Blu ray Disc (DOC)

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					?Blu-ray Disc (also known as Blu-ray or BD) is an optical disc storage media format.
It is mainly used in high-definition video and data storage. Blu- ray Disc has the same
physical dimensions as a standard DVD or CD.

A single-Layer Blu-ray has storage capacity of 25GB, while a dual- layer BD can store
up to 50GB of data. As the name suggests, a blue-violet laser is used to write data on a
Blu-ray disk, unlike the traditional method wherein red laser employed to store data
on DVDs.

A Blu-ray disk can hold 9-hour high definition video and standard-definition (SD)
video that can run 23 hours, on a 50GB disk. The BD-ROM movies will require a rate
of MBPS for data transfer, so the expected speed is 2x (72 Mbps). There is also a
scope for having much higher speed because of the larger numerical aperture (NA)
adopted by the BD. It implies that a Blu-ray disk will need less recording power and
lower disk rotation speed vis-à-vis conventional DVDs and HD DVDs. The sole
limiting factor for blu-ray is the capacity of the hardware.

The storage capacity of Blu-ray disk (BDs) is five times that of conventional DVDs.
BDs supports NPEG-2, MPEG-4 AVC and SMPTE VC-1 formats (codecs).

Blu-ray Disc uses a "blue" (technically violet) laser operating at a wavelength of 405
nm to read and write data. Conventional DVDs and CDs use red and near infrared
lasers at 650 nm and 780 nm respectively.

The blue-violet laser's shorter wavelength makes it possible to store more information
on a 12 cm CD/DVD sized disc. The minimum "spot size" on which a laser can be
focused is limited by diffraction, and depends on the wavelength of the light and the
numerical aperture of the lens used to focus it. By decreasing the wavelength,
increasing the numerical aperture from 0.60 to 0.85 and making the cover layer
thinner to avoid unwanted optical effects, the laser beam can be focused to a smaller
spot. This allows more information to be stored in the same area. For Blu-ray Disc,
the spot size is 580 nm In addition to the optical improvements; Blu-ray Discs feature
improvements in data encoding that further increase the capacity.


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