For an ideal antireflective effect, the surface of the transparent cover should have a gradi­ent of the index of refraction ranging from 1.0 for air to the index of refraction of the cover material. For the required very small indices of refraction no materials exist in nature. A solution is given by subwavelength surface-relief gratings with a continuous profile which form an effective refractive index gradient. This type of anti-reflective surface-relief grating is called "moth-eye” structure according to the example found in nature on the cornea of night-flying moths [17]. In Fig. 4 an artificial "moth-eye” structure made in photoresist with periods of 250 nm is shown. By replicating such types of antireflective surfaces into poly­mers or sol-gel films on glass, the transmittance of films or sheets can be increased sig­nificantly in the region of the solar spectrum (Fig. 5). While replication in polymers is a well established process, the replication in sol-gel films resulting in purely inorganic micro­structures is still under development. The sol-gel technology is especially interesting in the case of antireflective surfaces which are exposed directly to the incident solar radiation where polymer microstructures are not sufficiently durable.

Micro-structuring surfaces allows are very variable modification of the optical properties of the surfaces. Many solutions for optical components in solar applications are given by this approach in principle. The major difficulty in realisation is the big mismatch between the dimension of the functional structure and the area which has to be homogeneously struc­tured. Thus, the challenges is still to develop suitable structure origination and replication techniques. It has been shown that interference lithography is a quite versatile tool to originate the required structures on areas of up to half a square meter so far. Further in­crease of the homogeneously structured area is nevertheless still necessary for some of the applications.