As mentioned before, there are three kinds of coatings that will be discussed here: partially transparent absorbing coatings, opaque reflective coatings and non-static, optically switchable coatings that change their optical properties due to a change in a control parameter or ambient condition.

Absorbing coatings as described above can inhibit unwanted refractive effects of micro prisms. Reflective coatings offer the same option as absorbing coatings and they are additionally capable to improve or completely change the light shading abilities. Switchable coatings like gaschromic switchable WO34 films can be applied to the structures to offer new actively controllable features to the sunshading-system. Switchable mirrors that change from a transparent to a reflective state are capable to completely change the behavior of sun-shading devices.5 For certain structures, this can be achieved without disturbing or prohibiting the direct view through the glazing. As mentioned above, one easy option for reducing the refractive effect of prism arrays is a semitransparent coating on the prism-face responsible for the refracted image. This will reduce the refracted image without disturbing the direct view, as can be seen in figure 9. The graph is similar to figure 3 with some slight differences. In figure 9, the incident light hits the surface at an angle of 35° with respect to the sample normal. Therefore, the peak representing the direct image is located at -35° and the peak representing the refracted image at about -60°. In the bold curve, the prism surface facing the sun is coated with a Ni layer with transmittance of about 25%. The refracted image is reduced to approximately one fourth as expected, while the direct image is nearly unchanged. Of course, the hemispherical transmission is also reduced and especially for small angles, this could be an unwanted effect. The advantage of absorbing coatings in comparison to reflective coating is that the light guiding properties of the system stay unchanged and therefore can be easily used with existing designs.

If light absorption is unwanted, reflecting coatings can be used. However, the properties and effects of reflective coatings have to be considered already in the design process. In

these first tests, the coatings have been applied to existing structures to study the effects. Optimising the structure for use with face selective mirrors will be the next step. Below, first results of simulations are presented. A good example for the effects of a face selective mirror is shown in figure 10. In this case, the structure shown in figure 4 is coated with reflective Al coating. In contrary to figure 4, the coating was applied to the lower prism face oriented nearly like the normal of the sample. In figure 10, the measured hemispherical transmission for changing incidence angles is shown for the structure with and without a coating. For small angles, the reduction of transmission is small, because the mirror is nearly parallel to the incident light. For large angles, where for unchanged structure the transmittance of the light is increasing again, the coated prisms still reflect most of the incident light. Thus, a coating is not only capable of reducing distracting visual effects, but it becomes a new free parameter for designing microprism arrays for light shading applications.