Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks
Patent
1993-06-03
1995-12-26
Sugarman, Scott J.
Optics: eye examining, vision testing and correcting
Spectacles and eyeglasses
Ophthalmic lenses or blanks
351176, G02C 702
Patent
active
054792207
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an eyeglass lens, and more particularly to an eye glass lens for eyesight correction which is reduced in lens thickness thereby to decrease its weight and which is improved in optical properties by removing the aberration.
BACKGROUND ART
It is necessary for correcting astigmatic vision by an eyeglass lens that at least one surface of lens is formed as a refracting surface with curvatures different depending upon the direction, which will be called as an astigmatic surface. Conventionally, a cylindrical surface or a toric surface has been employed as the astigmatic surface. The toric surface is defined as follows (referring to FIG. 2).
Consider a curve expressed by z=f(y) on the YZ plane in FIG. 2. This curve may be a circle, a quadric curve, or any of other curves. A rotation axis M parallel to the Y axis is then taken to pass through a point R.sub.x on the Z axis, and the curve z=f(y) is revolved around the above rotation axis M to obtain a curved surface, which is referred to as the toric surface.
When the surface is cut by the YZ plane a curve appearing in the section is called as a Y principal meridian, whereas when the surface is cut by the XZ plane a curve appearing in the section is called as an X principal meridian. As apparent from the definition, the y principal meridian of toric surface is the curve expressed by z=f(y), and the X principal meridian is a circle with radius R.sub.x.
In this example the rotation axis is described as parallel to the Y axis, but in another case the rotation axis may be defined as being parallel to the X axis in the same manner.
The restriction from processing has heretofore limited the shape of toric surface actually employed only to those with f(y) being a circle. There are two types of shapes in this case depending upon a relative magnitude between R.sub.x and R.sub.y and upon the direction of rotation axis. In case that R.sub.x <R.sub.y, the surface is of a barrel type (as shown in FIG. 3A) if the rotation axis M is parallel to the Y axis, while it is of a tire type (as shown in FIG. 3B) if the rotation axis M is parallel to the X axis. In case that R.sub.x >R.sub.y, the situation is reverse. Although there are the two types of shapes depending upon whether the rotation axis is made parallel to the X axis or to the Y axis with a single combination of RX and R.sub.y, no other degree of freedom exists. Accordingly, provision of R.sub.x and R.sub.y completely determines the two types of surface shapes.
Also, a conventional eyeglass lens employs a combination of a spherical surface with a toric surface, which is unsatisfactory in respect of aberration correction. Thus, the combination had problems such as the uncorrectable residual aberration and a weight increase of lens. Then, there are various shapes of refracting surface proposed, trying to achieve a satisfactory aberration correction.
For example, Japanese Patent Laying-open Application No. 64-40926 discloses a low aberration eyeglass lens, which has a refracting surface expressed by the following equation with r a distance from the origin: ##EQU3## where n is an even number satisfying 4.ltoreq.n.ltoreq.10.
If A.sub.n =0 in Equation (a), the equation has only the first term, which represents a quadratic surface. C is determined with a radius of curvature at r=0, and the shape is determined by a value of K.
The first term in Equation (a) represents the following surfaces depending upon the value of K:
FIG. 4 shows how the radius of curvature changes depending upon the value of K if Equation (a) has only the first term. As seen from FIG. 4, the radius of curvature continuously increases with K<0, while it continuously decreases with K>0.
Although it monotonously increases or decreases in case of A.sub.n =0, various changes may be effected by using higher order terms.
FIG. 5 shows a change in radius of curvature with change of A.sub.8 value in case of K=-1. As seen, the change in radius of curvature may be adjusted by values of K and A.sub.
REFERENCES:
Patent Abstracts of Japan vol. 13, No. 250 (P-882) 12 Jun. 1989.
Katz, M., "Aspherical surfaces used to minimize . . . " Applied Optics, vol. 21, No. 16, Aug. 1982, N.Y. pp. 2982-2990.
Katada Toshiharu
Komatsu Akira
Yokoyama Osamu
Seiko Epson Corporation
Sugarman Scott J.
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