A beam generated from a laser light source are reflected by a beam splitter to proceed toward an optical disc. The beam focus onto the optical disc via a selective light-transmitting part installed in front of the optical disc and an objective lens. The beam is totally transmitted through an inner circular...http://www.google.de/patents/US5917800?utm_source=gb-gplus-sharePatent US5917800 - Optical pickup device for reproducing discs of two types with different densities by double beam focuses of different sizes
