In
computing and
optical disc recording technologies, an
optical disc is a flat, usually circular disc which encodes binary data in the form of
pits and lands on a special material on one of its flat surfaces. The encoding material sits atop a thicker substrate which makes up the bulk of the disc and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the entire disc surface and extending from the innermost track to the outermost track. The data is stored on the disc with a
laser or stamping machine, and can be accessed when the data path is illuminated with a
laser diode in an
optical disc drive which spins the disc at speeds of about 200 to 4000
RPM or more, depending on the drive type, disc format, and the distance of the read head from the center of the disc. The pits or bumps distort the reflected laser light, hence most optical discs characteristically have an
iridescent appearance created by the grooves of the reflective layer. The reverse side of an optical disc usually has a printed label, sometimes made of paper but often printed or stamped onto the disc itself. This side of the disc contains the actual data and is typically coated with a transparent material, usually
lacquer. Unlike the 3½-inch
floppy disk, most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems.
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