Computers can't make any real use of the analog world that surrounds us. For them to do their work, analog information has to be changed into digital data. Computers can then process the data, send it to your screen, a printer or the internet, or save it for later use. One of the ways computers store information is through the use of optical media.
A Quick Storage Device Primer
There are three main types of storage your computer can use. The most common type through most of the industry's history has been magnetic storage devices. These include obsolete tape drives and floppy disks, as well as the hard drives that still provide most computers' storage capacity. Solid-state storage devices include thumb drives, memory cards, and the high-speed solid-state drives or SSDs that are beginning to replace hard drives in many uses. A third is optical storage, typically in the form of removable discs.
How the Bytes Are Read
The difference between these storage media comes down to how data is digitally encoded and read. On magnetic media, the zeroes and ones that make up digital data are encoded as magnetic signals, grouped into tracks and sectors. Solid-state drives and USB thumb drives use a form of computer memory, which records data by charging or not charging a given portion of the chip to create the equivalent of zeroes and ones. With optical media, the same zeroes and ones are coded visually and read by lasers.
How Optical Storage Works
The discs used in optical drives consist of a rigid reflective layer to hold the data, sandwiched between a protective transparent layer and an opaque layer where the label goes. Lasers create the zeroes and ones by pitting or darkening microscopic areas on the reflective layer, making them nonreflective. When you play back the data, a given spot is either reflective or not, and the reflective and not-reflective spots make up the bits and bytes of digital information. Optical tape works similarly, except the layers are made of flexible tape rather than rigid discs.
CD-ROM, CD-R and CD-RW
The first optical disc to see widespread use was the compact disc, or CD. Early drives could only read manufactured disks and were referred to as CD-ROM or CD read-only memory. CD-R expanded on that basic technology, making it possible for the computer's disk drive to record on a single-use optical disc. Once written, the disc couldn't be rewritten. That refinement came later with CD-RW or rewritable CD drives and discs. These could be written and rewritten many times, much like hard drives or floppy drives, and held about 700MB of data.
DVD, DVD-R and DVD-RW
CDs were originally developed to hold album-length quantities of music, but DVDs needed a higher capacity to record and play back movies. They're similar to CDs in basic technology but pack together smaller reflective and nonreflective dots at higher density. They followed a similar evolution from nonrecordable DVDs through recordable DVDs to DVD-RW drives. DVDs can hold 4.7GB on a standard disk, and dual-layer disks – which have two reflective layers in the middle – can hold well over 8GB.
Blu-Ray Optical Media
The highest-capacity optical discs in widespread use are Blu-ray discs, which were created to store high-definition movies. The name comes from these drives' use of a blue laser rather than a red one, which has a shorter wavelength and therefore can be focused more tightly. This lets manufacturers pack data even more densely than on a DVD. Blu-ray discs can hold 25GB to 100GB of data when used for computer storage.
Archival Optical Media Products
Although ordinary discs hold plenty of data for consumer purposes, businesses and public institutions need archival-quality data storage in much higher capacities. These can be manufactured in the form of disc cartridges, which work much like DVD or Blu-ray discs but are protected by an external housing, or they can be in tape form. Usually, they're designed as write once read many, or WORM, media.
WORM media are essentially large-scale versions of the early CD-R concept: Data can be written just once on each tape or cartridge, but then it can be read over and over again. Advanced WORM products, such as IBM's tape cartridges for its model 3592 drives, have additional tamper-proofing features for both the data and the physical cartridges themselves, to provide an additional level of data security. Their capacity can range into the terabytes per cartridge, or thousands of GB.