Capacity of Blu-ray discs with a diameter of 12 cm
Capacity of mini Blu-ray discs (8 cm)
In one-layer Blu-ray disc (BD-ROM), polycarbonate is metallized with reflective layer (aluminium or silver alloy). That layer is coated with a cover layer (polycarbonate film, silicon, etc.) with a final addition of a solid transparent protective layer composed of resin (oxide silica), silicone, etc.
The structure of the two-layer Blu-ray discs is more complicated. The first layer is a substrate with pressed-in tracks (data layer 0), a reflective layer (silver alloy), a spacer layer (pressure sensitive adhesive). Second layer with pressed-in tracks (data layer 1) which is covered by a semi-reflective layer of silver alloy. Finally, a coating layer is applied and then a hard protective layer of resin or silicone etc.
With its structure, Blu-ray disc is more susceptible to scratches, visibility of fingerprints and dust catching. To prevent that the disc is coated with either a silicon layer or more often with a 2-5 μm transparent protective resin layer (silicon dioxide).