Progressive power ophthalmic lens

Abstract

An improved ophthalmic lens for presbyopia is disclosed in which the refractive power is progressively changed to provide a natural visual target arrangement. In the opthalmic ophthalmic lens, rotation of the head of a wearer for binocular lateral vision is taken into account to permit comfortable binocular lateral vision closer to vision with the naked eyes.

Description

This invention relates to orthogoanltarger target for the right eye and the curve D_L ' representing the visual target for the left eye, as shown in FIG. 6.

In the consideration of binocular vision, one of the eyes may be dominant over the other, that is, the "dominant eye" may be present. In such a case, it can be easily surmised that the effect of the position of the visual target corresponding to the eyeball of the "dominant eye" will be greater than that of the position of the visual targer target viewed with binocular vision. However, it may generally suffice to consider that a curve D" lying intermediate between the visual target curves D_L ' and D_R ' for the left and right eyes respectively, as shown in FIG. 6, provides the position of the visual target in the case of binocular vision.

It is to be noted that this curve D" differes differs greatly from the curve C in FIG. 2 showing the visual target position in the prior art ophthalmic lens.

In FIG. 6, a visual target Pi" is shown displaced rightward from the front-viewing position Po on the curve D". As this visual target Pi" moves infinitely rightward from the position Po, the angle α" of binocular vision for viewing the visual target located at the position Pi", that is, the angle <O_L Pi"O_R approaches progressively to zero. This progressive approach of the angle of binocular vision toward zero means that the relative convergence of the two eyes approaches progressively to zero. The angle of binocular vision attains finally the value of zero when β=90° or β_E =45° in the illustrated example. In summary, the relative convergence of the two eyes continues to progressively decrease as the lens wearer turns his eyes progressively in the lateral direction from the condition viewing a visual target located at a finite distance in front of him, and the relative convergence is finally reduced to zero when the eyeballs of the eyes of the lens wearer are directed to view a visual target located in the extreme lateral direction in which β =90°, that is, when the ophthalmic lenses are directed to view a visual target located in the lateral direction in which β_E is about 45°.

We will discuss the passing positions of the fixation lines on the ophthalmic lenses, that is, the positions on the ophthalmic lenses through which the wearer views a visual target disposed in the lateral direction.

Referring to FIG. 7, reference numerals 71 and 72 designate left and right ophthalmic lenses respectively when viewed from the side of a first surface facing a visual target.

A thick solid curve M--M' is shown on each of the lenses 71 and 72. This curve M--M' is obtained by connecting the passing positions of the fixation line of the corresponding eye of the lens wearer for both of distant vision and near vision when he views a visual target disposed directly in front of him. Thus, this curve M--M' coincides normally with the aforementioned umbilical meridian curve.

A straight line L--L' is also shown on each of the lenses 71 and 72. This line L--L' extends vertically through the passing positions of the fixation line of the associated eye of the lens wearer for distant vision and is called the meridian curve.

A line S--S' is also shown on each of the lenses 71 and 72. This line S--S' is obtained by vertically connecting the passing positions of the fixation line of the associated eye of the lens wearer when the fixation line is diverted toward the right through an angle of 45°.

When the lens wearer views, through the ophthalmic lenses shown in FIG. 7, a visual target disposed at a finite distance in front of him, the two eyes converge more or less, and the fixation lines pass on the umbilical meridian curves M--M' instead of the meridian curves L--L'. Suppose that this visual target moves away progressively rightward in the horizontal direction. Then, the fixation lines move progressively rightward through an angle of 45° until finally they pass on the lines S--S' described above. In the course of the above manner of fixation line movement, the moving distance of the fixation line of the right eye on the associated ophthalmic lens is longer than that of the left eye. Conversely, the moving distance of the fixation line of the left eye is longer than that of the right eye when the visual target moves away progressively leftward in the horizontal direction. Thus, when both of these two cases are considered, it can be concluded that the moving distance of the fixation line of each of the eyes on the associated ophthalmic lens is longer on the temporal side than on the nasal side when the lens wearer viewing with the two eyes a visual target disposed at a finite distance in front of him diverts the eyes laterally in the horizontal direction for binocular lateral vision. It is preferable that the lens 71 for the left eye and the lens 72 for the right eye are in the form of mirror images of each other, that is, they are symmetrical with each other on opposite sides of the nose.

It is also preferable that the fixation lines of the two eyes for binocular vision pass on the ophthalmic lenses at such positions at which the factors of refraction (mean refractive power, amount of astigmatism, directions of major axes of astigmatism, etc.) with respect to one of the eyes are approximately equal to those with respect to the other.

Therefore, it is preferable that the distributions of the factors of refraction in the ophthalmic lenses 71 and 72 shown in FIG. 7 are mirror images of each other; that the factors of refraction in each of the lenses 71 and 72 are so distributed as to be symmetrical with each other relative to the meridian curve L--L' in the horizontal direction in the region where the umbilical meridian curve M--M' overlaps the meridian curve L--L'; and that the factors of refraction in each of the lenses 71 and 72 change more gradually in the horizontal direction on the temporal side than on the nasal side in the region where the umbilical meridian curve M--M' is displaced more or less toward the nasal side relative to the meridian curve L--L'. It is also preferable that the factors of refraction in each of the lenses 71 and 72 are symmetrical with each other relative to the meridian curve L--L' in the zones spaced apart by a "predetermined distance" of, for example, 15 mm from the meridian curve L--L' in the horizontal direction. This "predetermined distance" can be determined for each of individual points on the curve L--L'.

A preferred embodiment of the present invention will now be described. Referring to FIG. 7, the left-eye ophthalmic lens made according to the present invention is generally designated by the reference numeral 71, and the view is taken from the side of a visual target.

A point O is the geometrical center of the lens 71, and another point N shown on the lens surface is located beneath the center O at a position spaced apart by a vertical distance of 14 mm from the horizontal line passing through the center O and a horizontal distance of 2.5 mm toward the nasal side from the meridian curve L--L'.

In the illustrated lens 71, the area upper than the horizontal line passing through the point O serves as the region for distant vision, and the area lower than the horizontal line passing through the point N serves as the region for near vision. The remaining area, that is, the area lower than the horizontal line passing through the point O and upper than the horizontal line passing through the point N provides the region for intermediate vision. The line L--L' represents the aforementioned meridian curve passing through the point O, and the curve M--M' represents the aforementioned umbilical meridian curve passing through both of the point O and the point N. The distribution of refractive power on this umbilical meridian curve M--M' is such that the refractive power on the portion M-O has a constant value D_F, the refractive power on the portion N-M' has a constant value D_N, and the refractive power on the portion O-N increases progressively from D_F to D_N. The straight lines S--S' and T'T' are located in a relation parallel and symmetrical with respect to the meridian curve L--L', and the horizontal distances from the meridian curve L--L' are equal to each other or 23 mm in the preferred embodiment of the present invention. The regions outer relative to the straight lines S--S' and T--T' provide planes symmetrical with each other relative to the meridian curve L--L' in the horizontal direction.

The shape of the surface of the lens 71 according to the present invention is defined by an envelope of a group of sectional curves when the lens is sectioned in the horizontal direction by planes passing through a plurality of arbitrarily selected points Gi on the umbilical meridian curve M--M'.

As described hereinbefore, the radius of curvature of each individual curve at the point Gi is so determined that the point Gi provides the umbilic. An arc will be the simplest form of the sectional curve. In fact, the initially employed form of the sectional curve was an arc in the embodiment of the present invention, and the shape of the sectional curve was modified by taking into account the distribution of the factors of refraction (means mean refractive power, amount of astigmatism, directions of major axes of astigmatism, etc.) described later. In this connection, the radius of curvature at the point Gi is preferably excepted from the modification so as to maintain the function of the point Gi as the umbilic. To find the two major radii of curvature and their axial directions at an arbitrarily selected point on an envelope of provisionally determined sectional curves in a manner as described above is well known as the Gauss' differential theory of surfaces.

The two major radii of curvature can be converted into the refractive power, whose unit is the diopter, by the following equation well known in this field of art:

D=(N-1)/R

where D is the refractive power whose unit is the diopter, R is the radius of curvature whose unit is the meter, and N is the index of refraction of the lens, having no unit. The arithmetic means mean of the two values of the refractive power thus calculated gives the mean refractive power, and the difference therebetween gives the amount of astigmatism. The axial directions of astigmatism coincide with the axial directions of the major radii of curvature above described. After the calculation of the distribution of the factors of refraction in the manner above described, the shape of the surface of the lens 71 is determined by modifying the factors of refraction in such a manner that the factors change less more gradually from the umbilical meridian curve M--M' toward the temporal side in the horizontal direction than from the umbilical meridian curve M--M' toward the nasal side in the horizontal direction in the region where the umbilical meridan curve M--M' is displaced more or less toward the nasal side relative to the meridian curve L--L', as described already. While the lens 71 for the left eye has only been referred to in detail hereinbefore, it is apparent that the same applies also to the lens 72 for the right eye.

Thus, the both sides of spectacles 71 and 72 can be shaped to have the visual-target viewing surfaces which are mirror images of each other. That is, their lens surface configurations are the same in the region provided for distant vision and are symmetrical with each other in the regions provided for intermediate vision and near vision. The distribution of the factors of refraction at the surface of the lens 71 for the left eye shown in FIG. 7 is such that the values of refractive power on curves T₁ --T₁ ', T₂ --T₂ ', T₃ --T₃ ' and T₄ -T₄ ' depicted on the temporal side are approximately equal in the horizontal direction to those of corresponding curves S₁ --S₁ ', S₂ --S₂ ', S₃ --S₃ ' and S₄ --S₄ ' depicted on the nasal side, respectively. The same applies also to the lens 72 for the right eye. At a glance on FIG. 7, it will be readily seen that, in the region in which the umbilical meridian curve M--M' is more or less displaced toward the nasal side relative to meridian curve L--L' in each of the lenses 71 and 72, the factors of refraction change less more gradually in the portion closer to the temporal side relative to the umbilical meridian curve M--M' than the portion closer to the nasal side relative to the umbilical meridian curve M--M'. The lens surface designed according to the present invention can be formed on a piece of suitable lens material by any one of suitable methods employed hitherto in this field of art.

By way of example, the lens surface according to the present invention may be divided into a matrix of 0.5 mm×0.5 mm squares, and the data of cutting depths at the individual intersections may be stored in a memory provided for a numerically-controlling milling machine, the lens material being then cut with the milling machine so as to obtain a relatively rough lens surface. The relatively rough lens surface may then be ground with a sheet of soft grinding cloth, followed by successive steps of polishing with abrasives of gradually reduced grain sizes, until finally the desired completely polished lens surface can be obtained.

Claims

1. An ophthalmic lens having two refractive surfaces, one of said refractive surfaces including an imaginary first meridian curve (M--M') called an umbilical meridian curve extending substantially in the vertical direction along said refractive surface when said refractive surface is viewed from a direction substantially orthogonal with respect thereto in the condition in which said lens stands in the same vertical direction as that mounted on a wearer, the distribution of the radius of curvature of said umbilical meridian curve (M--M') including a zone in which the radius of curvature decreases gradually from an upper portion toward a lower portion of said curve according to a predetermined rule, the radii of curvature at the intersections of orthogonal curves crossing at right angles with said umbilical meridian curve (M--M') in said refractive surface being substantially equal to the radii of curvature of said umbilical meridian curve (M--M') at said intersections respectively so that the astigmatism along said umbilical meridian curve (M--M') in said refractive surface is almost equal to zero, said umbilical meridian curve (M--M') dividing said refractive surface into two lateral areas closer to the nasal side and temporal side respectively when said lens is mounted on the wearer, said two lateral areas of said refractive surface being asymmetrical with each other, said refractive surface being such that, when a second meridian curve (L--L') extendiang extending in the vertical direction along said refractive surface to overlap, intersect or contact with said umbilical meridian curve (M--M') in an upper region of said refractive surface is imagined, said umbilical meridian curve (M--M') is displaced toward the nasal side relative to said second meridian curve (L--L') in a lower region of said refractive surface, while it is less gradually displaced toward the nasal side relative to said second meridian curve (L--L') in an intermediate region of said refractive surface, said intermediate and lower regions in which said umbilical meridian curve (M--M') is displaced more or less toward the nasal side relative to said second meridian curve (L--L') including at least one sectional curve which extends in the horizontal direction within a range of not more than 15 mm on opposite sides of said umbilical meridian curve (M--M') and along which the distribution of astigmatism on the nasal side relative to said umbilical meridian curve (M--M') is asymmetrical with that on the temporal side.

2. An ophthalmic lens having two refractive surfaces, one of said refractive surfaces including an imaginary first meridian curve (M--M') called an umbilical meridian curve extending substantially in the vertical direction along said refractive surface when said refractive surface is viewed from a direction substantially orthogonal with respect thereto in the condition in which said lens stands in the same vertical direction as that mounted on a wearer, the distribution of the radius of curvature of said umbilical meridian curve (M--M') including a zone in which the radius of curvature decrease gradually from an upper portion toward a lower portion of said curve according to a predetermined rule, the radii of curvature at the intersections of orthogonal curves crossing at right angles with said umbilical meridian curve (M--M') in said refractive surface being substantially equal to the radii of curvature of said umbilical meridian curve (M--M') at said intersections respectively so that the astigmatism along said umbilical meridian curve (M--M') in said refractive surface is almost equal to zero, said umbilical meridian curve (M--M') dividing said refractive surface into two lateral areas closer to the nasal side and temporal side respectively when said lens is mounted on the wearer, said two lateral areas of said refractive surface being asymmetrical with each other, said refractive surface being such that, when a second meridian curve (L--L') extending in the vertical direction along said refractive surface to overlap, intersect or contact with said umbilical meridian curve (M--M') in an upper region of said refractive surface is imaged, said umbilical meridian curve (M--M') is displaced toward the nasal side relative to said second meridian curve (L--L') in a lower region of said refractive surface, while it is less gradually displaced toward the nasal side relative to said second meridian curve (L--L') in an intermediate region of said refractive surface, said intermediate and lower regions in which said umbilical meridian curve (M--M') is displaced more or less toward the nasal side relative to said second meridian curve (L--L') including refractive surface portions which are symmetrical with each other relative to a plane including said second meridian curve (L--L') and which are included in two lateral areas spaced apart by not less than 17.5 mm from said second meridian curve (L--L') in the horizontal direction respectively.

3. An ophthalmic lens as claimed in claim 1 or 2, wherein the astigmatism along said umbilical meridian curve (M--M') is not less than zero but not more than 0.25 diopters.

4. An ophthalmic lens according to claim 1, wherein the distribution of astigmatism changes more gradually in the horizontal direction on the temporal side than on the nasal side in the region where the umbilical meridian curve (M--M') is displaced toward the nasal side relative to the meridian curve (L--L').