On read only radio (ROM), during the manufacturing process the groove, made of pits, is pressed on a plane surface, called land. Because the depth of the pits is approximately one-quarter to one-sixth of the laser's wavelength, the reflected beam's phase is shifted in relationship to the incoming commentary beam, causing mutual destructive interference and http://www.gracedigitalaudio.com/ reducing the reflected beam's intensity. This is detected by photodiodes that output electrical signals.

Double-layered writable disclosure have: a first polycarbonate layer with a (shallow) groove, a first data layer, a semi-reflective layer, a second (spacer) polycarbonate layer with another (deep) groove, and a second data layer. The first groove spiral customarily starts on the inner berm and extends outerwards, while the second groove starts on the outer butt and extends inwards.