IBM defines optical storage as "any storage method that uses a laser to store and retrieve data from optical media." Britannica notes that it "uses low-power laser beams to record and retrieve digital (binary) data." Compact disc (CD) and DVD are examples of optical media.
Optical storage is the storage of data on an optically readable medium. Data is recorded by making marks in a pattern that can be read back with the aid of light, usually a beam of laser light precisely focused on a spinning optical disc. An older example of optical storage that does not require the use of computers, is microform. There are other means of optically storing data and new methods are in development. An optical disc drive is a device in a computer that can read CD-ROMs or other optical discs, such as DVDs and Blu-ray discs. Optical storage differs from other data storage techniques that make use of other technologies such as magnetism, such as floppy disks and hard disks, or semiconductors, such as flash memory and RAM.
Optical storage can range from a single drive reading a single CD-ROM to multiple drives reading multiple discs such as an optical jukebox. Single CDs (compact discs) can hold around 700 MB (megabytes) and optical jukeboxes can hold much more. Single-layer DVDs can hold 4.7 GB, while dual-layered can hold 8.5 GB. This can be doubled to 9.4 GB and 17 GB by making the DVDs double sided, with readable surfaces on both sides of the disc. HD DVDs were able to store 15 GB with a single-layer and 30 GB with a dual-layer. Blu-ray discs, which won the HDTV optical format war by defeating HD DVDs, can hold 25 GB for single-layer, 50 GB for dual-layer and up to 128 GB for quad-layer discs. Optical storage include CDs and DVDs.