Patent ID: 7602694

Claim:
A dual-stack optical data storage medium for recording using a focused radiation beam having a wavelength λ and entering through an entrance face of the medium during recording, comprising: at least one substrate with present on a side thereof: a first recording stack named L 0 , comprising a recordable type L 0 recording layer, said first recording stack L 0 having an optical reflection value R L0 and an optical absorption value A L0 at the wavelength λ, a second recording stack named L 1 comprising a recordable type L 1 recording layer, said second recording stack L 1 having an optical reflection value R L1 and an optical absorption value A L1 at the wavelength λ, said second recording stack being present closer to the entrance face than the first recording stack, a transparent spacer layer sandwiched between the recording stacks, said transparent spacer layer having a thickness substantially larger than the depth of focus of the focused radiation beam, wherein the first recording stack and the second recording stack are formed such that A L1 ≦1−R min −√(R min /R L0 ) in which formula R min is the minimum required effective optical reflection value for each recording stack for the medium to be compliant with a read-only version of the medium.