Optical recording is a method of recording either data, audio content or video content in a form which is played back using light. This is usually done with a disc as the storage medium. The main advantage to optical recording is that there is less physical contact with the surface storing the data than with other forms of recording, theoretically giving it a longer lifespan.
In most forms of recording, optical recording methods have replaced magnetic-based methods. For example, in audio recording, the compact disc replaced the audio cassette. In video recording, the DVD replaced the videotape. In computer data storage, the CD and DVD replaced the audio cassette and floppy disk.
Optical recordings are only possible where data is in digital format, meaning it has been converted to binary. This binary code is then stored in physical form on the disc. On pre-recorded discs, a piece of data which is represented as a 1 in binary is stored by pressing a pit into the disc. A piece of data which is represented as a 0 in binary is stored by leaving the surface flat.
The data is then read by the disc spinning around as a laser is beamed onto the surface. The time it takes for the light to bounce back will vary depending on whether or not the surface is flat or pitted. This system is the reason that optical recording is normally stored on a disc: this is inherently the most efficient shape for an object that is spun.
Contrary to popular belief, the data is physically stored on the top side of the disc, which is the side with the printed label. The data is housed immediately below this label. The bottom side of the disc, which is the shiny side, is merely a transparent layer which protects the data surface from being contaminated.
Each type of optical discs is available in three formats. Read-only discs come with the data pre-recorded. Recordable discs do not have pits and flat sections, but rather use a dye which can be made either reflective or non-reflective to produce the same effect. Rewriteable discs use a metal compound which can be heated and melted repeatedly to produce different combinations of pits and flat surfaces.
There are some potential drawbacks to optical recording. One is that the protective dye on discs can fade, wiping part or all of the data. This can happen with extreme temperatures or because the dye was of low quality. Another problem is that scratches or stains can prevent the laser from reading the data. How severe the damage is will determine whether it is reversible.