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 applications 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, 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.