Optical disc

Lore & Background

The encoding material sits atop a thicker substrate, usually polycarbonate, that forms a dust defocusing layer. The encoding pattern follows a continuous spiral path covering the entire disc surface from innermost to outermost track. Data are stored with a laser or stamping machine and accessed when illuminated by a laser diode in an optical disc drive spinning at 200 to 4,000 RPM or more. Most optical discs exhibit iridescence due to the diffraction grating formed by their grooves. The reverse side typically has a printed label, and unlike floppy disks, most optical discs lack an integrated protective casing, making them susceptible to scratches and fingerprints. Blu-rays have a coating called durabis that mitigates these problems.

Reader's Guide

Optical discs are most commonly used for digital preservation, storing music, video, data and programs for personal computers, and offline hard copy data distribution due to lower per-unit prices. Libraries and archives enact optical media preservation procedures. For computer data backup and physical data transfer, optical discs are gradually being replaced by faster, smaller solid-state devices like USB flash drives. Additionally, music, movies, games, software and TV shows purchased or streamed over the Internet have reduced sales of audio CDs, video DVDs and Blu-rays. However, audio CDs and Blu-rays are still preferred by some for uncompressed audio or better image and sound quality without compression artifacts. Blu-rays may be the only option for playing large games without downloading over an unreliable connection, and as of 2020 they remain widely used by gaming consoles like the PlayStation 4 and Xbox One X. Optical discs are typically stored in special cases, should be handled by the edges, and cleaned in a straight line, not a circular pattern. Recordable discs should not be exposed to light for extended periods. Storage conditions should be dry and cool, with temperatures between -10 and 23 °C and humidity between 20 and 50%.

Did You Know?

• Optical discs can be reflective or transmissive, with light source and detector on the same or opposite sides.
• Write-once discs commonly use an organic dye recording layer; rewritable discs use a phase change alloy of silver, indium, antimony, and tellurium.
• Blu-ray recordable discs that use an organic dye layer are known as low-to-high (LTH) discs and can be made on existing CD and DVD production lines.