Patent ID: 7218603

Claim:
A multi-stack optical data storage medium for rewritable recording using a focused radiation beam entering through an entrance face of the medium during recording, comprising: a substrate with present on a side thereof: an L 0 recording stack comprising a phase-change type L 0 recording layer, said first recording stack being present at a position closest to the entrance face and having an optical transmission of T L0a when the phase-change layer is in the amorphous phase, and having an optical transmission of T L0c when the phase-change layer is in the crystalline phase, an L 1 recording stack, comprising a phase-change type L 1 recording layer, being present more remote from the entrance face than the L 0 recording stack, a transparent spacer layer between the recording stacks, said transparent spacer layer having a thickness substantially larger than the depth of focus of the focused laser-light beam, the medium further comprising pre-recorded information, characterized in that the pre-recorded information contains a flag whether formatting of the L 0 recording layer of the L 0 recording stack is needed depending on the transmission values T L0a and T L0c Of the L 0 stack.