A phase change disc, as its name implies, is a media that performs recording and overwriting through the reversible "phases" of its recording layer. In the recorded area, the recording material is in an amorphous (non-crystalline) phase, having a comparatively low reflectance ratio. Conversely, the erased areas or non-recorded areas are in a crystalline phase, with a comparatively high reflectance ratio. Playback is performed by reading the differences in the reflectance ratio of these two phases while tracking the groove.
The recorded areasare irradiated by a comparatively high-powered laser beam (recording power level),as per the arrow, using the appropriate level of modulation, thus causing the recording material to rise to a temperature approaching the fusion point. It then cools rapidly. Through this, it is frozen in a random form, causing it to become amorphous. The erased segments and the temperature is raised to a level where it can be converted back to crystallized form, then it is cooled gradually. This is how the amorphous phase is changed to the crystalline phase.