panels and diffuse reflecting panels in the tube, but an other solution is chosen in this paper.

fig. 17. The screens inside the tube.

fig. 18. An impression of the tube and the underground space.

The inner material of the tube is supposed to be made of highly specular material. Sunshades or other screens of diffuse reflecting materials are connected to the walls (fig.17). By lowering or lifting the screens it is possible to regulate the ratio diffuse and specular reflecting material in the tube. This concept is worked out by a graduate student. Figure 18 shows an impression of the tube and the underground space.

3. CONCLUSION

Measurements in the daylight chamber and calculations with the lighting simulation program Radiance have shown that in an underground space with one tube in the ceiling the use of daylight is an option. On the walls a daylight factor of about 1.5 % and on the

floor 4.5 % can be reached in case of a diffuse sky. Simulations with a clear sky show that most of the time protection against direct sunlight is necessary to avoid light spots of high illuminances. There are different possibilities to avoid the problems of the direct sunlight. The design concept in this paper is one solution.