Patent ID: 6026069
Filing Date: 2000-02-15
Classification: G11B

Abstract:
An optical record carrier having at least three information planes and two intermediate layers separating the information planes from one another, wherein the difference between d.sub.1 /n.sub.1 and d.sub.2 /n.sub.2 is at least three times the depth of field of a converging radiation beam for reading a selected one of the information planes by forming a focal spot on the selected plane; in which d.sub.1, d.sub.2 and n.sub.1, n.sub.2 are the thicknesses and refractive indices of the first and second intermediate layers, respectively, and the depth of field is .lambda./(2 NA.sup.2), where .lambda. and NA are the wavelength and numerical aperture of the radiation beam, respectively.