One type of switchable mirrors as they are researched at the Fraunhofer ISE consist of a layer of a Mg and Ni mixture5, where both elements are evaporated or sputtered at the same time. A second layer of Palladium (Pl) is needed as a catalyst for the hydrogen — activated switching process. This multiple layer is obviously much more complex than a single layer. Therefore, it has to be proved that it is also possible to attach a switchable mirror face selectively. Already showed above in figure 7 and figure 8, is a SEM image of an applied switchable mirror coating. The edges of the coating are sharp and the geometry is as expected, but in this way it is not possible to distinguish whether the structure of the layer itself is suitable for optical switching. In Table 1, the results of some measurements of the hemispherical transmission of the structure shown in figure 7 for several incidence angles in both states of the switchable mirror are presented. The effect of switching from the reflecting to the clear state reaches a maximum for high incidence angles, while for small angles the effect is small. This is the result one would expect when using this arrangement. Unfortunately, on high incident angles the measurement error was increasing due to the small available sample size and the available measurement setup itself. Therefore, this measurements can only give a qualitative idea of the possible results, but they are still suitable to demonstrate the feasibility of the concept. As mentioned above, the use of switchable mirrors need a new structure. The optical properties of the switchable mirrors, e. g. the minimisation of the absorptance in its clear state, are still subject for further research, and they are changing for different samples. So we had to work with varying parameters for the simulation of the coating. Figure 11 shows a first concept of a controllable light shading structure using switchable mirrors. The parabolic design was chosen because this should be easy to realise on a large scale with the existing technology of interference lithography. The coating is attached to the area marked with bold lines. The function of the structure is shown schematically. Light of low incident angles will pass through the structure in both states of the coating. For large incident angles in the reflective-state the light will be reflected, while for the transparent state most light will pass through. Therefore one can

control the intensity of the direct sun, with this system while diffuse light will still be transmitted. Figure 12 shows the result of a raytracing simulation of this design. The calculated hemispherical transmission as a function of the incidence angle is plotted. The solid line represents the structure with the coating in its mirror-state. For this first analysis, a constant reflectance of 0.9 and an absorptance of 0.1 has been assumed. Further work will include the exact optical properties, but these values proved to be a good estimation. By changing the structure parameters like the aspect ratio as well as the ratio of the coated area to the non-coated area, the angle of incidence, where the transmittance reaches its minimum can be adjusted. The dotted lines are the upper and lower limits for the transparent state, depending on the coating parameters that are to be expected. The upper line represents the transmission of the non-coated structure, i. e. zero absorptance of the mirror in the clear state, whereas the lower line was calculated with a coating of 50% transmittance in the clear state. It is estimated that a transmittance of more than 60% is reachable. The shown structure still has big potential for optimising e. g. to increase the transmission in the clear state. Not only the coating itself can reach a higher transmittance, but it should be also possible to increase the transmittance of the non-coated structure e. g. by changing the index of refraction and optimisation of the geometry. For this purpose detailed parameter studies will be carried out in future works. Beside the optimisation of the transmittance also several applications with different cut-off angles will be studied