In modern architecture, highly transparent facades and increasing daylight utilisation are very common. A high energy efficiency as well as thermal and visual comfort are strong requirements that can be fulfilled only with a sophisticated use of the solar irradiation. Microstructured light-guiding systems have the ability to guide or reflect the light incident under certain angles and are therefore a way to improve daylighting and solar control. Interference lithography1 offers the possibility to generate these microstructures in large scale. Another approach for solar control glazing are switchchable glazings like

gaschromic4 or thermotropic windows. It is possible to combine both concepts. The Fraunhofer ISE is focusing on two general types of microstructures for light guiding and sun shading systems2 The first concept, shown in figure 1, is a prismatic array where prisms and coplanar areas are alternating. In between two critical angles of incidence, most of the light entering the structure will be reflected due to total internal reflection at the lower face and the backside of the structure. The structure uses the effect of self shadowing of the coplanar areas for increasing incidence angles. This allows an outside view for coplanar sections and solar-control function of the prism elements. The critical angles are depending on some parameters such as the geometry of the prisms, the aspect ratio of the prism-section and the coplanar section and the index of refraction. These parameters can be chosen in a way that one can achieve a minimum of transmittance for directions that are corresponding to the incidence angles of the summer sun on a vertical south facade. There are several sets of parameters suitable for this requirements. Samples and prototype glazings have been manufactured for two different types of microsprism arrays for the use as sunshading devices in glazings.

The second concept is based on an array of one dimensional Compound Parabolic Concentrators (CPCs)3. CPCs focus radiation incident between certain aperture angles onto the lower exit of the structure. If this aperture is covered by a reflector, the element reflects all radiation incident between the aperture angles. Incident radiation at angles larger than the critical aperture angles is mostly transmitted. Because the rejection angles of CPCs are in the vicinity of the surface normal of the structured sheet, they are best suitable for tilted/tiltable applications like roof windows or venetian blinds. Figure 2 shows a replication of a CPC array of period approx. 9 pm with the tips (exit aperture) area- selectively coated with a metallic film.