Patent ID: 7573802

Claim:
A dual-stack optical data storage medium for write-once 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 L 0 comprising a write-once type first recording layer having a complex refractive index ñ L0 =n L0 −i.k L0 and having a thickness d L0 , said first recording stack L 0 having an optical reflection value R L0 and an optical transmission value T L0 , a second recording stack L 1 comprising a write-once type second recording layer having a complex refractive index ñ L1 =n L1 −i.k L1 and having a thickness d L1 , said second recording stack L 1 having an optical reflection value R L1 , all parameters being defined at the wavelength λ, said first recording stack being present at a position closer to the entrance face than the second 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 0.45≦T L0 ≦0.75 and 0.40≦R L1 ≦0.80 and k L0 <0.3 and k L1 <0.3.