Single lens having one aspherical surface

Claim 2

A single lens having one aspherical surface for forming, from a beam of radiation, a diffraction-limited scanning radiation spot on an information surface of a transparent record-carrier substrate which is traversed by the radiation beam and which has a thickness of approximately 1.2 mm, characterized in that the lens has a lens factor, F, such that ##EQU2## and a shape factor, C 2 /C 1 having the following relationship ##EQU3## for less magnifications between M=0 and M=-0.222, the values of the coefficients a, b and c for said magnifications and the lens factor F for various values of the numerical aperture N.A. and the refractive index n of the lens material being given by the following tables: ______________________________________ N.A. = 0.40N.A. = 0.45N.A. = 0.50 abcabcabc ______________________________________ M = 0 ##STR1## ##STR2## ##STR3## ______________________________________ M = -0.222 ##STR4## ##STR5## ##STR6## ______________________________________ where d is the axial thickness of the lens, f the focal length of the lens, C 1 the paraxial curvature of the aspherical surface, and C 2 the curvature of the other lens surface.

Claim 3

A single lens as claimed in claim 1, for which M=0, the numerical aperture N.A.=0.45 and the focal length f=4.5 mm, characterized in that the non-aspherical surface is a plane surface and the paraxial radius of the curvature R of the aspherical surface satisfied the requirement d/R=-5.8n.sup.2 +23.7n-22.9.

Claim 4

A single lens as claimed in claim 1, for which M=0, the numerical aperture N.A.=0.50 and the focal length f=4.5 mm, characterized in that the non-aspherical surface is a plane surface and the radius of curvature R of the aspherical surface satisfies the requirement d/R=-5n.sup.2 +20.5n-19.8.

Claim 5

A single lens as claimed in claim 1, 2 or 3, characterized in that the lens is formed by a glass body.

Claim 6

A single lens as claimed in claim 1, 2 or 3, characterized in that the lens comprises a glass body having one surface which is provided with a layer of a transparent plastic with an aspherical outer profile.

Claim 7

In an apparatus for reading and/or writing information on an optical record carrier having an information surface on a transparent substrate, said apparatus comprising a source for producing a beam of radiation which is directed through the transparent substrate and onto the information surface of the record carrier, and an objective for focusing the beam to a radiation spot on the information surface, the improvement wherein said objective comprises a single lens having one aspherical surface which is directed towards the radiation source, said lens having a lens factor, F, such that ##EQU4## and a shape factor, C 2 /C 1 having the following relationship ##EQU5## for lens magnifications between M=0 and M=-0.222, the values of the coefficients a, b and c for said magnifications and the lens factor F for various values of the numerical aperture N.A. and the refractive index n of the lens material being given by the following tables: ______________________________________ N.A. = 0.40N.A. = 0.45N.A. = 0.50 abcabcabc ______________________________________ M = 0 ##STR7## ##STR8## ##STR9## ______________________________________ M = -0.222 ##STR10## ##STR11## ##STR12## ______________________________________ where d is the axial thickness of the lens, f the focal length of the lens, C 1 the paraxial curvature of the aspherical surface, and C 2 the curvature of the other lens surface.

Claim 8

In an apparatus for reading and/or writing information on an optical record carrier having an information surface on a transparent substrate, said apparatus comprising a source for producing a beam of radiation which is directed through the transparent substrate and onto the information surface of the record carrier, and an objective for focusing the beam to a radiation spot on the information surface, the improvement wherein said objective comprises a single lens having one aspherical surface which is directed towards the radiation source, said lens having a lens factor, F, such that ##EQU4## and a shape factor, C 2 /C 1 having the following relationship ##EQU5## for lens magnifications between M=0 and M=-0.222, the values of the coefficients a, b and c for said magnifications and the lens factor F for various values of the numerical aperture N.A. and the refractive index n of the lens material being given by the following tables: ______________________________________ N.A. = 0.40N.A. = 0.45N.A. = 0.50 abcabcabc ______________________________________ M = 0 ##STR7## ##STR8## ##STR9## ______________________________________ M = -0.222 ##STR10## ##STR11## ##STR12## ______________________________________ where d is the axial thickness of the lens, f the focal length of the lens, C 1 the paraxial curvature of the aspherical surface, and C 2 the curvature of the other lens surface.

Claim 9

The apparatus according to claim 6 wherein M=-0.222.

Claim 10

The apparatus according to claim 6, including a collimating lens disposed in the path of the beam between said source and said objective and wherein M=0.

Claim 11

The apparatus according to claim 8, wherein the numerical aperture N.A.=0.45 and the focal length f=4.5 mm, the non-aspherical surface is a plane surface and the paraxial radius of the curvature R of the aspherical surface satisfied the requirement d/R=5.8n.sup.2 +23.7n-22.9.

Claim 12

The apparatus according to claim 8, wherein the numerical aperture N.A.=0.50 and the focal length f=4.5 mm, the non-aspherical surface is a plane surface and the radius of curvature R of the aspherical surface satisfies the requirement d/R=-5n.sup.2 +20.5n-19.8.