What is Blu Ray Disc

In February 2002, the leading optical storage companies formed the Blu-ray Disc Founders (BDF) and announced the initial specifications for Blu-ray Disc, a high-capacity CD/DVD-type optical disc format. By May 2002, the Blu-ray Disc (BD) specification 1.0 was released, and in April 2003, Sony released the BDZ-S77 for the Japanese market, the first commercially available BD recorder. In January 2006, the Blu-ray Disc Association also released a 2.0 specification for BD-RE discs. Blu-ray is a fully rewritable format that enables recording up to 25GB of data or up to 2 hours of high-definition video on a single-sided, single-layer 12cm diameter disc (which is the same as existing CDs and DVDs) using a 405nm blue-violet laser. Dual-layer BD recorders are also being developed and will record up to 50GB. Although backward compatibility with DVD and CD is not a requirement of the Blu-ray specification, it is a feature drive manufacturers can easily include. In fact, all the Blu-ray drives announced so far have full backward compatibility with DVD and CD formats.

One of the main application for higher-capacity optical storage is recording high-definition TV, which takes an incredible amount of storage. Current DVD recorders can't store enough data to handle high-definition video. Blu-ray, on the other hand, is designed to store up to 2 hours of high-definition video (or more than 13 hours of standard broadcast-quality TV) on a single-layer disc, or double that when dual-layer versions become available. As with DVD, Blu-ray uses the industry-standard MPEG-2 compression technology.

The Blu-ray Disc 1.0 specification includes the following formats:

• BD-ROM. Read-only for prerecorded content

• BD-R. Recordable for PC data storage

• BD-RW. Rewritable for PC data storage

As of January 2006, only the BD-RE format, which is a rewritable format for HDTV recording, had been revised. The current specification is BD-RE 2.0.

Standard CDs use a 780nm (infrared) laser combined with a 0.45 numerical aperture lens, whereas DVDs use a 650nm (red) laser combined with a 0.60 numerical aperture lens. Blu-ray uses a much shorter 405nm (blue-violet) laser with a 0.85 numerical aperture lens. Numerical aperture is a measurement of the light-gathering capability of a lens, as well as the focal length and relative magnification. The numerical aperture of a lens is derived by taking the sine of the maximum angle of light entering the lens. For example, the lens in a CD-ROM drive gathers light at up to a 26.7° angle, which results in a numerical aperture of SIN(26.7) = 0.45. By comparison, the lens in a DVD drive gathers light at up to a 36.9° angle, resulting in a numerical aperture of SIN(36.9) = 0.60. Blu-ray drives gather light at up to a 58.2° angle, resulting in a numerical aperture of SIN(58.2) = 0.85. Higher numerical apertures allow increasingly oblique (angled) rays of light to enter the lens and therefore produce a more highly resolved image.

The higher the aperture, the shorter the focal length and the greater the magnification. The lens in a CD-ROM drive magnifies roughly 20 times, whereas the lens in a DVD drive magnifies about 40 times. The Blu-ray lens magnifies about 60 times. This greater magnification is necessary because the distance between tracks on a Blu-ray disc is reduced to 0.32um, which is almost half of that of a regular DVD. Because of the very high densities involved, a simple cartridge is normally used to hold the disc, which prevents it from being impaired by dust, fingerprints, or scratches.

The most important features of a Blu-ray disc are summarized:

Blu-ray Disc Specifications

Specification	Value	Specification	Value
Capacity (single-layer)	23.3GB/25GB/27GB	Tracking pitch	0.32um
Capacity (dual-layer)	46.6GB/50GB/54GB	Shortest pit length	0.160/0.149/0.138um
Laser wavelength	405nm (blue-violet)	Recording density	16.8/18.0/19.5Gb/sq. in.
Lens numerical aperture	0.85	Data transfer rate	36Mbps
Cartridge dimensions	Approximately 129x131x7mm	Recording format	Phase change recording
Disc diameter	120mm	Tracking format	Groove recording
Disc thickness	1.2mm	Video format	MPEG2
Optical protection layer	0.1mm

Blu-ray, or perhaps some other blue-laser disc drive, will eventually become the replacement for today's DVD drives. For more information about Blu-ray, see the official licensing website at www.blu-raydisc.info/, as well as the unofficial Blue-ray site at www.blu-ray.com.