An optical recording technique that uses dyed plastic layers as the recording medium. WORM disks typically use a single dye polymer layer. Erasable disks use two layers: a top retention layer and a bottom expansion layer. A bit is written by shining a laser through the retention layer onto the expansion layer, which heats the area and forms a bump that expands into the retention layer. The retention layer bumps are the actual bits read by the unit. To erase a bit, a laser with a different wavelength strikes the retention layer, and the bump subsides.