By surface structures transmission angle /° Fig. 3: relative transmittance as a function of the transmission angle of two similar microprism structures shown in figure 4 (17 pm) and in figure 8 (100 pm). Normal incidence, prisms facing the light source. Negative angle of transmission is direction below horizon Diffraction and refraction

When using structures with small periods, visual effects due to diffraction and refraction can be very distractive for the use in glazings, even if the overall function of the system in terms of total transmittance may not be strongly affected by miniaturisation of the structures. To study period dependent effects and to find an optimal feature size, we created two structures comparable to those in figure 1 with a period of 100 pm and a period of 17 pm. As shown below, there are slight differences of the structures due to the production process (see figure 4 and figure 8), but it is still possible to discuss the effects of the different feature sizes qualitatively. In figure 3, the measured relative intensity of white light as a function of the angle of transmission for
normal incidence is shown. For the large period (100 pm) , two narrow, high peaks were measured. One, at 0°, represents light directly transmitted through the coplanar areas and the other, at approx. -25°, light refracted by the prisms. For the use in a glazing, as a consequence this means that the user would see two different sharp images of comparable intensity at one position. For the small period the image based on refraction is strongly reduced in maximum intensity. It is also much more diffuse, because now there are several separated diffraction orders the intensity is divided into. The different position of this image is caused by slightly different prism angles. On the other hand, the first orders of diffraction are clearly separated from the central peak (zero order). For the use in a glazing, this means that the image generated by the refracted light can hardly be seen, but instead there will be clearly separated diffraction orders. This means that beside the main image one will see at least two colourful images close by, which are much less in intensity compared to the refracted image, but that are still distracting because they cause the main image to be blurred. There are several options to reduce these effects. The diffractive effects are most effectively reduced by an increase of the feature size. For refractive effects, one option is a face-selective absorbing coating, which decreases the intensity of the refracted image while the intensity of the direct image is unaffected. Tests showed that an intensity reduction of 50% already provides a good subjective visual improvement.