Patent ID: 7110344

Claim:
An objective lens for recording and/or reproducing information of an optical information recording medium, comprising: at least one aspherical surface; wherein the objective lens is a single lens and satisfies the following formulas: 0.85<NA<0.98 0.80 <d/f <3.00 0.5<φ1<5.8 0.40<( X 1 −X 2)·( n −1)/( NA·f·√( 1 +|m |))<0.80 where NA is an optical information recording medium side numerical aperture, d is a thickness (mm) on an optical axis, f is a focal length (mm) for an object located at an infinite distance, φ1 is an effective diameter (mm) of a light source side surfaces, X1 is a difference (mm) in the direction of an optical axis between a plane that is perpendicular to the optical axis and is tangent to the vertex of a light source side surface and a light source side surface at the outermost periphery in the effective diameter (at a position on a light source side surface where the marginal ray at the above NA enters), and it is assumed that X1 takes a positive sign when it is measured in the direction toward the optical information recording medium from the tangent plane that serves as a reference point, and it takes a negative sign when it is measured in the direction toward the light source, X2 is a difference (mm) in the direction of an optical axis between a plane that is perpendicular to the optical axis and is tangent to the vertex of an optical information recording medium side surface and an optical information recording medium side surface at the outermost periphery in the effective diameter (at a position on an optical information recording medium side surface where the marginal ray at the above NA enters), and it is assumed that X2 takes a positive sign when it is measured in the direction toward the optical information recording medium from the tangent plane that serves as a reference point, and it takes a negative sign when it is measured in the direction toward the light source, n is a refractive index of the objective lens for a used wavelength, and m is a magnification of the objective lens.