Patent ID: 8339918

Claim:
An optical reading method for reading information recorded on an information recording layer by a PRML detection method by irradiating an optical recording medium, in which a distance from a light incident surface to the information recording layer is less than 100 μm, with a laser beam through an objective lens, wherein the distance from the light incident surface to an information recording layer which is the nearest from the light incident surface is 80 μm or less; and a constraint length n in the PRML detection method is set to an integer which satisfies 0.5×(λ/NA)/(LV/f)−1<n<0.5×(λ/NA)/(LV/f)+1, wherein λ represents a wavelength of a laser beam, NA represents a numerical aperture of the objective lens, LV represents a linear velocity of the optical recording medium when recording, and f represents a channel bit frequency when recording.