Patent ID: 8151289

Claim:
An optical disk comprising: a center hole; a first side being an entrance surface with respect to a light beam and a second side, opposite to the first side, being a non-entrance surface; a first area and a second area on the first side; a recording layer located at about 0.1 mm below the surface of the first side; and a projection extending from the surface of the first side on the second area, the projection being located in a diameter equal to or greater than 17 mm and equal to or less than 21 mm; wherein the second area is provided between the center hole and the first area, with an inner diameter of larger than 15 mm and an outer diameter of smaller than 44 mm, wherein the first area is provided with an inner diameter that is larger than the outer diameter of the second area, the projection is formed on a first portion of the second area, the first portion being located at a diameter of larger than 15 mm and nearer the center hole than the first area, and at least a second portion on which the projection is not formed is remained on the second area, the projection having an upper side and a bottom side, width of the upper side being smaller than the bottom side, and both lateral portions of the projection being gradually inclined from an end of the upper side to an end of the bottom side, and the height of the upper side of the projection is equal to or smaller than 0.6 mm, and a flank angle of the projection is greater than 90 degrees and less than 160 degrees, and the projection is configured to prevent the first side from direct contact with a surface on which the optical disk is placed.