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The term "optical storage" refers to physical computer storage discs or drives that use a visual method to record and read data. One of the most common types of optical storage is a compact disc (CD), on which information is stored in tiny points that can be read by a laser. Additional formats incorporate compression and features such as multiple layers within a single disc to create higher-capacity storage discs such as the digital video disc (DVD) and more experimental media, such as holographic storage discs. Although optical storage is relatively inexpensive and can hold a good amount of information, sometimes measured in gigabytes of storage, it also can be prone to write errors and may have a limited shelf life, reducing its viability as a very long-term or permanent storage option.
One of the most common forms of optical storage is a CD, which appears as a circular disk that has a reflective surface contained inside. In actuality, there is a fine line that winds outward from the center of the disc, encompassing nearly every available space on a single side. To store information, a process is used that makes small indentations in the bottom of the disk that reveal the reflective layer underneath. Each point on the disk relates to a single bit of information and can be read either as a positive value when the reflective surface can be seen or as a negative value when it is not exposed.
Some higher-density optical storage media, such as DVDs, actually use a system in which multiple layers of information are encoded in the same physical amount of space as a CD. This can allow much more data to be stored but also reduces the speed of reading and writing data. Even more advanced techniques, such as holography, can be used to contain several layers inside a three-dimensional (3D) storage device, achieving much higher data storage rates.
Although optical storage media have good performance in relation to other removable storage solutions, they also have some drawbacks. Namely, information that is being written to optical storage devices does not always undergo error checking and is sometimes prone to failure because of the sensitive nature of the drives. The way in which some optical discs are assembled also gives them a limited shelf life, because they rely on dyes or other volatile coatings to hold information that will degrade over time.