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Compact discs are made from a 1.2 mm thick disc of very pure polycarbonate plastic. A thin layer of Super Purity Aluminium is applied (or rarely gold, used for its data longevity, such as in some limited-edition audiophile CDs) to the surface to make it reflective, which is protected by a film of lacquer. The lacquer can be printed with a label. Common printing methods for compact discs are silkscreening and offset printing. CD data is stored as a series of tiny indentations (pits), encoded in a tightly packed spiral track of pits moulded into the top of the polycarbonate layer. The areas between pits are known as 'lands'. Each pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 ?m of length. The spacing between the tracks, the pitch, is 1.6 ?m. A CD is read by focusing a 780 nm wavelength semiconductor laser through the bottom of the polycarbonate layer. The difference in height between pits and lands leads to a phase difference between the light reflected from a pit and from its surrounding land. By measuring the intensity with a photodiode, one is able to read the data from the disc. The pits and lands themselves do not directly represent the zeroes and ones of binary data. Instead, Non-return-to-zero, inverted encoding is used: a change from pit to land or land to pit indicates a one, while no change indicates a zero. This in turn is decoded by reversing the Eight-to-Fourteen Modulation used in mastering the disc, and then reversing the Cross-Interleaved Reed-Solomon Coding, finally revealing the raw audio data stored on the disc.
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