Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks
Reexamination Certificate
1999-12-02
2001-02-13
Sugarman, Scott J. (Department: 2873)
Optics: eye examining, vision testing and correcting
Spectacles and eyeglasses
Ophthalmic lenses or blanks
Reexamination Certificate
active
06186627
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a progressive power lens wherein the dioptric power for distance vision is negative and a progressive surface is formed on the back surface thereof.
2. Description of the Related Art
Progressive power lenses which are utilized as spectacle lenses are known as lenses having a distance portion positioned on upper part of the lens, a near portion positioned on lower part of the lens, and an intermediate portion positioned between the near and distance portions with a continuous change of dioptric power from the near portion to the distance portion.
In such progressive power lenses, although the surface power of the distance portion and the surface power of the near portion are naturally different from each other, in the past, the surface power at a certain point on the main meridian in the main meridional plane was identical to the surface power at that same point in a plane perpendicular to the main meridional plane, in view of facilitating ease of manufacture thereof. In recent progressive power lenses, due to advancement in production technology, the surface power at a certain point in the plane perpendicular to the main meridional plane can be easily made different from the surface power at the certain point in the main meridional plane. Consequently, there have been various proposals made for the surface power for such progressive power lenses.
It is desirable, mainly from a cosmetic point of view, for progressive power spectacle lenses, and also any other types of spectacle lenses, to be formed (manufactured) as thin as possible. A spectacle lens can be thinned without changing the power thereof by employing a gentle base curve. However, if a gentle base curve is adopted for the purpose of thinning a progressive power lens, it is difficult to correct aberrations, particularly astigmatism. Due to this reason, an appropriate base curve is selected taking into account the correction for aberrations. However, due to such a conventional approach, the thickness of the progressive power lens cannot be made sufficiently small.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a back-surface progressive power lens having a progressive surface on the back surface thereof, wherein both the reduction of the thickness and the correction for aberrations can be achieved in a balanced combination.
The inventor of the present invention has discovered that, in a back-surface progressive power lens, if the surface power in the main meridional plane is stronger than that in a plane perpendicular to the main meridional plane (the sagittal plane) in the distance portion and if the surface power in the main meridional plane is weaker than that in the sagittal plane in the near portion, a balanced combination of the reduction of the thickness and the correction for aberrations can be achieved, particularly in a progressive power lens wherein the dioptric power for distance vision thereof is negative.
Namely, according to an aspect of the present invention, a progressive power lens is provided having a negative dioptric power for distance vision, wherein a progressive surface is formed on the back surface of the progressive power lens. The value of a surface power Pf
m
of the progressive surface in the main meridional plane at a first point on the main meridian in a distance portion of the progressive power lens is less than the value of a surface power Pf
s
of the progressive surface in a plane perpendicular to the main meridional plane at the first point (Pf
m
<Pf
s
). The value of a surface power Pn
m
of the progressive surface in the main meridional plane at a second point on the main meridian in a near portion of the progressive power lens is greater than the value of a surface power Pn
s
of the progressive surface in a plane perpendicular to the main meridional plane at the second point (Pn
m
>Pn
s
). The surface power P is defined by the following condition: P=(n−1)/r; wherein “n” represents the refractive index of the progressive power lens, and “r” represents the radius of curvature at one of the first point and the second point on the main meridian in the near portion and the distance portion, respectively.
In the conditions described in the specification, “P” generally designates a surface power; “n” and “f” suffixed to “P” designates the near portion and the distance portion, respectively; and “
m
” and “
s
” suffixed to “n” or “f” designate the meridional plane and the sagittal plane (a plane perpendicular to the meridional plane), respectively.
Preferably, the value of the surface power P
m
is identical to the value of the surface power P
s
at a point in the close vicinity of a specific point on the main meridian at which there is no prismatic power.
Preferably, the value S
N
of a dioptric power for near vision is negative (S
N
<0), and wherein the following relationship exists at a point within the near portion:
Pn
m
(−20)−Pn
s
(−20)>0.1;
wherein Pn
m
(−20) represents the surface power in the main meridional plane at a point, on the main meridian, 20 mm downward from a prism reference point of the progressive power lens in the near portion; and Pn
s
(−20) represents the surface power at a point, on the main meridian in a plane perpendicular to the main meridional plane, 20 mm downward from the prism reference point of the progressive power lens in the near portion.
With this relationship, a good field of view can be obtained throughout the entire near portion.
Preferably, the value S
N
of a dioptric power for near vision is greater than or equal to zero (S
N
≧0), and the following relationship exists at a point within the near portion: Pn
m
(−20)−Pn
s
(−20)>0.2; wherein Pn
m
(−20) represents the surface power in the main meridional plane at a point, on the main meridian, 20 mm downward from a prism reference point of the progressive power lens in the near portion; and Pn
s
(−20) represents the surface power at a point, on the main meridian, in a plane perpendicular to the main meridional plane, 20 mm downward from the prism reference point of the progressive power lens in the near portion.
At the point 20 mm downward from the prism reference point, a constant effect in the correction for astigmatism can be obtained if the value of “Pn
m
−Pn
s
” is greater than zero (Pn
m
−Pn
s
>0) in the case where the value S
N
of the dioptric power for near vision is equal to or greater than zero (S
N
≧0). It is preferable that the value of “Pn
m
−Pn
s
” is greater than 0.2 (Pn
m
−Pn
s
>0.2) to obtain a sufficient effect in the correction for astigmatism.
Preferably, the value S
F
of a dioptric power for distance vision is defined by the following condition:
−2≦S
F
<0;
and wherein the following relation exists at a point within the distance portion:
Pf
m
(15)−Pf
s
(15)<−0.1;
wherein Pf
m
(15) represents the surface power in the main meridional plane at a point, on the main meridian, 15 mm upward from a prism reference point of the progressive power lens in the distance portion, and Pf
s
(15) represents the surface power at a point, on the main meridian in a plane perpendicular to the main meridional plane, 15 mm upward from the prism reference point of the progressive power lens in the distance portion.
Preferably, the following condition is satisfied:
−2≦S
F
≦−1.
It is usually necessary to increase the amount of correction for aberrations as the value S
F
of the dioptric power for distance vision decreases in the case where the value S
F
of the dioptric power for distance vision is negative. In the case of “−2≦S
F
≦0”, a sufficient effect cannot be obtained if the value of “Pf
m
−Pf
s
” at the point 15 mm upward from the prism reference point is larger than −0.1. Preferably, “Pf
m
−Pf
s
” is greater than 0.1, especially in the case of “−2≦S
F
≦−1”.
Preferably,
Asahi Kogaku Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Sugarman Scott J.
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