Initially, optical discs were used to store music and computer software. The laser disc format stored analog video signals, but commercially lost to the VHS videotape cassette, due mainly to its high cost and non-re-recordability; other first-generation disc formats were designed only to store digital data and were not initially capable of use as a video medium.
Most first-generation disc devices had an infrared laser reading head. The minimum size of the laser spot is proportional to its wavelength, thus wavelength is a limiting factor against great information density, too little data can be stored so. The infrared range is beyond the long-wavelength end of the visible light spectrum, so, supports less density than any visible light colour. One example of high-density data storage capacity, achieved with an infrared laser, is 700MB of net user data for a 12 cm compact disc.
Second-generation
Second-generation optical discs were for storing great amounts of data, including broadcast-quality digital video. Such discs usually are read with a visible-light laser (usually red); the shorter wavelength and greater numerical aperture[2] allow a narrower light beam, permitting smaller pits and lands in the disc. In the DVD format, this allows 4.7 GB storage on a standard 12 cm, single-sided, single-layer disc; alternately, smaller media, such as the MiniDisc and the DataPlay formats, can have capacity comparable to that of the larger, standard compact 12 cm disc.
Third-generation
Third-generation optical discs are in development, meant for distributing high-definition video and support greater data storage capacities, accomplished with short-wavelength visible-light lasers and greater numerical apertures. The Blu-ray disc uses blue-violet lasers and focusing optics of greater aperture, for use with discs with smaller pits and lands, thereby greater data storage capacity per layer. In practice, the effective multimedia presentation capacity is improved with enhanced video data compression codecs such as H.264, and VC-1.
Fourth-generation
That is for now, the following formats go beyond the current third-generation discs and have the potential to hold more than one terabyte (1TB) of data: Holographic Versatile Disc, LS-R and Protein-coated disc
FROM: http://en.wikipedia.org/wiki/Optical_disc
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