 It was announced on April 15, 2004.The basic structure of Sony's paper disc is similar to that of a Blu-ray Disc in that the recording layer on which the data is stored lies under a 0.1 millimeter protective layer and on top of a 1.1 millimeter substrate.In a Blu-ray Disc, this substrate is made of polycarbonate plastic, which is replaced with paper in a paper disc.