Blu-ray Disc

Blu-ray Disc, known in Chinese as 蓝光光盘 and abbreviated as BD, is a high-density optical storage format that uses a blue-violet laser to read and record data.

Nine companies announced the basic specifications in 2002, and the 蓝光光盘 Association, responsible for subsequent development and licensing, was formed in the mid-2000s. The first consumer-grade pre-recorded movies and playback devices entered the market in 2006.

The 蓝光光盘 retains the 120-millimeter diameter and 1.2-millimeter thickness of its predecessor, but uses a blue-violet laser with a wavelength of approximately 405 nanometers and an objective lens with a numerical aperture of 0.85. The smaller focused spot further reduces the spacing between recording marks and tracks. The data layer is located approximately 0.1 millimeters from the reading surface—closer to the surface than on a DVD—so early bare-disc designs relied on hard coatings to resist scratches and contamination; this also allowed Blu-ray to dispense with the protective case used in the initial prototypes. Standard single-layer BDs have a capacity of 25 GB, while dual-layer discs hold 50 GB; these are commonly referred to as BD-25 and BD-50. BDXL extends recordable and rewritable discs to a three-layer 100 GB configuration and offers a four-layer 128 GB structure for write-once discs. Ultra HD Blu-ray uses 50 GB, 66 GB, or 100 GB read-only media; however, as it falls under BD-ROM Version 4, it features different recording densities, video specifications, and playback requirements, so not all 100 GB Blu-ray discs can be classified as a single application format. Media types include factory-pressed BD-ROM, write-once BD-R, and rewritable BD-RE. Early BD-R discs primarily used organic or inorganic recording layers, followed by the emergence of the LTH specification, which employs organic dyes and recording layers with different polarities; LTH requires explicit support from device firmware. BD-RE uses phase-change materials to alter the state of the recording layer. BDXL also requires a compatible optical drive; standard BD devices cannot automatically support multilayer discs simply because they can read 25 GB or 50 GB discs.

Pre-recorded Blu-ray discs typically use the BDMV application format and the UDF 2.5 file system. The primary audio and video data are stored in M2TS files within the BDMV/STREAM directory, while playlists, clip information, menus, subtitles, and interactive applications are managed by other directories and index files. M2TS is based on the MPEG-2 transport stream, with 4 bytes of time information prepended to each 188-byte transport packet, allowing for the multiplexing of video, multiple audio tracks, graphic subtitles, and auxiliary data. Playlists can rearrange different segments, so a program displayed on screen does not necessarily correspond to a single continuous file.

Blu-ray is not synonymous with 1080p or any specific encoding format.

The BD-ROM video specification supports MPEG-2 Video, MPEG-4 AVC/H.264, and SMPTE VC-1. Common high-definition modes include 1920×1080 at 23.976/24p and 50/59.94i, as well as 1280×720 at 50/59.94p; the specification also allows for standard-definition programming. The "Blu-ray" label on a disc merely indicates the medium and application system; it does not guarantee that the content is 1080p, progressive scan, or sourced from a native HD master. All three encoding formats were used in early releases, though AVC later became more common. Audio formats include LPCM, Dolby Digital, DTS, and their subsequent extensions. Dolby TrueHD and DTS-HD Master Audio can losslessly reproduce multichannel PCM, while Dolby Digital Plus and DTS-HD High Resolution Audio use lossy encoding. A player’s specified decoding capabilities, the formats permitted for use on the disc, and whether the device can actually output a bitstream are three distinct factors; connections that do not support extensions may sometimes only transmit the core audio, transcoded audio, or stereo PCM.

Commercial subtitles typically use the Presentation Graphics Stream, which stores fonts, positions, colors, and typography as time-stamped bitmaps. Basic menus can be implemented using HDMV navigation commands, while BD-J uses the Java platform to create pop-up menus, games, network features, and more complex interactions. BonusView defines local add-on features such as picture-in-picture, while BD-Live adds persistent storage and network access; these features require corresponding player configurations and disc content, and are not used in all Blu-ray releases.

Blu-ray 3D uses Multiview Video Coding to store stereoscopic video, where the base view is accessible to standard 2D players, and the dependent view combines with the base view to form the stereoscopic image. Region playback control divides movie BD-ROMs into three regions—A, B, and C—though some discs have no region restrictions. AACS is a common encryption and key management system for pre-recorded content, while BD+ can serve as an additional layer of protection; both region coding and content protection are independent of video resolution and audio formats.

Blu-ray, BDXL, Blu-ray 3D, and Ultra HD Blu-ray are different media formats or application branches within the same family. Standard Blu-ray players cannot read Ultra HD Blu-ray discs, and 3D playback requires corresponding hardware and display connections. Many Blu-ray players are also compatible with DVDs and CDs, but this backward compatibility stems from additional hardware configurations and is not automatically guaranteed by the discs’ identical physical form factors.