Optical disc

This article is about digital storage media such as CDs and DVDs. For a similarly-named anatomic feature of the eye, seeoptic disc.

"Optical media" redirects here. For transmission media for light, seeOptical medium.For broader definition, seeoptical storage.

Anoptical discis a flat, usuallydisc-shaped object that stores information in the form of physical variations on its surface that can be read with the aid of a beam of light. Optical discs can be reflective, where the light source and detector are on the same side of the disc, or transmissive, where light shines through the disc to be detected on the other side.

The bottom surface of a 12 cmcompact disc(CD-R), showing characteristiciridescence.

The optical lens of acompact discdrive.

LaserCard made by Drexler Technology Corporation.

Optical discs can store analog information (e.g.Laserdisc), digital information (e.g.DVD), or store both on the same disc (e.g.CD Video).

Their main uses are the distribution of media and data, and long-termarchival.

Design and technology

The encoding material sits atop a thicker substrate (usuallypolycarbonate) that makes up the bulk of the disc and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the entire disc surface and extending from the innermost track to the outermost track.

The data are stored on the disc with alaserorstamping machine,and can be accessed when the data path is illuminated with alaser diodein anoptical disc drivethat spins the disc at speeds of about 200 to 4,000RPMor more, depending on the drive type, disc format, and the distance of the read head from the center of the disc (outer tracks are read at a higher data speed due to higherlinear velocitiesat the sameangular velocities).

Most optical discs exhibit a characteristiciridescenceas a result of thediffraction gratingformed by their grooves.This side of the disc contains the actual data and is typically coated with a transparent material, usuallylacquer.

The reverse side of an optical disc usually has a printed label, sometimes made of paper but often printed or stamped onto the disc itself. Unlike the 31⁄2-inchfloppy disk,most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems. Blu-rays have a coating calleddurabisthat mitigates these problems.

Optical discs are usually between 7.6 and 30 cm (3.0 and 11.8 in) in diameter, with 12 cm (4.7 in) being the most common size. The so-calledprogram areathat contains the data commonly starts 25 millimetres away from the center point.A typical disc is about 1.2 mm (0.047 in) thick, while the track pitch (distance from the center of one track to the center of the next) ranges from 1.6μm(for CDs) to 320nm(forBlu-ray discs).

Recording types

An optical disc is designed to support one of three recording types: read-only (e.g.: CD andCD-ROM), recordable (write-once, e.g.CD-R), or re-recordable (rewritable, e.g.CD-RW). Write-once optical discs commonly have an organic dye (may also be a (Phthalocyanine)Azo dye,mainly used byVerbatim,or an oxonol dye, used byFujifilm) recording layer between the substrate and the reflective layer. Rewritable discs typically contain analloyrecording layer composed of aphase change material,most oftenAgInSbTe,an alloy ofsilver,indium,antimony,andtellurium.Azo dyes were introduced in 1996 and phthalocyanine only began to see wide use in 2002. The type of dye and the material used on the reflective layer on an optical disc may be determined by shining a light through the disc, as different dye and material combinations have different colors.

Blu-ray Disc recordablediscs do not usually use an organic dye recording layer, instead using an inorganic recording layer. Those that do are known as low-to-high (LTH) discs and can be made in existing CD and DVD production lines, but are of lower quality than traditional Blu-ray recordable discs.

Usage

Optical discs are often storedin special casessometimes called jewel cases and are most commonly used fordigital preservation,storing music (e.g. for use in aCD player), video (e.g. for use in aBlu-rayplayer), or data and programs forpersonal computers(PC), as well as offlinehard copydata distribution due to lower per-unit prices than other types of media. TheOptical Storage Technology Association(OSTA) promoted standardizedoptical storageformats.

Libraries and archives enactoptical media preservationprocedures to ensure continued usability in the computer's optical disc drive or corresponding disc player.

File operationsof traditionalmass storagedevices such asflash drives,memory cardsandhard drivescan be simulated using aUDFlive file system.

For computer data backup and physical data transfer, optical discs such as CDs andDVDsare gradually being replaced with faster, smaller solid-state devices, especially theUSB flash drive.This trend is expected to continue as USB flash drives continue to increase in capacity and drop in price.

Additionally, music, movies, games, software and TV shows purchased, shared or streamed over the Internet has significantly reduced the number of audio CDs, video DVDs and Blu-ray discs sold annually. However, audio CDs and Blu-rays are still preferred and bought by some, as a way of supporting their favorite works while getting something tangible in return and also since audio CDs (alongsidevinyl recordsandcassette tapes) contain uncompressed audio without the artifacts introduced bylossy compressionalgorithms likeMP3,and Blu-rays offer better image and sound quality than streaming media, without visible compression artifacts, due to higher bitrates and more available storage space.However, Blu-rays may sometimes betorrentedover the internet, but torrenting may not be an option for some, due to restrictions put in place byISPson legal or copyright grounds, low download speeds or not having enough available storage space, since the content may weigh up to several dozen gigabytes. Blu-rays may be the only option for those looking to play large games without having to download them over an unreliable or slow internet connection, which is the reason why they are still (as of 2020) widely used by gaming consoles, like thePlayStation 4andXbox One X.As of 2020, it is unusual for PC games to be available in a physical format like Blu-ray.

Discs should not have any stickers and should not be stored together with paper; papers must be removed from the jewel case before storage. Discs should be handled by the edges to prevent scratching, with the thumb on the inner edge of the disc. The ISO Standard 18938:2014 is about best optical disc handling techniques. Optical disc cleaning should never be done in a circular pattern, to avoid concentric cirles from forming on the disc. Improper cleaning can scratch the disc. Recordable discs should not be exposed to light for extended periods of time. Optical discs should be stored in dry and cool conditions to increase longevity, with temperatures between -10 and 23 °C, never exceeding 32 °C, and with humidity never falling below 10%, with recommended storage at 20 to 50% of humidity without fluctuations of more than ±10%.

Durability

Optical discs are not vulnerable to water.

Although optical discs are more durable than earlier audio-visual and data storage formats, they are susceptible to environmental and daily-use damage, if handled improperly.

Optical discs are not prone to uncontrollablecatastrophic failuressuch ashead crashes,power surges,or exposure to water likehard disk drivesandflash storage,since optical drives' storage controllers are not tied to optical discs themselves like with hard disk drives andflash memory controllers,and a disc is usually recoverable from a defective optical drive by pushing an unsharp needle into the emergency ejection pinhole, and has no point of immediate water ingress and no integrated circuitry.

Safety

As the media itself only is accessed through a laser beam, no internal control circuitry, it can not contain malicious hardware such as so-calledrubber-duckiesorUSB killers.

Malwareis unable to spread over factory-pressed media, finalized media, or-ROM(read-only memory) drive types whose lasers lack the strength to write data.Malware is conventionally programmed to detect and spread over traditionalmass storagedevices such asflash drives,externalsolid-state drivesandhard disk drives.

History

Main article:History of optical storage media

An earlier analog optical disc recorded in 1935 forLichttonorgel(sampling organ)

Lichttonorgel & optical disc system

The first recorded historical use of an optical disc was in 1884 whenAlexander Graham Bell,Chichester BellandCharles Sumner Tainterrecorded sound on a glass disc using a beam of light.

Optophonie is a very early (1931) example of a recording device using light for both recording and playing back sound signals on a transparent photograph.

An early analogue optical disc system existed in 1935, used on Welte'sLichttonorgelsampling organ.

An early analog optical disc used for video recording was invented byDavid Paul Greggin 1958and patented in the US in 1961 and 1969. This form of optical disc was a very early form of the DVD (U.S. patent 3,430,966). It is of special interest thatU.S. patent 4,893,297,filed 1989, issued 1990, generatedroyalty incomefor Pioneer Corporation's DVA until 2007 —then encompassing the CD,DVD,and Blu-ray systems. In the early 1960s, the Music Corporation of America bought Gregg's patents and his company, Gauss Electrophysics.

American inventorJames T. Russellhas been credited with inventing the first system to record a digital signal on an optical transparent foil that is lit from behind by a high-power halogen lamp. Russell's patent application was first filed in 1966 and he was granted a patent in 1970. Following litigation, Sony and Philips licensed Russell's patents (then held by a Canadian company, Optical Recording Corp.) in the 1980s.

Both Gregg's and Russell's disc are floppy media read in transparent mode, which imposes serious drawbacks. In theNetherlandsin 1969,PhilipsResearchphysicist,Pieter Kramer invented an opticalvideodiscin reflective mode with a protective layer read by a focusedlaserbeamU.S. patent 5,068,846,filed 1972, issued 1991. Kramer's physical format is used in all optical discs. In 1975, Philips and MCA began to work together, and in 1978, commercially much too late, they presented their long-awaitedLaserdiscinAtlanta.MCA delivered the discs and Philips the players. However, the presentation was a commercial failure, and the cooperation ended.

In Japan and the U.S.,Pioneersucceeded with the Laserdisc until the advent of the DVD. In 1979, Philips andSony,in consortium, successfully developed theaudio compact disc.

In 1979, Exxon STAR Systems in Pasadena, CA built a computer controlled WORM drive that utilized thin film coatings of Tellurium and Selenium on a 12 "diameter glass disk. The recording system utilized blue light at 457 nm to record and red light at 632.8 nm to read. STAR Systems was bought by Storage Technology Corporation (STC) in 1981 and moved to Boulder, CO. Development of the WORM technology was continued using 14" diameter aluminum substrates. Beta testing of the disk drives, originally labeled the Laser Storage Drive 2000 (LSD-2000), was only moderately successful. Many of the disks were shipped to RCA Laboratories (now David Sarnoff Research Center) to be used in the Library of Congress archiving efforts. The STC disks utilized a sealed cartridge with an optical window for protectionU.S. patent 4,542,495.

TheCD-ROMformat was developed by Sony andPhilips,introduced in 1984, as an extension ofCompact Disc Digital Audioand adapted to hold any form of digital data. The same year, Sony demonstrated aLaserDiscdata storage format, with a larger data capacity of 3.28 GB.

In the late 1980s and early 1990s, Optex, Inc. of Rockville, MD, built an erasable optical digital video disc systemU.S. patent 5,113,387using Electron Trapping Optical Media (ETOM)U.S. patent 5,128,849.Although this technology was written up in Video Pro Magazine's December 1994 issue promising "the death of the tape", it was never marketed.

In the mid-1990s, a consortium of manufacturers (Sony, Philips,Toshiba,Panasonic) developed the second generation of the optical disc, theDVD.

Magnetic disks found limited applications in storing the data in large amount. So, there was the need of finding some more data storing techniques. As a result, it was found that by using optical means large data storing devices can be made that in turn gave rise to the optical discs. The very first application of this kind was the compact disc (CD), which was used in audio systems.

Sony and Philips developed the first generation of the CDs in the mid-1980s with the complete specifications for these devices. With the help of this kind of technology the possibility of representing the analog signal into digital signal was exploited to a great level. For this purpose, the 16-bit samples of the analog signal were taken at the rate of44,100 samples per second.This sample rate was based on theNyquist rateof 40,000 samples per second required to capture the audible frequency range to 20 kHz without aliasing, with an additional tolerance to allow the use of less-than-perfect analog audio pre-filters to remove any higher frequencies.The first version of the standard allowed up to 75 minutes of music, which required 650 MB of storage.

TheDVDdisc appeared after the CD-ROM had become widespread in society.

The third generation optical disc was developed in 2000–2006 and was introduced as Blu-ray Disc. First movies on Blu-ray Discs were released in June 2006.Blu-ray eventually prevailed in ahigh definition optical disc format warover a competing format, theHD DVD.A standard Blu-ray disc can hold about 25 GB of data, a DVD about 4.7 GB, and a CD about 700 MB.

Comparison of various optical storage media

First-generation

From the start optical discs were used to store broadcast-quality analog video, and later digital media such as music or computer software. TheLaserDiscformat storedanalog videosignals for the distribution ofhome video,but commercially lost to theVHSvideocassetteformat, due mainly to its high cost and non-re-recordability; other first-generation disc formats were designed only to store digital data and were not initially capable of use as adigital videomedium.

Most first-generation disc devices had an infrared laser reading head. The minimum size of the laser spot is proportional to thewavelengthof the laser, so wavelength is a limiting factor upon the amount of information that can be stored in a given physical area on the disc. The infrared range is beyond the long-wavelength end of the visible light spectrum, so it supports less density than shorter-wavelength visible light. One example of high-density data storage capacity, achieved with an infrared laser, is 700 MB of net user data for a 12 cm compact disc.

Other factors that affect data storage density include: the existence of multiple layers of data on the disc, the method of rotation (Constant linear velocity(CLV),Constant angular velocity(CAV), or zoned-CAV), the composition of lands and pits, and how much margin is unused is at the center and the edge of the disc.

Types of Optical Discs:

• Compact disc(CD) and derivatives
  • Audio CD
  • Video CD(VCD)
  • Super Video CD
  • CD Video
  • CD-Interactive
• LaserDisc
• GD-ROM
• Phase-change Dual
• Double Density Compact Disc(DDCD)
• Magneto-optical disc
• MiniDisc(MD)
  • MD Data
• Write Once Read Many(WORM)

Second-generation

Second-generation optical discs were for storing great amounts of data, including broadcast-quality digital video. Such discs usually are read with a visible-light laser (usually red); the shorter wavelength and greaternumerical apertureallow a narrower light beam, permitting smaller pits and lands in the disc. In the DVD format, this allows 4.7 GB storage on a standard 12 cm, single-sided, single-layer disc; alternatively, smaller media, such as theDataPlayformat, can have capacity comparable to that of the larger, standard compact 12 cm disc.

• DVDand derivatives
  • DVD-Audio
  • DualDisc
  • Digital Video Express(DIVX)
  • DVD-RAM
  • DVD±R
• Nintendo GameCube Game Disc(miniDVD derivative)
• Wii Optical Disc(DVD derivative)
• Super Audio CD(SACD)
• Enhanced Versatile Disc
• DataPlay
• Hi-MD
• Universal Media Disc(UMD)
• Ultra Density Optical

Third-generation

Third-generation optical discs are used for distributinghigh-definition videoand videogames and support greater data storage capacities, accomplished with short-wavelength visible-light lasers and greater numerical apertures. Blu-ray Disc and HD DVD uses blue-violet lasers and focusing optics of greater aperture, for use with discs with smaller pits and lands, thereby greater data storage capacity per layer. In practice, the effective multimedia presentation capacity is improved with enhanced videodata compressioncodecssuch asH.264/MPEG-4 AVCandVC-1.

• Blu-rayand derivatives (up to 400 GB - experimental)
  • BD-R and BD-RE
  • High Fidelity Pure Audio
  • AVCHDandAVCREC
  • BDXL and Blu-ray 3D
  • 4K Blu-rayand 8K Blu-ray
• Wii U Optical Disc(25 GB per layer)
• HD DVD(discontinued disc format, up to 51 GB triple layer)
• CBHD(a derivative of the HD DVD format)
• HD VMD
• Professional Disc

Announced but not released:

• Digital Multilayer Disk
• Fluorescent Multilayer Disc
• Forward Versatile Disc

Fourth-generation

The following formats go beyond the current third-generation discs and have the potential to hold more than one terabyte (1TB) of data and at least some are meant for colddata storageindata centers:

• Archival Disc
• Holographic Versatile Disc

Announced but not released:

• LS-R
• Protein-coated disc
• Stacked Volumetric Optical Disc
• 5D DVD
• 3D optical data storage(not a single technology, examples areHyper CD-ROMandFluorescent Multilayer Disc)

In 2004, development of theHolographic Versatile Disc(HVD) commenced, which promised the storage of several terabytes of data per disc. However, development stagnated towards the late 2000s due to lack of funding.

In 2006, it was reported that Japanese researchers developed ultraviolet ray lasers with a wavelength of 210 nanometers, which would enable a higherbitdensity than Blu-ray discs.As of 2022, no updates on that project have been reported.

Folio Photonics is planning to release high-capacity discs in 2024 with the cost of $5 per TB, with a roadmap to $1 per TB, using 80% less power thanHDD.

Overview of optical types

Name	Capacity	Experimental	Years
LaserDisc(LD)	N/A		1971–2007
Write Once Read Many Disk(WORM)	0.2–6.0 GB		1979–1984
Compact disc(CD)	0.7–0.9 GB		1982–present
Electron Trapping Optical Memory (ETOM)	6.0–12.0 GB		1987–1996
MiniDisc(MD)	0.14–1.0 GB		1989–present
Magneto Optical Disc(MOD)	0.1–16.7 GB		1990–present
Digital Versatile Disc(DVD)	4.7–17 GB		1995–present
LIMDOW(Laser Intensity Modulation Direct OverWrite)	2.6 GB	10 GB	1996–present
GD-ROM	1.2 GB		1997–2006
Fluorescent Multilayer Disc		50–140 GB	1998-2003
Versatile Multilayer Disc(VMD)	5–20 GB	100 GB	1999-2010
Hyper CD-ROM	1 PB	100 EB	1999–present
DataPlay	500 MB		1999-2006
Ultra Density Optical(UDO)	30–60 GB		2000–present
Forward Versatile Disc(FVD)	5.4–15 GB		2001–present
Enhanced Versatile Disc(EVD)	DVD		2002-2004
HD DVD	15–51 GB	1 TB	2002-2008
Blu-ray Disc(BD)	25 GB 50 GB		2002–present
BDXL	100 GB, 128 GB	1 TB	2010–present
Professional Disc for Data(PDD)	23 GB		2003-2006
Professional Disc	23–128 GB		2003–present
Digital Multilayer Disk		22-32 GB	2004–2007
Multiplexed Optical Data Storage(MODS-Disc)		250 GB–1 TB	2004–present
Universal Media Disc(UMD)	0.9–1.8 GB		2004–2014
Holographic Versatile Disc(HVD)		6.0 TB	2004–2012
Protein-coated disc (PCD)		50 TB	2005–2006
M-DISC	4.7 GB (DVD format) 25 GB (Blu-ray format) 50 GB (Blu-ray format) 100 GB (BDXLformat)		2009–present
Archival Disc	0.3-1 TB		2014–present
Ultra HD Blu-ray	50 GB 66 GB 100 GB 128 GB		2015–present

Notes

Recordable and writable optical discs

Main article:Optical disc recording technologies

There are numerous formats of opticaldirect to disk recordingdevices on the market, all of which are based on using a laser to change thereflectivityof thedigital recordingmedium in order to duplicate the effects of the pits and lands created when a commercial optical disc is pressed. Formats such asCD-RandDVD-Rare "Write once read many"or write-once, whileCD-RWandDVD-RWare rewritable, more like amagnetic recordinghard disk drive(HDD).

Media technologies vary, for example,M-DISCmedia uses a rock-like layer to retain data for longer than conventional recordable media. While being read-only compatible with existing DVD and Blu-ray drives, M-DISC media can only be written to using a stronger laser specifically made for this purpose, which is built into fewer optical drive models.

Surface error scanning

Error rate measurement on a DVD+R. The error rate is still within a healthy range.

Optical media canpredictivelybe scanned for errors andmedia deteriorationwell before any data becomes unreadable.Optical formats include some redundancy forerror correction,which works until the amount of error exceeds a threshold. A higher rate of errors may indicate deteriorating and/or low quality media, physical damage, an unclean surface and/or media written using a defective optical drive.

Precise error scanning requires access to the raw, uncorrected readout of a disc, which is not always provided by a drive. As a result, support of this functionality varies per optical drive manufacturer and model. On ordinary drives without this functionality, it is possible to still look for unexpected reduction in read speed as an indirect, much less reliable measure.

Error scanning software includesNero DiscSpeed,k-probe,Opti Drive Control(formerly"CD speed 2000") andDVD info ProforWindows,and QPxTool forcross-platform.

Error types

There are different types of error measurements, including so-called"C1","C2"and"CU"errorson CDs,and"PI/PO (parity inner/outer) errors"and the more critical"PI/PO failures"on DVDs.Finer-grain error measurements on CDs supported by very few optical drives are calledE11,E21,E31,E21,E22,E32.

"CU"and"POF"represent uncorrectable errors on data CDs and DVDs respectively, thusdata loss,and can be a result of too many consecutive smaller errors.

Due to the weaker error correction used onAudio CDs(Red Bookstandard) andVideo CDs(White Bookstandard),C2 errorsalready lead to data loss. However, even with C2 errors, the damage is inaudible to some extent.

Blu-raydiscs use so-calledLDC(Long Distance Codes) andBIS(Burst Indication Subcodes) error parameters. According to the developer of theOpti Drive Controlsoftware, a disc can be considered healthy at anLDCerror rate below 13 andBISerror rate below 15.

Optical disc manufacturing

Main article:CD manufacturing

Optical discs are made using replication. This process can be used with all disc types. Recordable discs have pre-recorded vital information, like manufacturer, disc type, maximum read and write speeds, etc. In replication, acleanroomwith yellow light is necessary to protect the light-sensitive photoresist and to prevent dust from corrupting the data on the disc.

A glass master is used in replication. The master is placed in a machine that cleans it as much as possible using a rotating brush and deionized water, preparing it for the next step. In the next step, a surface analyzer inspects the cleanliness of the master before photoresist is applied on the master.

The photoresist is then baked in an oven to solidify it. Then, in the exposure process, the master is placed in a turntable where a laser selectively exposes the resist to light. At the same time, a developer and deionized water are applied to the disc to remove the exposed resist. This process forms the pits and lands that represent the data on the disc.

A thin coating of metal is then applied to the master, making a negative of the master with the pits and lands in it. The negative is then peeled off the master and coated in a thin layer of plastic. The plastic protects the coating while a punching press punches a hole into the center of the disc, and punches excess material.

The negative is now a stamper - a part of the mold that will be used for replication. It is placed on one side of the mold with the data side containing the pits and lands facing out. This is done inside an injection molding machine. The machine then closes the mold and injects polycarbonate in the cavity formed by the walls of the mold, which forms or molds the disc with the data on it.

The molten polycarbonate fills the pits or spaces between the lands on the negative, acquiring their shape when it solidifies. This step is somewhat similar torecord pressing.

The polycarbonate disc cools quickly and is promptly removed from the machine, before forming another disc. The disc is then metallized, covered with a thin reflective layer of aluminum. The aluminum fills the space once occupied by the negative.

A layer of varnish is then applied to protect the aluminum coating and provide a surface suitable for printing. The varnish is applied near the center of the disc, and the disc is spun, evenly distributing the varnish on the surface of the disc. The varnish is hardened using UV light. The discs are then silkscreened or a label is otherwise applied.

Recordable discs add a dye layer, and rewritable discs add a phase change alloy layer instead, which is protected by upper and lower dielectric (electrically insulating) layers. The layers may be sputtered. The additional layer is between the grooves and the reflective layer of the disc. Grooves are made in recordable discs in place of the traditional pits and lands found in replicated discs, and the two can be made in the same exposure process.In DVDs, the same processes as in CDs are carried out, but in a thinner disc. The thinner disc is then bonded to a second, equally thin but blank, disc using UV-curableLiquid optically clear adhesive,forming a DVD disc.This leaves the data in the middle of the disc, which is necessary for DVDs to achieve their storage capacity. In multi layer discs, semi reflective instead of reflective coatings are used for all layers except the last layer, which is the deepest one and uses a traditional reflective coating.

Dual layer DVDs are made slightly differently. After metallization (with a thinner metal layer to allow some light to pass through), base and pit transfer resins are applied and pre-cured in the center of the disc. Then the disc is pressed again using a different stamper, and the resins are completely cured using UV light before being separated from the stamper. Then the disc receives another, thicker metallization layer, and is then bonded to the blank disc using LOCA glue. DVD-R DL and DVD+R DL discs receive a dye layer after curing, but before metallization. CD-R, DVD-R, and DVD+R discs receive the dye layer after pressing but before metallization. CD-RW, DVD-RW and DVD+RW receive a metal alloy layer sandwiched between 2 dielectric layers. HD-DVD is made in the same way as DVD. In recordable and rewritable media, most of the stamper is composed of grooves, not pits and lands. The grooves contain awobble frequencythat is used to locate the position of the reading or writing laser on the disc. DVDs use pre-pits instead, with a constant frequency wobble.

Blu-ray

HTL(high-to-low type) Blu-ray discs are made differently. First, asilicon waferis used instead of a glass master.The wafer is processed in the same way a glass master would.

The wafer is then electroplated to form a 300-micron thick nickel stamper, which is peeled off from the wafer. The stamper is mounted onto a mold inside a press or embosser.

The polycarbonate discs are molded in a similar fashion to DVD and CD discs. If the discs being produced are BD-Rs or BD-REs, the mold is fitted with a stamper that stamps a groove pattern onto the discs, in lieu of the pits and lands found on BD-ROM discs.

After cooling, a 35 nanometre-thick layer of silver alloy is applied to the disc usingsputtering.Then the second layer is made by applying base and pit transfer resins to the disc, and are pre-cured in its center.

After application and pre-curing, the disc is pressed or embossed using a stamper and the resins are immediately cured using intense UV light, before the disc is separated from the stamper. The stamper contains the data that will be transferred to the disc. This process is known as embossing and is the step that engraves the data onto the disc, replacing the pressing process used in the first layer, and it is also used for multi layer DVD discs.

Then, a 30 nanometre-thick layer of silver alloy is then sputtered onto the disc and the process is repeated as many times as required. Each repetition creates a new data layer. (The resins are applied again, pre-cured, stamped (with data or grooves) and cured, silver alloy is sputtered and so on)

BD-R and BD-RE discs receive (through sputtering) a metal (recording layer) alloy (that is sandwiched between two dielectric layers, also sputtered, in BD-RE), before receiving the 30 nanometre metallization (silver alloy, aluminum or gold) layer, which is sputtered. Alternatively, the silver alloy may be applied before the recording layer is applied. Silver alloys are usually used in Blu-rays, and aluminum is usually used on CDs and DVDs. Gold is used in some "Archival" CDs and DVDs, since it is more chemically inert and resistant to corrosion than aluminum, which corrodes intoaluminum oxide,which can be seen indisc rotas transparent patches or dots in the disc, that prevent the disc from being read, since the laser light passes through the disc instead of being reflected back into the laser pickup assembly to be read. Normally, aluminum does not corrode since it has a thin oxide layer that forms on contact with oxygen. In this case, it can corrode due to its thinness.

Then, the 98 micron-thick cover layer is applied using UV-curableliquid optically clear adhesive,and a 2 micron-thick hard coat (such asDurabis) is also applied and cured using UV light. In the last step, a 10 nanometre-thicksilicon nitridebarrier layer is applied to the label side of the disc to protect against humidity.Blu-rays have their data very close to the read surface of the disc, which is necessary for Blu-rays to achieve their capacity.

Discs in large quantities can either be replicated or duplicated. In replication, the process explained above is used to make the discs, while in duplication, CD-R, DVD-R or BD-R discs are recorded and finalized to prevent further recording and allow for wider compatibility.(SeeOptical disc authoring). The equipment is also different: replication is carried out by fully automated purpose-built machinery whose cost is in the hundreds of thousands of US dollars in the used market,while duplication can be automated (using what's known as an autoloader) or be done by hand, and only requires a small tabletop duplicator.

Specifications

Base (1×) and (current) maximum speeds by generation

Generation	Base	Max
	(Mbit/s)	(Mbit/s)	×
1st (CD)	1.17	65.6	56×
2nd (DVD)	10.57	253.6	24×
3rd (BD)	36	504	14×
4th (AD)	?	?	14×

Capacity and nomenclature

Designation		Sides	Layers (total)	Diameter	Capacity
				(cm)	(GB)
BD	SS SL	1	1	8	7.8
BD	SS DL	1	2	8	15.6
BD	SS SL	1	1	12	25
BD	SS DL	1	2	12	50
BD	SS TL	1	3	12	100
BD	SS QL	1	4	12	128
CD–ROM 74 min	SS SL	1	1	12	0.682
CD–ROM 80 min	SS SL	1	1	12	0.737
CD–ROM	SS SL	1	1	8	0.194
DDCD–ROM	SS SL	1	1	12	1.364
DDCD–ROM	SS SL	1	1	8	0.387
DVD–1	SS SL	1	1	8	1.46
DVD–2	SS DL	1	2	8	2.66
DVD–3	DS SL	2	2	8	2.92
DVD–4	DS DL	2	4	8	5.32
DVD–5	SS SL	1	1	12	4.70
DVD–9	SS DL	1	2	12	8.54
DVD–10	DS SL	2	2	12	9.40
DVD–14	DS DL/SL	2	3	12	13.24
DVD–18	DS DL	2	4	12	17.08
DVD–R 1.0	SS SL	1	1	12	3.95
DVD–R (2.0), +R, –RW, +RW	SS SL	1	1	12	4.7
DVD-R, +R, –RW, +RW	DS SL	2	2	12	9.40
DVD–RAM	SS SL	1	1	8	1.46
DVD–RAM	DS SL	2	2	8	2.65
DVD–RAM 1.0	SS SL	1	1	12	2.58
DVD–RAM 2.0	SS SL	1	1	12	4.70
DVD–RAM 1.0	DS SL	2	2	12	5.16
DVD–RAM 2.0	DS SL	2	2	12	9.40

See also

• Disc Description Protocol
• List of optical disc manufacturers
• Universal Disk Format(UDF)

External links

• "Inventor of the Week Archive: The Digital Compact Disc".Massachusetts Institute of Technology. December 1999. Archived fromthe originalon 2008-06-19.Retrieved2007-07-13.
• Dudley, Brier (November 29, 2004)."Scientist's invention was let go for a song".The Seattle Times.Retrieved2007-07-13.
• "David Gregg and the Optical Disk".About.com. Archived fromthe originalon 2020-08-19.Retrieved2007-07-13.
• Byers, Fred R. (2003).Care and Handling of CDs and DVDs — A Guide for Librarians and Archivists(PDF)(Report).National Institute of Standards and Technology.Archived fromthe original(PDF)on September 23, 2008.RetrievedJuly 17,2009.
• Romeyn, Jacob."50th-anniversary -of-the-optical-disc".Archived fromthe originalon 2015-02-08.Retrieved2011-06-28.
• "Optical Storage Technology Association".
• O'Kelly, Terence."Reference Guide for Optical Media"(PDF).Memorex Inc. – via A.N.T. Audio.
• "The history of ideas" the optical disc as a "unique" carrier of information in the systems management ".European Society of the History of Science. Archived fromthe originalon 2015-05-18.Retrieved2013-06-11.
• "Thomson-CSF's transmissive videodisc".
• "Know Your Digital Storage Media: a guide to the most common types of digital storage media found in archives".US: University of Texas at San Antonio.
• Longevity of Recordable CDs, DVDs and Blu-rays — Canadian Conservation Institute (CCI) Notes 19/1