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14.1 How is a BD-ROM disc constructed?
14.2 How is a BD-R disc constructed?
14.3 How is a BD-RE disc constructed?
14.4 What is the purpose of the hard coat on a BD disc?
14.5 What are the weights and physical dimensions of BD discs?

14.1 How is a BD-ROM disc constructed?
A BD-ROM (prerecorded) is a sandwich of multiple layers of materials and either one or two data layers (single or dual-layer). A single-layer (SL) disc begins with the substrate (polycarbonate) molded with a spiral track of pits and lands extending from the inside to the outside diameter of the disc. Added to this substrate are a reflective layer (aluminum or silver alloy), a cover layer (polycarbonate film, silicone and others) and finally a transparent protective hard coating (silicon dioxide resin, silicone and others). To seal and prevent it from absorbing water, which might warp the disc, the substrate is also topped with a moisture barrier (silicon nitride and others).

The structure of a dual-layer (DL) disc is more complex. First comes the substrate with its track of pits and lands [data layer 0], a reflective layer (silver alloy), a spacer layer (pressure sensitive adhesive, photo curable acrylate adhesive and others) imprinted with a second track of pits and lands [data layer 1], a semi-reflective layer (silver alloy), a cover layer and then a protective hard coating.

It is interesting to note that laser light from the player, drive or recorder does not travel through the BD substrate to access information on the disc. Thus it may be possible to employ materials other than the optical-grade polycarbonate plastic as used in CD, DVD and other disc construction. Indeed, polycarbonate is currently employed, but manufacturers have demonstrated that various resins (polyphenylene oxide/PPO and others) and even paper may be possibilities for the future.

14.2 How is a BD-R disc constructed?
A BD-R (recordable) disc can be Low-to-High (LTH) single-layer (SL) or High-to-Low (HTL) single (SL) or dual-layer (DL) and a BD-R XL disc High-to-Low (HTL) triple (TL) or quadruple-layer (QL). A High-to-Low (HTL) SL disc begins with the substrate (polycarbonate) molded with a shallow spiral groove extending from the inside to the outside diameter of the disc. Next, a reflective layer (silver alloy), a dielectric layer (zinc sulfide or zirconium dioxide and silicon dioxide, carbon), followed by a single recording layer (bismuth nitride and germanium nitride, palladium-doped tellurium suboxide, tellurium) or a pair of layers (copper alloy and silicon) that combine during recording, another dielectric layer (zinc sulfide or zirconium dioxide and silicon dioxide, carbon), a cover layer (polycarbonate film, silicone and others) and then a transparent protective hard coating (silicon dioxide resin, silicone and others).

Alternatively, a Low-to-High (LTH) SL disc employs an organic dye (azo, cyanine, phthalocyanine) recording layer, similar to CD-R and DVD±R discs, which is surrounded by an interlayer (tantalum, niobium, molybdenum, vanadium and others) and interface/barrier layer (silicon dioxide, zinc sulfide and silicon dioxide and others).

A DL disc is more complex. It consists of a substrate imprinted with the shallow spiral groove, a reflective layer, a dielectric layer, a recording layer pair [data layer 0], another dielectric layer, a spacer layer (pressure sensitive adhesive, photo curable acrylate adhesive and others) imprinted with a second groove, a dielectric layer, a second recording layer pair [data layer 1], another dielectric layer then the cover layer and hard coating.

BD-R disc design is still in the process of evolving as manufacturers experiment with new materials and designs in order to reduce costs, improve manufacturing efficiency, enhance performance, increase capacity and extend longevity, as well as offer differentiation while maneuvering around rival patents.

14.3 How is a BD-RE disc constructed?
A BD-RE (rewritable) disc can be either single (SL) or dual-layer (DL) and a BD-RE XL disc triple-layer (TL). A SL disc begins with the substrate (polycarbonate) molded with a shallow spiral groove extending from the inside to the outside diameter of the disc. Next, a reflective layer (silver alloy), a dielectric layer (zinc sulfide and silicon dioxide), followed by a phase change alloy recording layer (either germanium, tellurium and antimony or indium, silver, tellurium and antimony), another dielectric layer (zinc sulfide and silicon dioxide), a cover layer (polycarbonate film, silicone and others) and then a transparent protective hard coating (silicon dioxide resin, silicone and others). Additional layers (germanium nitride, silicon carbide, zirconium oxide, tin telluride, antimony telluride and others) may be incorporated above or below the dielectric layers.

A DL disc consists of a substrate molded with a shallow spiral groove, a reflective layer, a dielectric layer, a phase change alloy layer [data layer 0], another dielectric layer, a spacer layer (pressure sensitive adhesive, photo curable acrylate adhesive and others) imprinted with a second groove, a dielectric layer, a second phase change alloy layer [data layer 1], another dielectric layer then the cover layer and hard coating. Additional layers may be incorporated above or below the dielectric layers.

14.4 What is the purpose of the hard coat on a BD disc?
The design of a BD disc makes it more vulnerable than a CD or DVD to the effects of scratches, fingerprints and dust (increased error rates, loss of tracking control, etc.). To make the laser entrance (incident) surface more robust to withstand the bumps, scrapes and grime that come from day to day use and handling, the disc’s cover layer is either manufactured from a resilient material or, more commonly, coated with a thin (approx. 2 to 5µm) transparent protective hard coat (silicon dioxide resin, silicone and others).

Formally, BD test specifications require that the exterior laser incident surface of the disc offer a minimum degree of durability so manufacturers evaluate their discs by subjecting them to "Taber Abrasion" accelerated wear testing (according to ISO 9352:1995. Plastics – Determination of Resistance to Wear by Abrasive Wheels and ASTM D1044 – 08 Standard Test Method for Resistance of Transparent Plastics to Surface Abrasion). As such, discs are rotated for a set number of revolutions under two mild abrasive wheels (see figure 14.1) at a predetermined load (pressure) and then meet certain quality criteria for various measured optical signals.

Beyond scuffs, the hard coat helps protect against damage from accidental impact from the optical pickup of the drive, player or recorder. To meet specifications, discs must exhibit minimum performance when subjected to an impact resistance test. This involves striking a plastic (polyoxymethylene) pin against the rotating disc under specified conditions and ensuring that a set data error rate is not exceeded.

Typically, the hard coat also repels fingerprints and dust thereby reducing the impact of surface contaminants on reading and writing performance and allow for straightforward cleaning. Generally speaking, the hard coat is engineered to have a low surface energy so fingerprints tend to form droplets rather than expand or stain. And dust resistance comes by way of employing conductive materials to create low surface resistivity thereby reducing the build up of electrostatic charge so dust and other particles are less likely to cling.

To meet specifications, manufacturers must evaluate the sensitivity of their discs to fingerprints and other grime. This is accomplished by applying an Artificial Finger Print (AFP), created by coating a scuffed silicone rubber stamp with a suspension of methoxy propanol (solvent), triolein (a skin oil component) and various sized (1.6 to 2.3 µm median diameter) inorganic particles (silicon dioxide, iron oxide, aluminum oxide, calcium oxide, magnesium oxide) and pressing it against the disc surface. The disc is then tested to ensure that a specific data error rate is not exceeded when reading or recording through the AFP.

Tests and technologies aside, BD discs, of course, are not indestructible and should always be handled with care and treated according to manufacturer’s instructions.

14.5 What are the weights and physical dimensions of BD discs?
According to official physical specifications set by the Blu-ray Disc Association, a BD disc must weigh between 12 to 17 g (12 cm) and 5 to 8 g (8 cm) or, roughly speaking, 1/2 to 1/4 oz respectively (somewhat lighter than its DVD and CD counterparts). A BD also spins faster than does a DVD or CD so, to limit noise and vibration, improve safety, as well as to reduce wear and strain on drives, players and recorders, tighter restrictions are imposed applying to a BD disc’s moment of inertia and dynamic unbalance/imbalance relative to its rotational axis.

Subtle differences aside, BD discs otherwise have the same general physical features, size and appearance as CDs and DVDs (see figure V-2). Each has an outer diameter of 120 mm (4.72 in) or 80 mm (3.15 in), a center hole spanning 15 mm (0.59 in) and employs multi-layer construction reaching a nominal thickness of 1.2 mm (0.05 in).

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