Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LENS FOR PRESB~OPIA FREE FROM SHORT DISTANCE
AND INTERMEDIATE DISTANCE ABERRATION
SUMMARY OF THE INVENTION
This invention relates to a lens for presbyopia
free from short distance and intermediate distance
aberration for use in correcting an old-age eyesight.
In the lens for presbyopia with a front lens face
having a smaller radius of curvature than a rear lens
face, a lens face has a refractive index successively
corrected as the lens face extends radially outwardly
away from a geometric center of the lens so that
lateral magnifications for all principal rays always
e~ual a lateral magnification for a paraxial range.
This construction is entirely free from distortional
aberration, and secures a greatly enlarged range of
distinct vision.
BACKGROUND OF THE INVENTION
When presbyopia develops with the crystalline lens
in the eyeball has a weakening adjusting power, accom-
modation for seeing a close object becomes impossible.
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In this case, generally, spectacles having convexlenses may be used to compensate for the adjusting
power.
An example of known convex lenses for presbyopia
is shown in Fig. 5. This is a lens 50 for presbyopia
with a front face 5OF having a radius of curvature r(l)
smaller than a radius of curvature r(2) of a rear face
5OR thereof.
Specifically, take a lens of two degrees or
diopters for example, the smaller radius of curvature
r(l) is set to 116.754mm, and the larger radius of
curvature r(2) to 218.667mm.
In the illustrated known lens for presbyopia, the
above radii of curvature r(l) and r(2) are both fixed
radially outwardly from a geometric center 51 of the
lens 50. Therefore, the ratio of size (lateral magni-
fication) between an object and a virtual image seen
through the lens 50 varies with the position of the
object, which results in a distortional aberration.
This phenomenon is the more salient the higher is the
degree of the lens.
In addition, as shown in hatching in Fig. 5, the
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conventional lens 50 provides a narrow range of dis-
tinct vision. When, for example, the user wears lenses
of two degrees, a corrected near point on an optical
axis a is at a distance of 300mm (which is a distance
from the diaphragm of an eyeball 52), and a corrected
far point is at a distance of 504mm (the focal length
of the lens) assuming that the far point is at infinity
when seen in the naked eye of the user. The range of
distinct vision is narrow with the near point at 300mm
and the far point at 504mm also when the looking
direction of the eye ball forms an angle 01 or ~2 of 30
degrees with the optical axis a. This phenomenon is
the more salient the higher is the degree of the lens.
Thus, when the lens 50 of the conventional con-
struction is used, the range of distinct vision islimited to short distances, and an image becomes
deformed by the distortional aberration. This results
in the disadvantages of the eyes becoming fatigued
after a long period of use in the absence of a comfort-
able visual sense.
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OBJECTS OF ~HE INVENTION
A principal object of this inventlon is to providea lens for presbyopia free from short distance and
intermediate distance aberration, which involves no
distortional aberration and realizes a greatly in-
creased range of distinct vision.
Another object of this invention is to provide a
lens for presbyopia free from short distance and
intermediate distance aberration, which, when used,
involves no deformation of an object and realizes a
comfortable visual sense to cause little fatigue of the
eye after a long period of use.
Other objects of this invention will be apparent
from the following description of the preferred embodi-
ment.
BRIEF DESCRIPTION OF THE ~RAWINGS
The drawings show an embodiment of this invention,in which:-
Fig. 1 is an explanatory view of a lens for
presbyopia free from short distance and intermediatedistance aberration according to this invention,
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Fig. 2 is an explanatory view showing a state of a
front face of the lens before a correction,
Fig. 3 is an explanatory view showing a state of
the front face of the lens after the correction,
Fig. 4 is an explanatory view showing a range of
distinct vision of the lens according to this inven-
tion, and
Fig. 5 is an explanatory view showing a range of
distinct vision of a conventional lens.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A lens with a front face having a corrected
refractive index will be described in detail as an
embodiment of this invention with reference to the
drawings.
The drawings show a lens for presbyopia free from
short distance and intermediate distance aberration.
In Fig. 1, a lens 11 for presbyopia defines a front
face llF having a radius of curvature smaller than a
radius of curvature r~2) of a rear face llR thereof.
The radius of curvature r'(l) of the front face llF of
the lens 11 is successively corrected as the front face
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2043038
extends radially outwardly away from a geometric center
of the lens 11, so that the radius of curvature r'(l)
progressively increases to have a lateral magnification
for all principal rays equaling a lateral magnification
for a paraxial range.
Figs. 1 and 2 show a radius of curvature r(l)
prior to the correction in phantom lines. Figs. 1 and
3 show the radius of curvature r'(l) after the correc-
tion in solid lines. The radius of curvature r(l) is
corrected such that the radius of curvature r'(i) is
progressively greater than the radius of curvature r(l)
as the front face llF of the lens 11 extends radially
outwardly away from the geometric center 12 of the
lens.
A specific construction of the lens 11 for
presbyopia will be described hereinafter, taking a lens
of two degrees fro example.
When the lens 11 is used as shown in Fig. 1, a
distance E'P' on an optical axis a between the front
face of the diaphragm of an eyeball 13 and the rear
; face Rll of the lens 11 is set to 18mm, the lens 11 has
a thic~ness d(l) on the optical axis a set to 3mm, a
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distance d(0) on the optical axis a between the front
face Fll of the lens 11 and an object 14 is 300mm, the
radius of curvature r(2) of the rear face llR of the
lens 11 is 218.667mm, and the radius of curvature r(l)
of the front face Fll of the lens 11 is 116.754mm. A
lateral magnification ~ for a paraxial range is 2.44136
which is derived from the refractive index of air N'(0)
which is 1 and the refractive index N'(l) of the lens
11 formed of a transparent acrylic resin which is
1.492. A distance on the optical axis a between the
rear face llR of the lens 11 and a virtual image 15 is
-731.11mm.
Next, when the height y(0) of the object 14 is
lOmm, the radius of curvature r'(l) of the front face
llF which is 116.?54mm is corrected to be
116.8539962768555mm so that the virtual image 15 has a
- - height y'(3) equaling the above lateral magnification
~, whereby the virtual image 15 having a height y(3)
prior to the correction is changed to the height y'(3)
which is 24.413mm. Then, the lateral magnification
is y'(3)/y(0) = 2.44134.
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That is, the radius of curvature of the front face
Fll of the lens 11 is increased from r~1~ to r'(1) so
that a horizontal length x(1) and a vertical length
y(1) of coordinates of an intersecting point between a
principal ray and the curved face as shown in Fig. 2
are corrected to be a horizontal length x'(1) and a
vertical length y'(1) of coordinates of the intersect-
ing point between the principal ray and the curved
face.
Next, when the height y(0) of the object 14 is
20mm, the radius of curvature r'(1) of the front face
llF is corrected to be 116.9539947509766mm so that the
virtual image 15 has a height y'(3) equaling the above
lateral magnification ~, whereby the virtual image 15
having the height y(3) prior to the correction is
changed to a height y'(3) which is 48.44166mm. Then,
the lateral magnification B is y'(3)iy(0) = 2.44166.
Next, when the height y(0) of the object 14 is
30mm, the radius of curvature r'(1) of the front face
llF is corrected to be 117.453987121582mm so that the
virtual image 15 has a height y'(3) equaling the above
lateral magnification B, whereby the virtual image 15
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having the height y(3) prior to the correction is
changed to a height y'(3) which is 73.24796mm. Then,
the lateral magnification B is y'(3)/y(0) = 2.4416.
Thereafter, the height y(0) of the object 14 is
successively increased lOmm, and the radius of curva-
ture r'(1) of the front face llF is increased as above
until y(0) equals 170mm. Only numeric values will be
set out for expediency of explanation.
When y(0) is 40mm,
r'~1) = 118.1539764404297mm,
y'(3) = 97.6602mm, and
~ = 2.4415.
When y(0) is 50mm,
r'(1) = 118.9539642333984mm,
y'(3) = 122.077mm, and
~ = 2.44154.
When y(0~ is 60mm,
r'(1) = 120.0539474487305mm,
y'(3) = 146.481mm, and
~ = 2.44136.
When y(0) is 70mm,
r'(1) = 121.1539306640625mm,
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y'(3) = 170.903mm, and
~ = 2.44147.
When y(0) is 80mm,
r'(1) = 122.5539093017578mm,
S y'(3) - 195.306m~, and
~ = 2.44133.
When y(0) is 90mm,
r'(1) = 123.9538879394531mm,
y'(3) = 219.731mm, and
B = 2.44145.
When y(0) is lOOmm,
r'(1) = 125.6538619995117mm,
y'(3) = 244.129mm, and
~ = 2.44129.
When y(0) is llOmm,
r'(1) = 127.3538360595703mm,
y~(3) = 268.551mm, and
~ = 2.44137.
When y(0) is 120mm,
r'(1) = 129.25390625mm,
y'(3) = 292.957mm, and
~ = 2.44131.
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When y(0) is 130mm,
r'(1) = 131.1540222167969mm,
y'(3) = 317.386mm, and
~ = 2.44143.
When y(0) is 140mm,
r'(1) = 133.254150390325mm,
y'~3) = 341.793mm, and
~ = 2.44138.
When y(0) is 150mm,
r'(1) = 135.4542846679687mm,
y'(3) = 366.194mm, and
~ = 2.4413.
When y(O) is 160mm,
r'(1) = 137.6544189453125mm,
y'(3) = 390.615mm, and
~ = 2.44134.
When y(0) is 170mm,
r'(1) = 139.9545593261719mm,
y'(3) = 415.024mm, and
~ = 2.44132.
In this way, the radius of curvature r'(1) of the
front face F11 of the lens 11 is successively increased
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so that the lateral magnifications for all the princi-
pal rays equal the lateral magnification B for the
paraxial range. Since the front face Fll of the lens
has a unique, aspherical structure, the distortional
aberration is completely eliminated to produce the
effect of realizing a lens free from aberration.
In addition, a greatly increased range of distinct
vision is secured as shown in hatching in Fig. 4. Take
the lens 11 of two degrees for example, the near point
on the optical axis a is at 300mm and the far point at
504mm. In a state in which principal rays b form an
angle ~1 or 02 of 30 degrees with the optical axis a,
the range of distinct vision is greatly increased with
the near point at about 373mm and the far point at
about 780mm. This effect is the more salient the
higher is the degree of the lens.
Thus, when the lens 11 of the above construction
is used, there occurs no fluctuation of the image due
to movement of the eyeball, and a clear field of view
is secured covering from a short distance range to an
intermediate range. Thus, a comfortable visual sense
is secured and the fatigue of the eye resulting from a
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long period of use is reduced.
Further, since, as described above, the radius of
curvature of the front face F11 of the lens 11 is
successively increased (i.e. the refractive index of
the lens is successively reduced), the front face F11
projects only a small amount, i.e. has a small swell.
The lens may be thinned by the corresponding amount,
which produces the effects of securing a brighter field
of view and allowing the lens 11 to be lightweight.
In the foregoing embodiment, the radius of curva-
ture of the front face of the lens is successively
increased to successively decrease the refractive index
of the lens. The correction is not limited to the
front face of the lens, but may of course be effected
to the rear face only or to both the front and rear
faces.
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