Patent ID: 7842366

Claim:
A multi-layer optical information recording medium comprising four or more information recording layers on which signal information is recorded with any one of information pits whose concave-convex surfaces or phases change and recording marks whose reflectances or phases change, wherein the information recording layers are denoted by the m-th information recording layers, where m is any integer that is 0 or larger, an information recording layer that is farthest from an objective lens of a reproduction optical pickup being denoted by a 0th information recording layer, spacers being interposed between the information recording layers, the spacers being made of a transparent material and having nearly constant thicknesses, wherein when the thickness of the spacer interposed between the m-th information recording layer and an (m+1)th information recording layer is denoted by t m , the thickness of the spacer interposed between the (m+1)-th information recording layer and a (m+2)-th information recording layer is denoted by t m+1 , the difference between the thicknesses t m and t m+1 of the spacers is denoted by Δt m , the sum of the thicknesses of the spacers interposed between the m-th information recording layer and a (m+a)-th, where a is any integer that is 2 or larger, information recording layer is denoted by (t m +t m+1 + . . . +t m+a−1 ), the sum of the thicknesses of the spacers interposed between the (m+a)-th information recording layer and a (m+b)-th, where b is any integer that is 2 or larger, is denoted by (t m+a +t m+a+1 + . . . +t m+b−1 ), and the difference between the sum (t m +t m+1 + . . . +t m+a−1 ) and the sum (t m+a +t m+a+1 + . . . +t m+b−1 ) is denoted by Δt m−sum , then the thicknesses of the spacers are designated so that Δt m and Δt m−sum satisfy formulas (1) and (2): Δ t m >A CCT ×R m+1 ×R m+2 ×T m+1 2 ×nλ /(2 NA 2 ) (1) Δ t m−sum >A CCT ×R m+a ×R m+b ×T ab 2 ×nλ /(2 NA 2 ) (2) where A CCT represents a crosstalk index, R m+1 , R m+2 , R m+a , and R m+b represent an intensity reflectances associated with reflective films formed on the (m+1)-th layer, (m+2)-th layer, (m+a)-th layer, and (m+b)-th layer, respectively, T m+1 represents an intensity transmissivity of the spacer t m+1 , T ab represents an intensity transmissivity from the (m+a)-th information recording layer to the (m+b)-th information recording layer, n represents a refractive index of the spacer material, and NA represents a numerical aperture of the objective lens, wherein the crosstalk index (A CCT ) ranges from 31.6 to 100, and wherein the intensity reflectances associated with reflective films are due to multiple reflections and decrease in a direction closer to an objective lens, and the differences in thicknesses between the spacers can be decreased in the direction closer to the objective lens based on the decreasing intensity reflectances associated with the reflective films.