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Structure of the medium, data recording Micrograph of recesses and fields Diagram of a cross-section of a pressed plate A standard compact disc, often referred to as an audio CD, stores the digital audio record in a "red book" standard to distinguish it from later variants. Compact discs are made of a 1.2 mm thick polycarbonate plate with a diameter of 12 cm, covered with a thin layer of aluminum (aluminum), in which information is contained (in the form of a combination of micro-grooves and places without them). They are read with a semiconductor laser (AlGaAs) with a wavelength of about 780 nm. The recording creates a spiral path from the center to the edge of the record.