Plasma pre-treatment and coating of PMMA Fresnel lenses

Most Fresnel lenses for projection systems are made from Polymethylmethacrylate (PMMA). Such lenses, e.g. for LCD-projectors, are flat discs whereas one surface is plane and the other microstructured. Special coatings are necessary to reduce stray light and "hot spots" in the projected image, finally. Plasma-Ion Assisted Deposition (Plasma-IAD) has been applied to realize dielectric multilayers on PMMA. Additionally, a low pressure plasma pre-treatment has been carried out as the first step of the coating process which inducing an irreversible modification of surface composition that leads to excellent adhesion conditions for coating. The special problem for coating the microstructured surface is the inclination angle of the small Fresnel facets which increases from the center to the outer area of microstructured lens. Consequently, the vapor incidence angle changes in the same way. Another problem is to keep the temperature at the polymer surfaces low during coating process. Thin microstructured polymer lenses show lower dimensional stability compared to flat or compact samples. Several coating designs have been investigated to get a uniform moderate antireflective effect and in result an optimized light impression for the observer.