Aspherical single lens

Claim 2

An aspherical single lens whose first and second surfaces each are formed by an aspherical surface and in which, when the distance from an arbitrary point on said aspherical surface to the tangential plane of the vertex of said aspherical surface is x and the distance from said arbitrary point to the optic axis is H and the reference radius of curvature of the νth surface is R.sub.ν and the cone constant of the νth surface is K.sub.ν and the aspherical coefficient of the νth surface is A.sub.νi (i=3, 4, . . . ), said aspherical surface is an aspherical surface represented by the following equation and which satisfies the following conditions (1), (2) and (3): ##EQU6## where F is the focal length of said aspherical single lens, D is the on-axis thickness of said aspherical single lens, and N is the refractive index of said aspherical single lens for the wavelength used.

Claim 3

An aspherical single lens according to claim 1, further satisfying the following conditions (4) and (5): ##EQU7##

Claim 4

An optical head comprising: a light source; and an optical system for directing a light beam from said light source to an information recording medium; said optical system comprising an aspherical single lens whose first and second surfaces each are formed by an aspherical surface and in which, when the distance from an arbitrary point on said aspherical surface to the tangential plane of the vertex of said aspherical surface is x and the distance from said arbitrary point to the optic axis is H and the reference radius of curvature of the νth surface is R.sub.ν and the cone constant of the νth surface is K.sub.ν and the aspherical coefficient of the νth surface is A.sub.νi (i=3, 4, . . . ), said aspherical surface is an aspherical surface represented by the following equation and which satisfies the following conditions (1), (2) and (3): ##EQU8## where F is the focal length of said aspherical single lens, D is the on-axis thickness of said aspherical single lens, and N is the refractive index of said aspherical single lens for the wavelength used.

Claim 5

An optical head comprising: a light source; and an optical system for directing a light beam from said light source to an information recording medium; said optical system comprising an aspherical single lens whose first and second surfaces each are formed by an aspherical surface and in which, when the distance from an arbitrary point on said aspherical surface to the tangential plane of the vertex of said aspherical surface is x and the distance from said arbitrary point to the optic axis is H and the reference radius of curvature of the νth surface is R.sub.ν and the cone constant of the νth surface is K.sub.ν and the aspherical coefficient of the νth surface is A.sub.νi (i=3, 4, . . . ), said aspherical surface is an aspherical surface represented by the following equation and which satisfies the following conditions (1), (2) and (3): ##EQU8## where F is the focal length of said aspherical single lens, D is the on-axis thickness of said aspherical single lens, and N is the refractive index of said aspherical single lens for the wavelength used.

Claim 6

An optical head according to claim 3, further satisfying the following conditions (6) and (7): -0.47<-0.16 (6) 1.5<D 1/F NA<2.5 (7).