Blu-ray Disc

In February 2002, nine of the leading optical storage companies announced the initial specifications for Blu-ray Disc, a high-capacity CD/DVD-type optical disc format.

Blu-ray is a fully rewritable format that enables recording up to 27GB of data or up to 2 hours of high-definition video on a single-sided, single-layer 12cm diamater disc using a 405nm blue-violet laser. Blu-ray discs will be contained in a cartridge that protects them from dust and fingerprints.

Licensing for the technology began in February 2003, with the first products due in late 2003 or early 2004. The companies who created and license the Blu-ray Disc specification include:

  • Hitachi, Ltd.

  • LG Electronics, Inc.

  • Matsushita Electric Industrial Co., Ltd.

  • Pioneer Corporation

  • Royal Philips Electronics

  • Samsung Electronics Co., Ltd.

  • Sharp Corporation

  • Sony Corporation

  • Thomson Multimedia

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 disc. As with DVD, Blu-ray uses the industry-standard MPEG-2 compression technology.

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 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:

  • Recording capacity - 23.3GB/25GB/27GB

  • Laser wavelength - 405nm (blue-violet)

  • Lens numerical aperture - 0.85

  • Cartridge dimensions - Approximately 129x131x7mm

  • Disc diameter - 120mm

  • Disc thickness - 1.2mm

  • Optical protection layer - 0.1mm

  • Tracking pitch - 0.32um

  • Shortest pit length - 0.160/0.149/0.138um

  • Recording density - 16.8/18.0/19.5Gb/sq. in.

  • Data transfer rate - 36Mbps

  • Recording format - Phase change recording

  • Tracking format - Groove recording

  • Video format - MPEG2

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 Web site at