Patent ID: 7172797
Filing Date: 2007-02-06
Classification: G11B,Y10T

Abstract:
1. An optical information medium for recording by means of a focused radiation beam having a radiation wavelength · and a numerical aperture NA, said medium having a substrate, a stack of layers provided thereon, the stack comprising at least a first recording stack and k radiation beam transmissive layers, each radiation beam transmissive layer having a refractive index n i and an average thickness d i ·m and 1·i·k and k·2, characterized in that the average thickness d k of transmissive layer k satisfies the equation and D(n) represents the thickness versus refractive index function in ·m, of a single layer radiation beam transmissive layer causing minimal spherical wavefront aberration in the focal point of the focused radiation beam, said focal point being at the recording layer of the first recording stack.