Blu-ray discs are the third generation of optical discs intended for the storage of digital data. Data is stored in the spiral-shaped track 0.1 mm below the surface of the disc, with a lateral stop distance of 0.35 micron. Laser light is used to read Blu-ray discs with a wavelength of 405 nm. Technology was developed by the Japanese company Sony, with other firms like Philips being involved also. The name Blue-ray is associated with the color of light used for reading the data.
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As CDs, Blu-ray discs have a diameter of 12 cm, and a smaller 8 cm version with a thickness of 1.2 mm. Blu-ray discs can hold up to an incredible 25 GB on a single-layer, and 50 GB on dual-layer discs, up to 80 GB for double-sided dual-layer variants. With the recording location 0.1 mm below the surface, it is possible to produce a hybrid disc with DVD and Blu-ray recording on one side of the disc. Blu-ray reading devices for are developed with respect to compatibility with CD and DVD, ie. They can read of all three types of discs.
