Toric ophthalmic lenses

Details Toric ophthalmic lenses Toric ophthalmic lenses

1999-09-23

2002-05-07

Sugarman, Scott J. (Department: 2873)

Optics: eye examining, vision testing and correcting

Spectacles and eyeglasses

Ophthalmic lenses or blanks

C351S169000, C351S176000

Reexamination Certificate

active

06382789

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a method for determining an individual ophthalmic lens adapted to a wearer for whom an astigmatism has been prescribed; such lenses are also called toric ophthalmic lenses; they differ from ophthalmic lenses described as being spherical, the latter being intended to be worn by persons with no prescription for astigmatism. The method can be applied both to single-focus or multifocal lenses.
Multi-focal ophthalmic lenses are well-known; among these multi-focal lenses one can distinguish lenses known as progressive lenses, and lenses that are more specifically dedicated to near vision.
Multi-focal lenses present a particular problem for wearers needing correction of astigmatism. The astigmatism supplied to the wearer is the resultant of three components:
local cylinder of the progressive surface, characterised by its amplitude (or modulus) and its axis;
the prescribed cylinder and its axis;
oblique astigmatism.
Currently, to correct a spectacle wearer suffering from astigmatism, a lens is provided the front face of which is optimized in the case of a spherical prescription, and the rear face of which is a simple torus. Thus, account is not taken of the deterioration introduced by the torus; at best, one can play on the oblique astigmatism by adjusting the base value of the front face. For economic reasons, one cannot multiply the number of basic values existing already.
Multifocal progressive ophthalmic lenses are now well known. They are used for correcting longsightedness and allow the spectacle wearer to view objects over a wide range of distances without needing to take his glasses off. Such lenses typically comprise a far vision region, situated at the top of the lens, a near vision region at the bottom of the lens, the far and near vision regions being joined by an intermediate region, with a main meridian of progression passing through the three regions.
French Patent 2,699,294 describes the different elements of such a progressive multifocal ophthalmic lens in its introductory part, and mentions the works carried out by the applicant to improve the comfort of wearers of such lenses. Reference should be made to that document for more details on these various points.
Applicant has also proposed, for example in U.S. Pat. Nos. 5,270,745 or 5,272,495 to introduce a variation into the meridian, and notably to place it off-center with respect to a near vision control point, as a function of power addition and ametropy.
Applicant has also proposed, in order to better satisfy the visual requirements of presbytic (longsighted) persons and to improve comfort of progressive multifocal lenses, various improvements (French Patents 2,683,642, 2,699,294 and 2,704,327).
Lenses also exist that are more specifically dedicated to near vision; these lenses do not have a far vision region with a defined reference point like one finds in conventional progressive lenses. Such lenses are prescribed as a function of the power the wearer needs for near vision, independently of far vision power. Such a lens is described in an article in the “
Opticien Lunetier”
of April 1988, and is sold by the applicant under the Essilor Delta trademark; this lens is simple to use and just as easy to adapt to as a progressive lens, and is attractive to the population of presbytic persons not fitted with progressive lenses. This lens is also disclosed in French Patent application 2,588,973. It has a central portion which is equivalent to a single-focus lens which one would normally employ for correcting longsightedness, in order to ensure satisfactory near vision. It additionally has a slight decrease in power in the upper portion thereby ensuring the wearer has sharp vision also beyond the usual field of near vision. Finally, the lens has a point, at a value of power equal to the nominal power for near vision, a region of greater power in the lower portion of the lens, and a region of lower power in the upper portion of the lens.
Usually, multifocal lenses, whether they be progressive or dedicated to near vision, have one non-spherical multifocal face, for example the side facing the spectacle wearer, and one spherical or toric face, known as the prescription face. This spherical or toric face allows the lens to be adapted to the users's ametropy, so that a multifocal lens is generally only defined by its non-spherical surface. As is well known, such a non-spherical surface is generally defined by the altitude of all its points. One also uses parameters constituted by the maximum and minimal curvatures at each point, or more frequently, their half-sum and difference. This half-sum and difference multiplied by a factor n−1, n being the refractive index of the material of the lens, are known as mean sphere and cylinder.
For progressive multifocal lenses, one thus defines, by choosing a (power addition, base) pair, a set of non-spherical multifocal faces. Usually, one can thus define 5 basic values and 12 power addition values, giving a total of 60 multifocal faces. In each basic value, an optimization is performed for a given power, i.e. for a spherical prescription face having a given curvature.
The use within one of these multifocal faces of a spherical or toric prescription face having a power close to the prescription face considered for optimization makes it possible to meet all the requirements of wearers of progressive multifocal lenses. This known method makes it possible, starting from semi-finished lenses, of which only the multifocal face has been shaped, to prepare lenses that are adapted to each wearer, by simply machining one spherical or toric prescription face.
A similar method is used for optimization and prescription of lenses dedicated to near vision.
This method has the disadvantage of only being an approximation; consequently, the results obtained with a prescription face that is different from that used for optimization are worse than those corresponding to the prescription face employed for optimization.
U.S. Pat. No. 5,444,503 discloses a lens having a multifocal surface and a prescription surface. Compared to the prior art, which suggests defining the prescription service in order to obtain a given power at the far vision reference point, it is proposed, in that Patent, to define the prescription surface of the lens as a function of the power required by the wearer in a plurality of elementary surfaces. For this, the said United States.
Patent involves calculating aberration over the whole surface, and causing a continuous parametered surface to vary, for example a surface defined by splines, using known mathematical optimization algorithms. In practice, beyond the statement of principle, that Patent proposes using, in order to optimize the prescription surface, the distance to the cornea in an elementary surface, the object distance in an elementary surface, the inclination of the lens in the frame, the shape of the frame, and the curvature of the lens. That Patent says nothing regarding the effective calculation of the prescription surface. According to that document, their solution would make it possible to overcome the defects originating from replacement of the rear face used for optimization, by a rear face close to it.
That solution has the disadvantage of complicating lens manufacture: it involves determining, and machining, a non-spherical rear face. In this case, one should optimise and machine two complex surfaces. The proposed method does not appear to be founded on physiological data.
International application WO-A-96/13748 further discloses the use, for multifocal lenses, of a non-toric prescription surface, in order to limit defects with respect to the prescription surface employed for optimization. That Patent discloses prescription surfaces the main cross sections of which are circles having a radius defined by a given equation, the parameters of the equation depending on the wearer's sphere and cylinder. The solution disclosed in that document suffers from the same disadvantages as those described

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