Dual stack optical data storage medium and use of such medium

摘要

A dual-stack optical data storage medium ( 20 ) is described for read out using a focused radiation beam ( 29 ) with a wavelength of 400-410 nm and a Numerical Aperture (NA) of 0.84-0.86. The medium has a substrate ( 21 ) and a first stack of layers named L 0 ( 22 ) comprising a first information layer and a second stack of layers named L 1 ( 23 ), comprising a second information layer. A radiation beam ( 29 ) transparent spacer layer ( 24 ) is present between L 0 and L 1. A transmission stack named TS 0 with a thickness d_TS0 and an effective refractive index n_TS0 contains all layers between L 0 and an entrance face ( 26 ) of the medium ( 20 ). A transmission stack named TS 1 with a thickness d_TS1 and an effective refractive index n_TS1 containing all layers between L 1 and the entrance face ( 26 ). The spacer layer ( 24 ) has a thickness selected from the range 20-30 mum, the thickness d_TS0 in dependence on the refractive index n_TS0 and the thickness d_TS1 in dependence on the refractive index n_TS0 are within a specified area. In this way a reliable read out of both the first and the second information layer of respectively L 0 and L 1 is achieved.