An optical data storage system uses an optical medium made of a stack of spaced-apart optical disks. Each disk in the stack is made of light-transmissive material, such as polycarbonate when the system is a CD-ROM system, and has a partially light-transmissive data layer on at least one of its faces....http://www.google.de/patents/US5373499?utm_source=gb-gplus-sharePatent US5373499 - Multilayer optical disk and system having multiple optical paths include identical total disk substrate thickness
