Design method of aspheric ophthalmic lens

Technical Field

The present invention relates to a design method of an ophthalmic lens to be worn on or set in an eye in use, such as an intraocular lens and a contact lens.

Background Art

Heretofore, there have been known ophthalmic lenses such as an intraocular lens to be set in an eye in place of a crystalline lens and a contact lens to be placed on a cornea. Those ophthalmic lenses mostly have spherical surfaces. However, there has recently been proposed an ophthalmic lens having an aspheric surface optically designed to reduce the effect of aberration of an eye to obtain shaper images.

Such aspheric ophthalmic lens is designed to reduce the effect of eye aberration as mentioned above. This lens is produced by optical design with reference to an optical axis of the eye, but without consideration of a visual axis of the eye.

Summary of Invention

Technical Problem

The present invention has a purpose to provide a design method of aspheric ophthalmic lens capable of reducing the effect of aberration of an eye by taking into account a visual axis of the eye.

Solution to Problem

To achieve the above purpose, the present invention provides a method of designing an aspheric ophthalmic lens capable of reducing aberration of an eye, comprising: a step of modeling an optical system including a cornea and an ophthalmic lens by inclining the optical system about a rotation point by a predetermined angle in consideration of a displacement angle between an optical axis and a visual axis of the eye; a step of calculating aberration resulting from the modeled optical system; and a step of designing a shape of at least one of a front surface and a back surface of the ophthalmic lens to reduce the aberration resulting from the modeled optical system.

Further developments of the present invention are given in the dependent claims.

Brief Description of Drawings

Fig. 1 is a schematic view showing displacement between an optical axis and a visual axis of an eye.

Description of Embodiments

A detailed description of a preferred embodiment of the present invention will now be given referring to the accompanying drawing. In this embodiment, an ophthalmic lens is exemplified as an intraocular lens ("IOL") which serves as an alternative to a crystalline lens of an eye. In this embodiment, the IOL is optically designed in a way of setting a model by inclining an optical system including a cornea and the IOL at a predetermined angle in consideration of displacement between an optical axis and a visual axis of an eye, and designing the IOL by use of this model to reduce the effect of aberration of the eye.

Fig. 1 is a schematic view showing displacement between an optical axis and a visual axis of an eye 1 viewed from above. A central fovea is slightly deviated to an ear side than the optical axis of the eye 1. It is therefore well known that the visual axis is not coaxial with the optical axis of the eye and is inclined at a predetermined angle with respect to the optical axis of the eye. However, it is difficult to exactly determine the visual axis. In this embodiment, accordingly, it is assumed that a point on the optical axis located closer to a fundus from a corneal vertex by a predetermined distance (e.g., 7.2 mm) is a rotation point of the eye and an axis horizontally inclined by a predetermined angle θ relative to the optical axis is treated as the visual axis. A displacement angle θ between the optical axis and the visual axis in this embodiment is set at 5° but it is not limited thereto. This displacement angle θ may be set in a range of 1° to 10° and more preferably 3° to 7° based on for example an empirical rule.

The central fovea is slightly deviated to the ear side relative to the optical axis as mentioned above. More specifically, the central fovea is displaced to the ear side relative to the optical axis and also located slightly above a horizontal plane including the optical axis. Thus, the visual axis also has a slight displacement angle with respect to the optical axis in a vertical direction. When the IOL is to be optically designed by taking into account the displacement angle in the vertical direction, the displacement angle can be set to be preferably 3° or less, more preferably in a range of 1° to 2° with respect to the optical axis by centering the rotation point, in addition to the aforementioned horizontal displacement angle.

For performing optical design of the IOL, an existing optical simulation software (e.g., ZEMAX) is used. The optical design in this embodiment is conducted by modeling the optical system including the cornea and the IOL. Modeling such optical system requires a corneal aspheric polynomial coefficient, a curvature (a radius of curvature) of a cornea, a refractive index of the cornea, a distance from the cornea to the IOL, and power or diopter (refractive power) of the IOL. As design information to determine the power of the IOL, a curvature (a radius of curvature) of each of front and back lens surfaces, a refractive index of the IOL, and central thickness of the IOL are required. As information on a corneal shape, an average of quantitatively obtained data on human cornea may be used or information adopted in an existing model eye may be used. The corneal curvature may further be divided into a curvature of the corneal front surface and a curvature of the corneal back surface. In this embodiment, the aforementioned information for optical design is input by use of the optical simulation software, and the optical system including the cornea and the IOL is modeled by being inclined to a nose side by a predetermined angle (5°) about the rotation point and to a chin side by a predetermined angle (1°) about the rotation point. In this way, the axis relatively decentered by the predetermined angle about the rotation point with respect to the optical axis of the optical system including the cornea and the IOL is assumed to be the visual axis. Then, the IOL is optically designed into an aspheric shape to minimize aberration generated in the optical system with reference to the axis assumed to be the visual axis.

To determine aberration (wave aberration) generated in the modeled optical system, a Zernike polynomial is generally used. The Zernike polynomial can be described by the following mathematic expression (2).

$$\mathrm{w}\left(\rho \theta \right)={\displaystyle \mathrm{\sum }_{\mathrm{i}\mathrm{=}\mathrm{1}}^{\mathrm{15}}}{\mathrm{a}}_{\mathrm{i}}{\mathrm{Z}}_{\mathrm{i}}$$

Where, Zi is an i-th Zernike term, and a_i is a weighting factor in this term. Table 1 shows 1^st to 15^th Zernike terms and aberrations, which are defined by the following expressions respectively.

i

w_i(ρ, θ)

1

1

Piston

2

2 ρcosθ

Inclination x

3

2 ρsinθ

Inclination y

4

$\sqrt{3}\left(2{\rho }^2-1\right)$

Defocus

5

$\sqrt{6}\left({\rho }^2\sin 2\theta \right)$

Primary Astigmatic Aberration

6

$\sqrt{6}\left({\rho }^2\cos 2\theta \right)$

Primary Astigmatic Aberration

7

$\sqrt{8}\left(3{\rho }^3-2\rho \right)\sin \theta$

Coma Aberration

8

$\sqrt{8}\left(3{\rho }^3-2\rho \right)\cos \theta$

Coma Aberration

9

$\sqrt{8}\left(3{\rho }^3\sin 3\theta \right)$

3 Foil

10

$\sqrt{8}\left(3{\rho }^3\cos 3\theta \right)$

3 Foil

11

$\sqrt{5}\left(6{\rho }^4-3{\rho }^2+1\right)$

Spherical Aberration

12

$\sqrt{10}\left(4{\rho }^4-3{\rho }^2\right)\sin 2$

Secondary Astigmatic Aberration

13

$\sqrt{10}\left(4{\rho }^4-3{\rho }^2\right)\cos 2$

Secondary Astigmatic Aberration

14

$\sqrt{10}\left(4{\rho }^4\cos 4\theta \right)$

4 Foil

15

$\sqrt{10}\left(4{\rho }^4\sin 4\theta \right)$

4 Foil

An IOL having a general spherical shape uses the 4^th term. Furthermore, the aberration in the modeled optical system including the cornea and the IOL is usually reduced by bringing (approximating) a value obtained by use of the 11^th term close to zero. In this embodiment, to determine the aberration generated in the state where the optical system is inclined about the rotation point by the predetermined angle, the coefficient a_i is a value of the weighting coefficient based on the inclination by the predetermined angle.

In order to reduce the aberration in the thus modeled optical system, at least one of the front surface and the back surface of the IOL is designed as an aspheric shape. Each curved shape of the front surface and the back surface of the IOL can be described by the following mathematic expression (1).

$$\mathrm{z}\frac{\left(\mathrm{1}\mathrm{/}\mathrm{R}\right){\mathrm{h}}^{\mathrm{2}}}{\mathrm{1}\mathrm{+}\sqrt{\mathrm{1}\mathrm{-}{\left(\frac{\mathrm{1}}{\mathrm{R}}\right)}^{\mathrm{2}}\left(\mathrm{Q}\mathrm{+}\mathrm{1}\right){\mathrm{h}}^{\mathrm{2}}}}\mathrm{+}{\mathrm{A\; h}}^{\mathrm{4}}\mathrm{+}{\mathrm{B\; h}}^{\mathrm{6}}\mathrm{+}{\mathrm{C\; h}}^{\mathrm{8}}\mathrm{+}{\mathrm{D\; h}}^{\mathrm{10}}$$

Where "z" is a sag amount, "R" is a radius of curvature, "h" is a distance from a lens center, "Q" is a conic constant, and "A" to "D" are aspheric polynomial terms. In the case where the lens surface is treated as a spherical surface, not an aspheric surface, the conic constant and the aspheric polynomial terms are set to zero.

The values of the coefficients A to D used in the mathematic expression (1) are appropriately changed to bring (approximate) the value of the 11^th term representing the spherical aberration in the Zernike polynomial to as close to zero as possible in the optical system modeled by use of the optical simulation software. This is repeated to determine the curved shape of the front and/or back surface of the IOL. It is also possible to determine the coefficients A to D by using a similar method taking into consideration the terms representing other aberrations such as the astigmatic aberration and the coma aberration as well as the term representing the spherical aberration in the Zernike polynomial.

The above embodiment is explained about the design method of the IOL to be used in place of the crystalline lens. The present invention is not limited thereto and may be applied to an ophthalmic lens such as a contact lens to be used for eyes.

<Example 1>

Example 1 for performing the IOL optical design using the aforementioned design method is explained below. Herein, the IOL of a refractive power of 20.0D is designed to design the IOL front surface into an aspheric shape. The optical simulation software used herein was ZEMAX. Various data needed for the design were set as follows:

Corneal radius of curvature:

7.575 mm,

Corneal refractive index:

1.3375,

Corneal conic constant (Q):

-0.14135,

Distance from cornea to iris (pupil):

3.6 mm,

Distance from iris to IOL front surface:

0.9 mm,

Radius of curvature of IOL front surface:

13.36 mm,

Radius of curvature of IOL back surface:

-28.59 mm,

Refractive index of IOL base material:

1.519, and

Design wavelength:

587.56 nm.

Based on the above data, the optical system including the cornea and the IOL obtained by use of the optical simulation software is inclined about the rotation point (7.2 mm from the corneal vertex) by 5° in the horizontal direction (to the nose side) and 1° in the vertical direction (to the chin side), thereby creating a starting model.

With such optical system model, the optical design was made by correcting the conic constant and 4^th and 6^th aspheric polynomial coefficients in the expression (1) with respect to the IOL front surface so that the 11^th term in the above nominal becomes as close to zero as possible. This result is shown in Table 2. In Table 2, "Cornea" represents an aberration amount (µm) of only a set cornea, "Cornea + IOL" represents an aberration amount at the starting time, and "After Correction" represents an aberration amount indicated in the finally decided IOL optical design. Furthermore, the 4^th and 6^th aspheric polynomial coefficients finally decided are -5.518272E-04 and -2.919535E-05 respectively. The conic constant (Q) in the IOL front surface at that time was 0.767322 and the central thickness of the lens was 0.756 mm.

Cornea

Cornea + IOL

After Correction

1^st

-0.595323468

-0.743662252

-0.067382968

2^nd

0.412809469

-0.141036249

0.410480174

3^rd

0.082296591

-0.028170886

0.081863017

4^th

-0.002823135

0.001322866

-0.008098441

5^th

v0.01663683

0.039674746

0.023109376

6^th

-0.040039552

0.095506008

0.055641174

7^th

0.030649127

-0.010223299

0.029700738

8^th

0.153740727

-0.051178866

0.148927832

9^th

-0.000106208

-0.0008264

-0.000115897

10^th

-0.000158442

-0.001232564

-0.000172908

11^th

0.272336146

0.344518689

0.026917963

12^th

8.34338E-07

0.002699888

-0.000258909

13^th

1.4689E-07

0.001121567

-0.000107671

14^th

-3.81915E-07

8.30225E-06

-4.45372E-06

15^th

-3.64288E-07

8.361E-06

-4.47135E-06

As shown in Table 2, in the modeled state where the optical system including the cornea and ophthalmic lens is inclined about the rotation point by the predetermined angle in consideration of the displacement angle between the optical axis and the visual axis of the eye, the IOL optical design could be made so as to minimize the aberration of the cornea.

While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.