Aspherical spectacle lens with geometric center spaced from...

Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks

Reexamination Certificate

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C351S175000, C351S177000

Reexamination Certificate

active

06796653

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a single-vision aspherical spectacle lens to correct eyesight and a processing method thereof.
In general, a spectacle lens is custom-made to meet the customer's specification. However, it takes long time to process both front and back surfaces after receiving the customer's order. Therefore, semifinished lens blanks whose front surfaces are finished are stockpiled and a back surface of the selected semifinished lens blank is processed according to the customer's specification in order to shorten delivery times. The lens whose front and back surfaces are processed is an uncut finished lens. The uncut finished lens is edged according to a shape of a frame to obtain an edged lens.
During the processing of the spectacle lens, it is necessary to define a framing reference point that is a reference point when the lens is installed on a frame. The framing reference point is coincident with a pupil position of a user when the spectacle lens is installed on a frame. The framing reference point is coincident with an optical center and is located on an optical axis when the lens does not include a prism for correcting hereophoria (visual axes are deviated from each other during idle period). Further, when the lens includes the prism, the framing reference point is coincident with a prism reference point at which the design prismatic power is obtained.
A semifinished lens blank
1
has a circular outline shape as shown in FIG.
7
. In general, a back surface of the lens blank
1
is processed under the condition where a geometrical center
2
is coincident with the framing reference point
3
to ease the processing. The uncut finished lens is, as shown in
FIG. 8
, edged according to a shape of a frame to be an edged lens
4
. The framing reference point
3
, which is coincident with the geometrical center
2
, will be in agreement with a pupil position
5
of a user.
However, when the frame size is too large or an interpupillary distance is too short, the framing reference point
3
is largely decentered from a boxing center
4
′ of the edged lens
4
as shown in FIG.
9
. The boxing center
4
′ is the center of a recrangle that is circumscribed around the edged lens
4
. In such a case, when the framing reference point
3
is coincident with the geometrical center
2
of the semifinished lens blank
1
as described above, the planed shape of the edged lens
4
will be protruded from the semifinished lens blank
1
, which makes the processing impossible.
Therefore, a decentering processing is known as a prior art to process spherical lenses whose front and back surfaces are spherical. In the decentering processing, the semifinished lens blank
1
is processed under the condition where the framing reference point
3
is decentered from the geometrical center
2
of the semifinished lens blank
1
as shown in FIG.
10
. As a result, the planed shape of the edged lens
4
will remain within the semifinished lens blank
1
even if the framing reference point
3
is decentered from the boxing center
4
′ of the edged lens
4
.
During cutting or grinding process in the decentering processing, as shown in
FIG. 11
, the semifinished lens blank
1
is attached to a blocking jig
6
of a processing device, and a prism spacer
10
having a wedge shape is inserted between the blocking jig
6
and a rotating member (not shown) to incline the front surface
1
a
of the semifinished lens blank
1
. In another example, the semifinished lens blank
1
is attached to the blocking jig
6
such that the geometrical center
2
thereof is decentered from the rotation axis
7
as shown in FIG.
12
.
The back surface
1
c
of the uncut finished lens
1
′ processed by the decentering processing is shown as broken lines in
FIGS. 11 and 12
. The framing reference point
3
is decentered from the geometrical center
2
of the uncut finished lens
1
′. When the spectacle lens under the processing is a spherical lens, since the optical axis, which is perpendicular to both of the front and back surfaces
1
a
and
1
c
of the uncut finished lens
1
′, intersects the front surface
1
a
at the framing reference point
3
, the optical performance of the decentering lens processed by the decentering processing is equal to that of the non-decentering lens whose framing reference point
3
is coincident with the geometrical center
2
of the uncut finished lens
1
′.
On the other hand, when the spectacle lens under the processing is an aspherical lens whose front surface is aspherical, the situation becomes different. As shown in
FIG. 13
, a semifinished lens blank
11
for an aspherical lens has a front surface
11
a
that is finished as a rotationally symmetrical aspherical surface and a back surface
11
b
. The back surface
11
b
is processed to be a spherical surface or a toric surface to obtain an uncut finished lens. The symmetry axis
12
of the aspherical front surface
11
a
intersects the front surface
11
a
at the geometrical center
13
of the semifinished lens blank
11
. In order to reduce cost by limiting the number of molding dies, there was no other choice but to conform the symmetry axis
12
to the geometrical center
13
.
However, since the above-described conventional aspherical lens is designed to deliver the best optical performance under the condition where the symmetry axis
12
of the aspherical surface
11
a
intersects the framing reference point
3
that is coincident with the optical center, if the symmetry axis
12
is decentered from the framing reference point
13
, the optical performance will be significantly degraded.
Namely, if the conventional semifinished lens blank
11
for the aspherical lens is processed by the decentering processing that is same as for the spherical lens, an uncut finished lens
11
′ as shown in
FIG. 14
will be formed. Since the optical axis
16
that is perpendicular to both of the front and back surfaces
11
a
and
11
c
of the uncut finished lens
11
′ and intersects the framing reference point
15
will be decentered from the symmetric axis
12
that intersects the geometrical center
13
of the uncut finished lens
11
′, the optical performance will be significantly degraded.
FIGS. 15 and 16
are graphs showing average refractive power error and astigmatism within 50 degrees of visual angle, respectively, of the conventional aspherical lens whose framing reference point
15
is located on the symmetric axis
12
of the aspherical surface
11
a
. On the other hand,
FIGS. 17 and 18
are similar graphs of the aspherical lens whose framing reference point
15
is decentered from the symmetric axis
12
as shown in FIG.
14
. Analysis of these graphs shows that the decentering processing is virtually impossible because of the large aberrations.
Accordingly, the conventional aspherical lens employed for a large-size frame cannot be processed by the decentering processing, which requires a lens blank of large size.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an aspherical spectacle lens, which is capable of using a lens blank of small size even if a lens is employed for a large-size frame and is capable of keeping high optical performance. A further object of the present invention is to provide the processing method of the above aspherical spectacle lens.
For the above object, according to the present invention, there is provided an improved single-vision aspherical spectacle lens to correct eyesight, which includes:
a front surface; and
a back surface,
wherein at least one of the front and back surface is aspherical, a framing reference point that is coincident with a pupil position of a user when the lens is installed on a frame is decentered from a geometrical center of an uncut circular lens (a semifinished lens blank or an uncut finished lens).
With this construction, a lens blank of small-size can be employed for manufacturing a spectacle lens for a large-size frame by devi

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