Optical Discs

An entirely different method of recording data is used for optical disks. These include the various kinds of CD and DVD discs. You may guess from the word "optical" that it has to do with light. You'd be exactly right! Laser light, in fact. Optical disks come in several varieties which are made in somewhat different ways for different purposes_[43].

How optical disks are similar

• Formed of layers

• Data in a spiral groove on starting from the center of the disk

• Digital data (1's and 0's)

• 1's and 0's are formed by how the disk absorbs_[44] or reflects_[45] light from a tiny laser.

The different types of optical disks use different materials and methods to absorb and reflect the light.

How It Works (a simple version)

An optical disc is made mainly of polycarbonate (a plastic). The data is stored on a layer inside the polycarbonate. A metal layer reflects the laser light back to a sensor.

To read the data on a disk, laser light shines through the polycarbonate and hits the data layer. How the laser light is reflected or absorbed is read as a 1 or a 0 by the computer.

In a CD the data layer is near the top of the disc, the label side.

In a DVD the data layer is in the middle of the disc. A DVD can actually have data in two layers. It can access the data from 1 side or from both sides. This is how a double-sided, double-layered DVD can hold 4 times the data that a single-sided, single-layered DVD can.

Materials

The materials used for the data (recording) and metal (reflecting) layers are different for different kinds of optical disks.

CD-	DVD-	Type	Data Layer	Metal Layer
CD-ROM   (Audio/video PC software)	DVD-ROM   (Video/audio, PC use)	Read Only	Molded[46]	Aluminum (Also silicon, silver, or gold in double-layered DVDs)
CD-R	DVD-R DVD+R	Recordable (once!)	Organic dye[47]	Silver, gold, silver alloy
CD-RW	DVD-RW DVD+RW DVD+RAM	Rewritable (write, erase, write again)	Phase-changing metal alloy[48] film	Aluminum

Read Only:

The most common type of optical disk is the CD-ROM, which stands for Compact Disc - Read Only Memory. It looks just like an audio CD but the recording format is quite different. CD-ROM discs are used for computer software.

DVD used to stand for Digital Video Device, but now it doesn't really stand for anything at all! DVDs are used for recording movies.

The CDs and DVDs that are commercially produced are of the Write Once Read Many (WORM) variety. They can't be changed once they are created.

The data layer is physically molded into the polycarbonate. Pits_[49] (depressions) and lands (surfaces) form the digital data. A metal coating (usually aluminum) reflects the laser light back to the sensor. Oxygen can seep_[50] into the disk, especially in high temperatures and high humidity_[51]. This corrodes_[52] the aluminum, making it too dull_[53] to reflect the laser correctly.

CD-ROM and DVD-ROM disks should be readable for many, many years (100? 200?), but only if you treat them with respect.