Contact Lens Providing a Central Spherical Region Bounded by a Circumscribing Aspherical Region

The present invention relates to a contact lens comprising a rear surface presenting a spherical region described by an arc passsing through the vertex.

Such a known contact lens comprises a spherical rear surface having a diameter of approximately 7 to 9 mm, and a marginal zone adjacent thereto. This type of lens offers the advantage of a large optical zone. The seating of this lens is less favorable as the cornea has an aspherical surface. Furthermore, contact lenses with an elliptical rear surface are known. These lenses have a better seating than lenses with a spherical rear surface. Finally, contact lenses are known with a plurality of mutually adjacent optical zones. At each boundary between respectively neighboring optical zones a kink or bend occurs which leads to reflexes and imaging disturbances.

It is the object of the invention to provide a contact lens of the above-described type, which comprises a central spherical region and an aspherical region adjacent thereto, which has particularly good compatibility and adaptability properties.

This object is achieved with a contact lens of the type described by way of introduction, which according to the invention is characterized by the fact that an aspherical region, which can be represented by a curve, is adjacent to the spherical region, said curve and said arc having a substantially equal radius of curvature and a substantially equal gradient in the area of transition. This design achieves that no step occurs at the boundary between the spherical and the aspherical region.

Due to the small spherical central region, the contact lens is given imaging properties which are comparable to those achieved with purely spherical rear surfaces. Especially with an offset lens seating or with the movement of a lens after a palpebral twitch this fact becomes apparent by a stable acuity of vision. With a cornea which strongly flattens in its periphery, e.g. in the event of keratoconus, an optimum acuity of vision can be achieved as well with such a lens design, which is not possible with other aspherical rear surfaces such as ellipsoids or paraboloids with a high eccentricity. Furthermore, it is possible to vary the vertex curvature radius of the spherical region and the eccentricity separately from one another so that a better adaptation to the individual corneal shape will be possible. This results in its turn in a better exchange of tear fluid and good spontaneous and long-term compatibility.

Further features and expedient aspects of the invention will proceed from the description of an embodiment with reference to the figures.

In the drawing:

FIG. 1 is a section through a contact lens; and

FIG. 2 shows a curve representing the rear surface of the contact lens, beginning from the vertex thereof.

The rear surface 2 is defined by a curve 1 in a XY coordinate system. The rear surface of the contact lens is achieved by the rotation of this curve 1 about the y-axis. The coordinates of the vertex S of the curve are x=0, y=0. x=0, y=r.sub.o applies to the center of curvature M. The following quantities are defined at any point P (X/Y) of curve 1:

.alpha. is the angle which the normal encloses with the y-axis at point P. .alpha. is thus the gradient angle of the tangent at pint P.

y' is the tangent gradient tan .alpha..

The sagittal radius r.sub.s =length of the normal from point P onto the y-axis.

.epsilon. is the linear eccentricity defined by the equation: ##EQU1## Within the angular area .alpha..sub.1 the surface is an exact sphere with r.sub.s =r.sub.o and .epsilon.=0. .alpha..sub.1 ranges between 10.degree. and 20.degree., and preferably at 15.degree. approximately. The curve of the arc in the area from the vertex to the point of transition A, with x=a and y=b, is governed by the following relationship:

In this relationship, the power series of the sphere is in congruence with the gradient and the radius of curvature at point A, which means that .epsilon.=0. With an angle .alpha..sub.2 the eccentricity reaches the value .epsilon..sub.k. The constants c.sub.o through c.sub.3 are governed by the following relationships: ##EQU3## A preferred embodiment will be given below, wherein r.sub.o =8 mm, .alpha..sub.1 =14.degree., .alpha..sub.2 =30.degree. and .epsilon..sub.k =0.6.

In this embodiment applies: ##EQU4##