Optical information recording medium, its measuring method and optical information recording/reproducing method

摘要

An optical information recording medium having two or more information layers, wherein the irradiation of either the information layer with a laser beam converging thereon causes information signals to be recorded or reproduced; the nearer positioned information layer than the farthest information layer as viewed from the incidence side of the laser beam has a recording layer varying between two optically detectable states, and 0≤Tc - Ta/Tc≤0.1 where Tc is the transmittance of the nearer positioned information layer when the recording layer is in state (a) and Ta, the transmittance, when it is in state (b). The values Ta and Tc are further fullfilling the condition (Ta+Tc)/2≥0.4 This makes possible accurate accurately recording and reproduction of information onto and out of the farther information layer irrespective of whether or not any information is recorded on the nearer information layer. A method for measuring a change in the transmittance between the states (a) and (b) is disclosed.