Blu-ray disc

A key aspect of any novel optical disc format is compatibility with existing, widely used formats. For BD this means that the optical disc drive must be able to read and write BDs as well as DVDs and CDs, i.e. the dual CD/DVD compatibility of present day drives must be enlarged to triple CD/DVD/BD-compatibility. The main obstacle originates from the differences between CD, DVD, and BD in the amount of spherical aberration resulting from focusing through the substrate. This spherical aberration gap can be bridged by adding a compatibility plate to a BD-only objective lens. This plate has a continuous curved surface with intermittent discontinuous steps in the shape of rings