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COptical Technical Information |
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There are basically three types of media that comprise CD and DVD discs:
Stamped Discs (commonly referred to as “replication”) are primarily used for high volumes of discs targeted at consumer markets. CD stamped discs often contain audio from musicians. DVD stamped discs are used for high volume distribution of feature length movies. Replicating stamped discs in large quantities can also be used for other volume distribution projects containing audio, video, data or software. Stamped discs offer the most economical method to distribute content on optical media – but in high-volumes.
Rewritable Discs allow written content and data to be erased, modified, updated or rewritten. Rewritable discs are often used for back-up of select computer data that will be updated and rewritten again later. Using rewritable media for duplication is the least economical method and is not conducive to most disc duplication projects. As such, no further discussion of rewritable media will be discussed.
Write-Once Discs are popular and used extensively for duplicating master content for disc distribution projects that require one to five-hundred discs - sometimes more. Write-once discs are ubiquitous and are used to record and playback many forms of digital content or data. Because many PCs are shipped standard with disc reader's or existing PCs can be equipped with a disc writer at low cost, write-once disc formats such as CD-R and DVD-R have evolved into industry standards. Many DVD consumer players can playback CD-R or DVD-R discs that include images, digital photographs, audio, video and other types of content . CD-R discs have been embraced as an industry standard for many years. DVD-R discs for general purpose use were introduced in 2000. 
The DVD ForumDVD-R discs are endorsed by the official DVD Forum. As a write-once recording media, DVD-R discs are used extensively in critical commercial applications like medical image archive. The format has broad acceptance across consumer electronics and IT industry. The DVD Forum is an industry consortium that was founded in 1995 under the original name “DVD Consortium”. Ten companies founded the organization.
Non-DVD Forum FormatsIn 2001 alternative DVD write-once and rewritable formats were announced: DVD+R and DVD+RW. The plus (+DVD) format functionality and storage capacities are virtually the same as for DVD –R and –RW. However, the plus (+ DVD) formats are NOT endorsed by the official DVD Forum and lacks widespread acceptance in commercial applications.
CD & DVD Physical Disc StucturesOptical discs are produced with various physical layers of materials and coatings. A standard CD-R disc is usually constructed with around five physical layers (not including adhesive layers):
A standard single-sided DVD-R disc with one data recording layer can have 7 or 8 physical layers:
Substrate LayersThe bottom polycarbonate substrate layer acts as the disc’s foundation. Metallic and chemical coatings are “sputtered” in varying thicknesses on the bottom layer. Polycarbonate is a strong plastic used in many high-impact resistant applications. CD media uses a single bottom polycarbonate substrate layer with coatings layered on top. For CD’s the important reflective layer is closer to the top surface near the label/printing surface. As a result, CD discs can be more vulnerable to data-loss and damage if the top surface is gouged, smashed, heavily scratched or deforms slightly. Alternatively, the reflective and recording dye layers for DVD-R are sandwiched between two polycarbonate substrates that are bonded together with an adhesive. Some disc manufacturers employ sub-standard materials, old equipment or use inadequate procedures that can prevent adhesive being applied correctly to outer edges of DVD discs or perhaps fail to overlap the edges on CD discs with lacquer. Any form of optical interference – either on the disc’s bottom polycarbonate readout surface or contamination within layers themselves – can impede precise focusing of laser light on the recording layer where the disc’s data resides - resulting in errors or an unreadable disc.
Reflective LayersA metallic reflective layer is used within all CD and DVD discs to reflect laser light to the reader pick-up for conversion into meaningful data. There are basically five materials used for reflective layers on CD and DVD media: aluminum, copper, silver, silver alloy and gold. Aluminum is widely used in replication for high-volume stamped discs. It offers “adequate” reflectivity characteristics and is the least expensive material. Aluminum can oxidize within relatively short periods of time through oxygen absorption into the polycarbonate substrate from moisture (H2O) or sulfur dioxide (SOX) -- a pollutant and byproduct of fossil-fuel combustion. Instead of retaining a polished mirror-like surface, it becomes dull and loses the ability to accurately reflect light. Silver or silver-alloy offers the greatest reflectivity but can react with some recording dye formulations. Silver can also loose reflectivity due to environmental factors. Gold offers less reflectivity than silver but is very stable over time, does not corrode and is less likely to react with adjacent coatings. Discs utilizing gold on reflective layers are more expensive. Regardless of metallic materials used on reflective layers, they must be applied evenly and have sufficient thickness to reliably reflect laser light. CD-R and DVD-R discs that use gold on reflective layers can be difficult to determine or confirm. Unfortunately, deceptive disc packaging may use the word 'gold'– having no linkage whatsoever to disc reflectivity. Instead, the term may infer the 'gold colored' lacquer used on the top surface label. In other cases, generic disc manufacturer claims may use tinted polycarbonate substrates, making it extremely difficult to confirm metallics used on reflective layers.
Recording Dye LayersThe organic dye layer for recordable media is the most important, complex, and costliest step in disc manufacturing. The dye layer records and stores the digital bits that represent the actual content of a disc. The photo-reactive dye-layer sits between the overlying reflective layer and the readout surface at the bottom of the polycarbonate substrate. During recording, laser light travels through the bottom substrate and penetrates the dye. Altering transparency of the dye at a precise spot burns a dark “pit”. Hence, the term “burning a disc” refers to the laser burning dark pits in the photosensitive dye and altering its reflective characteristics to represent digital ones and zeros. When discs are read or played-back, low-power laser-light hits the dye-layer and is either absorbed or passes through to the reflective layer, where it bounces back to a photoreceptor. This contrast between reflected or absorbed light is detected and interpreted as digital bits. Most dye formulations are proprietary, patented and must accomplish multiple goals. In order for recordable media to playback correctly by disc readers, its optical characteristics must appear the same as stamped media. Dyes must bond well with polycarbonate and remain stable with solvents used to produce dyes themselves, along with other factors. Most manufacturers that produce recording dyes constantly adjust and fine-tune dye formulations to improve various characteristics. Manufacturers of recordable discs can use different combinations of dyes on recording layers with various metals on reflective layers.
All organic recording dyes degrade and eventually breakdown. It’s a natural process and generally occurs over a long period of time. Some dyes are optimized for high-speed recording and playback performance, while others are formulated for long-term stability. New, modified and improved dye formulations can be announced by manufacturers at any time. Some disc manufacturers change dye formulations without prior notice.
Disc Surface Printing LayersDisc manufacturers commonly add a final top-layer for surface printing and disc identification. The final surface coating is usually optimized for thermal, ink-jet or silkscreen printing.
Protective Coating Layer on CD DiscsA final coating should be applied to CD discs with an edge-to-edge lacquer to protect the underlying metallic reflective layer and prevent moisture absorption.
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