Technical background to the invention Contact lenses intended for the correction of common vision defects, such as myopia, hypermetropia, astigmatism or presbyopia, are typically individualised and prescribed to users on the basis of visual acuity tests.

These tests, among which we would mention for example the optotype test, are however typically carried out under specific and special conditions, and the value of the dioptre correction which results is therefore necessarily an averaged value. As a consequence, contact lenses prescribed on this basis are often unable to adapt to multiple environmental conditions (such as luminance, or the distance of the visual target) and subjective conditions (such as concentration) which can affect a person's visual acuity, even significantly.

By way of example, it is known that a contact lens for the correction of myopia having a dioptric power of for example-3 and capable of optimally correcting the visual defect of an individual placed under medium-high luminance conditions, is not able to correct the defect in the same individual in ,., such an optimum way under the conditions of reduced luminance.

In fact under these conditions the human eye, having a larger pupil diameter, generally needs a higher level of correction.

Spherical contact lenses have a curvature of the optic zone (and therefore a dioptric power) which is constant with distance from the optical axis of the lens and are therefore in themselves inappropriate to respond fully to the abovementioned requirement.

In the technical field in question there are however known lenses, referred to as spherical lenses, whose curvature is not maintained constant as the distance from the optical axis of the same changes.

This type of contact lens is typically used to correct particular optical defects, such as for example spherical aberrations, or for the correction of visual defects such as presbyopia and astigmatism.

Aspherical contact lenses comprising an optical zone in which different regions intended for long distance vision and close vision respectively are arranged concentrically one with respect to the other are for example known.

Each optical region has a different curvature and eccentricity to respond better to the different requirements of vision and is connected to the adjacent regions through suitable profiles. The change in the dioptric power of the lens as the distance from the optical axis varies is therefore generally curvilinear.

Examples of lenses of this type are described in International Patent Application no. 96/21878, or in United States Patents nos. 6,116, 735 and 5,796, 462.

_ It is also known-that spherical contact lenses can be made with a hyperbolic or parabolic profile, as taught for example in United States Patent no. 5,220, 359.

The latter are however generally designed to-correct visual defects on the basis of a specific dioptric power and are not very versatile in that they have little adaptability to the different use requirements of final users.

From what has been said above it is in fact obvious that two individuals having the same"theoretical"dioptric deficiency, as determined for example by the optotype test, may in reality need lenses having a different dioptric power when they use the contact lenses in contexts which are appreciably. different from each other, such as those of office work (with good luminance and close visual targets) and night driving (with reduced luminance and distant visual targets). These different user requirements may be evaluated by the optician, who nevertheless needs to have available contact lenses which can then respond adequately to them.

Description of the invention The problem underlying this invention is that of providing an spherical contact lens which is structurally and functionally designed to meet this

requirement and overcome the abovementioned limitations with respect to the cited known art.

In the context of this problem it is a principal aim of the invention to provide an spherical lens whose properties are easily understandable and manageable by the optician in relation to the user's requirements.

This problem is resolved and this aim is achieved by this invention through an spherical contact lens constructed in accordance with the following claims.

Brief description of the drawings Other advantages and characteristics of the present invention will become clear from the following detailed description which is given with reference to the appended drawings which are provided purely by way of non-limiting example and in which : - Figure 1 is a diagrammatical perspective view of an spherical contact lens constructed according to this invention, - Figures 2 to 4 are graphs representing the change in dioptric power with distance from the optical axis of the lens in Figure 1, and some variant embodiments of the same.

Preferred embodiment of the invention In Figure 1, an spherical contact lens constructed in accordance with <, this invention is indicated as a whole by 1.

Lens 1 is made of polymer material based on HEMA or its derivatives and has an inner concave surface 2 and an opposite outer convex surface 3.

The geometrical and optical characteristics of lens 1 are substantially symmetrical with respect to an optical axis X which passes perpendicularly through lens 1 at its centre.

On lens 1 there is also a circular optical zone 4, defined around the optical axis X and designed to be placed over the pupil of a user when in use.

In order to ensure more particularly that the user's pupil and optical zone 4 are concentric, lens 1 is preferably of the soft type.

Optical zone 4 in turn comprises an inner region 5, which is circular and aspherical, and an outer region 6, which is substantially spherical, concentric with inner region 5 and in an annular arrangement surrounding the same.

Inner region 5 has a diameter of generally between 3 and 5 mm, preferably 4 mm, against an overall diameter of optical zone 4 which is typically between 6 and 9 mm.

In view of the fact that in human beings the aperture of the pupil on average lies between a diameter of 1. 5 mm and a diameter of 5 mm, inner region 5 in practice constitutes the effective portion of optical zone 4, through which external light enters the eye.

While the dioptric power of outer region 6 is substantially constant, thanks to suitable shaping of outer surface 3 each point on inner region 5 has a dioptric power which is strictly dependent on its-distance from the optical axis X.

According to a principal feature of the invention, the correlation linking the dioptric power of a point in inner region 5 and its distance from optical axis X is substantially linear.

-In particular, it is preferred that the dioptric power should be a <BR> maximum (considered on a relative scale, in which, therefore, -2 is greater than-3) at the centre of the optical zone, where the distance from the optical '.. axis is zero, and a minimum at the perimeter of inner region 5, where the distance from the optical axis is a maximum. The dioptric power is then kept equal to this minimum value throughout outer region 6.

The amount of change in dioptric power between the centre of inner region 5 and its perimeter lies between 0.5 and 3.5 dioptres, preferably between 0.5 and 2.5 dioptres, as it is found that changes in dioptric power of less than 0.5 dioptres do not bring about appreciable improvements in visual acuity, while changes greater than 3.5 dioptres can disturb and adversely affect vision.

Lens 1 is designed for the correction of myopia and has a change in dioptric power as indicated by line A in the graph in Figure 2, with a maximum of-2. 5 dioptres at the centre of inner region 5 and a minimum of-3 dioptres

at the perimeter of the same, a value which is then maintained constant throughout outer region 6.

Lens 1 may advantageously be used for the correction of myopia in individuals having"theoretical"values of myopia which differ from each other and different requirement contexts and types of use, as more clearly stated in the following.

The graph shown in Figure 2 illustrates the changes in dioptric power (indicated by P), respectively indicated by A, B and C, in relation to the distance from the optical axis (indicated by R) of three different spherical contact lenses constructed according to this invention.

The lenses corresponding to graphs A, B and C have in common the fact that they have a difference of 0.5 dioptres between the maximum and minimum dioptric power values and the same diameter for the inner region (4 mm), as-a result of which the curves are parallel to each other. The latter are however spaced apart with a spacing of 0.25 dioptres, in accordance with the normal standards for the construction of graduated lenses having different dioptric powers.

All the lenses in which the change in dioptric power is shown in Figure 2 can be conveniently used in individuals having a"theoretical"degree of myopia (that is measured using the conventional tests) of 3, but at the same <. time having different use requirements.

The lens with the change shown by curve A (corresponding to lens 1) has a dioptric power of-2.5 at the centre of the optical zone and-3 dioptres at the periphery of the inner region and may be particularly suitable for an individual having a"theoretical"degree of myopia of 3 who uses contact lenses mainly to look at close objects under good lighting conditions and/or an individual of advanced age. In such cases in fact the degree of dioptric correction necessary is as a rule slightly less than that measured by the standard test, in that the pupil diameter is reduced and does not require all the correction identified by the tests. This will however be necessary under lower light conditions, when the pupil diameter is large, equal to or greater than 4 mm, corresponding to the diameter of the inner region.

As the. pupil diameter increases, the mean value of the dioptric power of the lens relating to that diameter falls, but in a non-linear way. It is known in fact that the surface area of the optical zone affected by an increase in pupil diameter increases with the square of the diameter, with the result that the fraction of the surface area of the lens having greater dioptric power (that close to the centre) rapidly loses importance with the increase in diameter, so that when the pupil is completely open the dioptric power of the lens is substantially similar to that of the minimum defined at the periphery of the inner region.

As an alternative to the lens now described, a lens having a change in dioptric power according to curve C in Figure 2, and therefore having a dioptric power equal to-3 at the centre of the optical zone and-3.5 at the periphery of the inner region, may find advantageous application in young individuals, or in individuals using contact lenses mainly under low light conditions, in which the pupil diameter is a maximum and the degree of correction required is greater than that specified by the standard visual capacity tests.

The lens corresponding to the change in curve B in Figure 2, having a dioptric power of-2. 75 at the centre of the optical zone and-3.25 dioptres at the periphery of the inner zone, has characteristics which are intermediate between the two lenses described above.

< By analogy with the examples of applications described above, it has been found that lens 1 could also conveniently be used in individuals having a "theoretical"degree of myopia equal to 2.75 or 2.5.

Figures 3 and 4 show changes in the dioptric power of lenses similar to those described in relation to the graph in Figure 2, except the overall amount of the variation in dioptric power along inner region 5 of the optical zone is equal to 1 and 1. 5 dioptres respectively.

Also the lenses identified by the graphs in Figures 3 and 4 can be used in accordance with the personalisation criteria briefly indicated above to correct a"theoretical"myopia of grade 3, depending upon the nature of use mentioned above.

Of course, as there is a more marked difference in dioptric power between the centre and the periphery of the inner region of the optic zone a greater number of lenses which can be associated with the same gradation of dioptric correction is available, with an advantageous increase in the possibilities for diversifying therapeutic strategies.

Although in the examples described the value of 1. 5 dioptres has been stated to be the maximum change in dioptric power, sufficient for most cases of medium myopia, there is also the possibility that contact lenses with greater variations, up to 3.5 dioptres, could also be constructed, for example for cases of severe ametropia, as better described below.

The abovementioned examples are directed towards contact lenses intended for the correction of myopia, however the concept underlying the invention can also be applied in a similar way to lenses for the correction of hypermetropia and emmetropia.

Contact lenses according to the invention also have a positive effect on visual acuity in terms of reducing the effects of spherical aberration of the human eye. As the latter is in fact a positive diopter, the focus of the peripheral rays is normally closer to the lens than the focus determined by the paraxial-rays, and the lenses according to the invention, having a greater dioptric power at the centre rather than the periphery, tend to bring such foci '< together, advantageously reducing the circle of minimum confusion.

In this way it is therefore also possible to correct visual defects due to mild astigmatism.

Contact lenses according to the invention also have advantageous application in the correction of severe ametropia, requiring up to 25 dioptres of dioptric power (as an absolute value). In these cases in fact it is known that the use of a lens provided with a"theoretical"dioptric power has the disadvantage that it substantially adversely effects perception of the size of images (for example, in the case of lenses for the correction of severe myopia, these tend to be diminished) and to significantly disturb close vision. In the case of severe myopias there is a tendency therefore to use lenses with a greater dioptric power (and therefore with a lesser degree of correction) than

the"theoretical"determined by the standard tests in order to find a difficult compromise between clearness of vision, the correct size of images and an acceptable capacity for close vision. In this way however overall visual acuity remains penalised, in particular it has an adverse effect on capacity for night vision.

With the spherical contact lenses according to the invention, using lenses with a high variation in dioptric power (1. 5-3.5), it is however possible to have a central zone of the lens with a greater dioptric power (hypocorrected) in order to provide satisfactory close vision and to attenuate the effect of diminishing images, and a peripheral zone with a lesser dioptric power to provide sufficiently clear vision in far vision and under low light conditions.

The spherical contact lenses according to the invention also have an advantageous application in the correction of accommodative insufficiencies of the eye and mild presbyopia.

This invention therefore resolves the problem stated above with reference to the known state of the art, at the same time offering many other advantages, among which the possibility of personalising the contact lens on the basis of parameters evaluated by the optician in terms of the subjective characteristics of the user and the manner in which the lens will be used.

,., The contact lenses according to the invention may also advantageously find application in post-surgical treatments, in the treatment of latent strabismus and in visual training.

In particular is should be noted that with the lenses according to the invention opticians have at their disposal a wide range of lenses which helps them to better identify those which best meet the needs and the characteristics of the user, and that the linearity of the correlation between the dioptric power of the lens and distance from the optical axis permits simple and correct handling of such individualisation.

The latter aspect makes it possible for example for an optician to prescribe the correct contact lens merely by making a visual acuity test under conditions of maximum and minimum pupil opening.