Key information Section 3. Optical storage, the typical optical disc

Optical storage, the typical optical disc, stores information in deformities on the surface of a circular disc and reads this information by illuminating the surface with a laser diode and observing the reflection. The deformities may be permanent (read only media), formed once (write once media) or reversible (recordable or read/write media). Optical disc storage is non-volatile.

Optical disks can store information at much higher densities than magnetic disks. Thus, they are ideal for multimedia applications where images, animation and sound occupy a lot of disk space. Besides, they are not affected by magnetic fields. This means that they are secure and stable, e.g. they can be transported through airport metal detectors without damaging the data. However, optical drives are slower than hard drives. While there are hard drives with an average access time of 8 ms, most CD-ROM drives have an access time of 150 to 200 ms.

The following forms of optical storage are currently in common use:

CD, CD-ROM, DVD, BD-ROM (read only memory): Read only storage, used for mass distribution of digital information (music, video, computer programs), you cannot change data stored on them. CD-R, DVD-R, DVD+R BD-R (recordable): Write once storage, used for tertiary and off-line storage; write-once devices which let you duplicate CDs.

• CD-RW, DVD-RW, DVD+RW, DVD-RAM, BD-RE (Blue Ray) (rewritable): Slow write, fast read storage, used for tertiary and off-line storage, the disks enable you to write onto them in multiple sessions, like a hard disk and can be erased and reused many times.
• DVDs (digital versatile discs) are similar in size to CDs (both are 1.2 mm thick), but they differ in structure and capacity. DVDs have more tracks and more pits (tiny holes) per track, and can store from 4.7 GB to 17 GB of data, movies, high-definition sound, etc.,; so they will probably replace CDs.
• Ultra Density Optical or UDO is similar in capacity to BD-R or BD-RE and is slow write, fast read storage used for tertiary and off-line storage.

Magneto-optical disc storage is optical disc storage where the magnetic state on a ferromagnetic surface stores information. The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is non-volatile, sequential access, slow write,

fast read storage used for tertiary and off-line storage. Consequently, MO disks are rewritable, i.e. they can be written to, erased, and then written again. They usually come in two formats: 5.25" cartridges can hold more than 5.2 GB; 3.5" floptical disks have a capacity of 230 MB to 1.3 GB. They are ideal

for back-up and portable mass storage. 3D optical data storage has also been proposed.