WO2002064058A2

WO2002064058A2 - Contact lens

Info

Publication number: WO2002064058A2
Application number: PCT/GB2002/000576
Authority: WO
Inventors: Tony Hough
Original assignee: Umist Ventures Limited
Priority date: 2001-02-13
Filing date: 2002-02-12
Publication date: 2002-08-22
Also published as: GB0103598D0

Abstract

A method of designing a contact lens comprising specifying the corrective power required for distance vision, specifying the corrective power required for near vision, and specifying the location and width of a near vision zone of the contact lens, thereby providing a step function which characterises the corrective power to be provided by the lens, the method further comprising applying a continuously varying function to the step function to provide a corrective power distribution which varies smoothly.

Description

CONTACT LENS

The present invention relates to a method of contact lens design and to a contact lens.

Presbyopia is the inability of the eye lens to bend sufficiently to focus on objects in the near field of vision. The flexibility of the eye lens reduces with age, and presbyopia is typically found in people over 40. A person suffering from presbyopia (a presbyope) will require reading glasses in order to focus on objects located in the near field of vision. Presbyopia is a progressive condition, and stronger reading glasses will be required as the presbyopia worsens.

Bifocal spectacles and bifocal contact lenses are designed to provide presbyopes with correction for the near field of vision and also with correction for distance vision. Typically, bifocal glasses include a lower lens portion which provides correction for the near field of vision, and an upper lens portion which provides correction for distance vision.

Bifocal contact lenses have been developed in recent years. There are several ways in which two or more powers of correction may be incorporated within the optic zone of a contact lens. In the case of rigid contact lenses the normal fitting characteristics of the lenses are such that a normally fitted rigid lens moves over the eye. When a wearer looks down, the rigid contact lens naturally translates over the eye. This allows near vision correction to be provided in a lower portion of the lens, and distance vision correction to be provided in a central (paraxial) region of the lens.

In the case of modern soft contact lenses there is virtually no movement of the contact lens on the eye during normal wear; the lens drapes onto the cornea and remains almost static. Bifocal optics must therefore be incorporated within the central optic zone of the contact lens and must assume no translation on downgaze. There have been numerous suggestions as to how this is best achieved.

Recently developed bifocal contact lens designs rely on the principle of 'simultaneous vision', where the lens provides optical information from both near and far fields of vision simultaneously, and the brain selects optical information of interest. This corresponds to normal vision, where one selects an object which is to be observed, and adjusts the focus of one's eyes accordingly.

The quality of an image obtained with simultaneous vision contact lenses is usually degraded by comparison to normal vision. There are also many technical issues related to the management of power distribution within the optic zone caused by the normal variations in pupil diameter in different lighting conditions (pupils become smaller in bright light, larger in dimly lit environments). A further complication is that the range of pupil size in a given light level varies with age.

There are two main categories of bifocal soft contact lens: a) lenses which have a single aspheric power gradient; and b) lenses which have two or more discrete zones which are designed to correct near and distance (or additionally intermediate) powers.

Typically, a lens with an aspheric power gradient is provided with a very compact region of near vision correction at a centre of the lens. The correction decreases smoothly in a radially outward direction, to a middle and outer region of the lens which provides distance correction. The middle and outer regions generally comprise more than 90% of the lens surface area.

Lenses with discrete multiple concentric zones are provided with a central zone which is designed to correct distance, and then a series of annular zones which provide alternately near and distance correction in a concentric arrangement. Each near vision correction annular zone has an axial thickness which is greater than the thickness of adjacent distance vision correction annular zones. Mathematically it would be convenient to provide a step transition between the zones. However, it is not possible to provide step transitions when mass producing contact lenses.

While all current designs of contact lens can be manufactured using computer controlled lathes, the preferred method of manufacture is by moulding. Moulding is preferred because it can provide very high volumes of lenses at reasonable unit cost. The moulding process typically comprises milling a steel or other suitable metal master of the lens surfaces, making a polypropylene or other similar material mould using the steel master, then using the polypropylene mould to cast the lens. The engineering of the master and the polypropylene mould cause the transitions of adjacent zones to become imprecise. The use of the polypropylene mould means that it is not possible to provide a precise step transition. To overcome this problem it is common practice to round corners of the step transition. It is also known to apply a mathematical joining function between zones to provides a transition that is easier to manufacture. This adjustment of the step function is added in response to manufacturing considerations, and will generally result in some degrading of the vision provided by the contact lens.

It is an object of the present invention to provide a method of designing a contact lens which overcomes or mitigates the above disadvantage.

According to a first aspect of the invention there is provided a method of designing a contact lens comprising specifying the corrective power required for distance vision, specifying the corrective power required for near vision, and specifying the location and width of a near vision zone of the contact lens, thereby providing a step function which characterises the corrective power to be provided by the lens, the method further comprising applying a continuously varying function to the step function to provide a corrective power distribution which varies smoothly.

The invention is advantageous because it does not merely round the corners of a step transition or apply a mathematical joining function, but instead a single unitary approach of applying a continuously varying function to a step function to provide a corrective power distribution which varies smoothly (this in turn provides a contact lens profile which varies smoothly).

Preferably, the continuously varying function is applied to the step function using the Levenberg-Marquardt method. Preferably, the continuously varying function is a Gaussian. Other suitable functions will be apparent to those skilled in the art.

Suitably, more than one near vision zone is included in the lens design by specifying more than one location and width of near vision zones of the contact lens.

Suitably, two or more near vision zones are made to overlap, by specifying zone widths and locations which overlap, or by specifying zone widths and locations which are sufficiently close that they will overlap following application of the continuously varying function.

Suitably, the method further comprises specifying the corrective power required for intermediate vision, and specifying the location and width of an intermediate vision zone of the contact lens.

Suitably, the corrective power distribution is displayed on a monitor or other output means to allow an operator to compare the corrective power distribution with pupil sizes that occur under different lighting conditions.

Suitably, the corrective power distribution is converted to a physical lens geometry to be used in the manufacture of the lens.

Suitably, the physical geometry is modified by determining a radial position at which there is a local maximum of corrective power, and inserting an annular region of fixed corrective power extending radially outwards from that position.

Suitably, a distance vision zone is provided at the centre of the lens, and a near vision zone is disposed concentrically around the distance vision zone.

Suitably, a second distance vision zone is disposed concentrically around the near vision zone.

Suitably, a near vision zone is provided at the centre of the lens, and a distance vision zone is disposed concentrically around the near vision zone. Suitably, a pair of contact lenses are designed, a first contact lens of the pair being in accordance with claim 10 and a second contact lens of the pair being in accordance with claim 12.

According to a second aspect of the invention there is provided a contact lens designed in accordance with the first aspect of the invention.

According to a tliird aspect of the invention there is provided a contact lens having a first central region which provides distance vision correction, a second region disposed concentrically around the first region which provides near vision correction, and a third region disposed concentrically around the second region which provides distance vision correction, wherein the power distribution of the contact lens has the form of a continuously varying function.

The lens according to the third aspect of the invention may be designed using the method according to the first aspect of the invention and any of the suitable features thereof.

The inventors have realised that a contact lens in which the power distribution varies continuously (instead of having step transitions with rounded corners), provides superior vision because all of the surface area of the lens (within the optic zone of the eye) provides useful optical information.

A specific embodiment of the invention will now be described by way of example only with reference to the accompanying drawing which is a graph which represents the corrective power of a contact lens designed according to the invention.

A method of designing a contact lens provides a curve representative of the variation of corrective power across a cross section of the contact lens, in response to parameters input by an operator. The input parameters to the model are the required distance power, the reading addition and two control parameters which are referred to herein as position and width. The term 'reading addition' refers to the corrective power required for near vision over and above the corrective power required for distance vision. The position parameter specifies the radial position (measured from the centre of the lens) of a middle part of a near vision zone. The width parameter specifies the width of the near vision zone.

An example of the input parameters is shown graphically in figure 1 : a) distance power = +3.00dioptres; b) reading addition = +2.00dioptres; c) position = 2.50mm; d) width = 1.8mm.

Taken together, these parameters describe a 'step function' of the corrective power that will be required by the wearer of the lens.

Instead of merely rounding the corners of the step function or applying a mathematical joining function to the step function (as is seen in the prior art), the invention uses a single unitary holistic approach of applying a continuously varying function to the entire step function. Referring again to figure 1, a Gaussian function is applied to the step function, and this results in the smoothly varying function labelled 'a'.

The Gaussian function is fitted to the step function using the Levenberg-Marquardt algorithm. The Levenberg-Marquardt algorithm uses second derivatives to determine Gaussian function parameters which provide an optimal fit to the step function (the fit is to nodes of the step function).

The core step in determining the parameters that provide the best fit of the Gaussian is the minimisation a so-called merit function, usually denoted by χ². This minimisation is achieved by means of an iterative process which utilises second derivatives of the Gaussian function. The Levenberg-Marquardt algorithm is known in the art, and is described in reference books, for example Marquardt, D.W. 1963, Journal of the Society for Industrial and Applied Mathematics, Nol 11 , pp . 431 -441.

The Gaussian parameters determined by the model may be used to calculate the corrective power of the lens at an arbitrary point X as follows:

The power atX= the central power + a GaussianMixture correction

This code is used to plot the power distribution across the optic zone of the contact lens, as shown in figure 1.

The smoothly varying function shown in figure 1 is advantageous because it does not include any step transitions, or anything which is close to being a step transition. This avoids the manufacturing problems mentioned above, and also avoids degrading vision at transitions between zones. Subjective vision provided by the lens is improved because all of the surface area of the lens (within the optic zone of the eye) provides useful optical information.

A further advantage of the function shown in figure 1 is that it provides a significant amount of intermediate vision. Referring to figure 1, there are two annular regions which provide an intermediate vision correction of around +4.00dioptres, the first region is at a radial distance of around 1.7mm from the centre of the lens and the second region is at a radial distance of around 3.2mm from the centre of the lens.

The embodiment of the invention illustrated in figure 1 provides a near vision zone having only one Gaussian peak. The model is not limited to the design of lenses having a single Gaussian peak, and more complex lens designs having multiple Gaussian peaks may be generated by providing additional input parameters. For example, a series of concentric zones may be provided by inputting a series of width and position parameters. Overlapping zones, which may include more than one Gaussian peak, may be provided by inputting overlapping width and position parameters, or by inputting width and position parameters which are sufficiently close that the zones will overlap when the Gaussian function is applied. This may be used for example to provide a corrective power which has a rate of change from distance to near that is quicker than the change from near to distance on the other side of the peak.

A zone of the contact lens which provides intermediate vision may be specified by inputting an intermediate addition (for example +1.00dioptres) together with position and width parameters. The Gaussian function is applied to the resulting step function as described above.

It is possible to modify the form of the power correction of the lens so that the shape on one side of the slope of selected Gaussian forms is different to the other. This is achieved by using overlapping Gaussian forms.

Once the corrective power function of a lens has been generated using the model, this is converted to the physical geometry of the lens using well known conversion techniques. The physical geometry of the lens will be smoothly varying because its form is determined by a smoothly varying corrective power function.

The physical geometry of the lens may be modified by determining a radial position at which there is a local maximum of corrective power, and inserting an annular region of fixed corrective power extending radially outwards from that position. The remainder of the lens geometry located radially outwards of that position is translated to the outermost edge of the annular region. Although the resultant lens geometry includes an annular region of fixed corrective power, this region is introduced after the generation of the correction power geometry using the continuously varying function (as described above).

The model is easy to use, and allows a clinical lens designer to calculate the distribution of distance and reading power for a given pupil size. It will be appreciated that the Gaussian function is not the only smoothly varying function that may be used by the method. Any other suitable smoothly varying function may be used, for example a binomial function.

It is preferred to have the distance power in the centre of the contact lens, but the model will also allow reading power to be central. A pair of lenses may be designed in which a first lens has a central distance power zone and a second lens has a central reading power zone.

The corrective power distribution may be displayed on a monitor or other output means to allow an operator to compare the corrective power distribution with pupil sizes that occur under different lighting conditions.

Claims

1. A method of designing a contact lens comprising specifying the corrective power required for distance vision, specifying the corrective power required for near vision, and specifying the location and width of a near vision zone of the contact lens, thereby providing a step function which characterises the corrective power to be provided by the lens, the method further comprising applying a continuously varying function to the step function to provide a corrective power distribution which varies smoothly.

2. A method according to claim 1, wherein the continuously varying function is applied to the step function using the Levenberg-Marquardt method.

3. A method according to claim 1 or claim 2, wherein the continuously varying function is a Gaussian.

4. A method according to any preceding claim, wherein more than one near vision zone is included in the lens design by specifying more than one location and width of near vision zones of the contact lens.

5. A method according to claim 4, wherein two or more near vision zones are made to overlap, by specifying zone widths and locations which overlap, or by specifying zone widths and locations which are sufficiently close that they will overlap following application of the continuously varying function.

6. A method according to any preceding claim, further comprising specifying the corrective power required for intermediate vision, and specifying the location and width of an intermediate vision zone of the contact lens.

7. A method according to any preceding claim, wherein the corrective power distribution is displayed on a monitor or other output means to allow an operator to compare the corrective power distribution with pupil sizes that occur under different lighting conditions.

8. A method according to claim 1, wherein the corrective power distribution is converted to a physical lens geometry to be used in the manufacture of the lens.

9. A method according to claim 8, wherein the physical geometry is modified by determining a radial position at which there is a local maximum of corrective power, and inserting an annular region of fixed corrective power extending radially outwards from that position.

10. A method according to any preceding claim, wherein a distance vision zone is provided at the centre of the lens, and a near vision zone is disposed concentrically around the distance vision zone.

11. A method according to claim 10, wherein a second distance vision zone is disposed concentrically around the near vision zone.

12. A method according to any of claims 1 to 9, wherein a near vision zone is provided at the centre of the lens, and a distance vision zone is disposed concentrically around the near vision zone.

13. A method according to claim 10 and claim 12, wherein a pair of contact lenses are designed, a first contact lens of the pair being in aqcordance with claim 10 and a second contact lens of the pair being in accordance with claim 12.

14. A contact lens designed in according with any of claims 1 to 13.

15. A contact lens having a first central region which provides distance vision correction, a second region disposed concentrically around the first region which provides near vision correction, and a third region disposed concentrically around the second region which provides distance vision correction, wherein the power distribution of the contact lens has the form of a continuously varying function.

16. A method of designing a contact lens substantially as hereinbefore described with reference to the accompanying figure.

17. A contact lens substantially as hereinbefore described with reference to the accompanying figure.