Abstract:
In a method of producing tolerance markings on a contact lens for correcting astigmatism which has a given cylinder, the tolerance markings comprise at least two straight line segments in the peripheral part of the contact lens with an angle 2α T  between them. The angle 2α T  between the two straight line segments of the tolerance markings depends on the cylinder of the contact lens.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The invention concerns contact lenses. It is more particularly concerned with lenses that are not circumferentially symmetrical, such as lenses for correcting astigmatism. To be more precise, it concerns a method of marking angular tolerances to facilitate putting in such lenses. 
     2. Description of the prior art 
     Contact lenses for correcting astigmatism have the particular feature that at least one of their faces is not circumferentially symmetrical about the optical axis. Consequently, they must be placed on the eye of the wearer in the correct angular position, failing which the correction of vision is imperfect. 
     Conventional lenses for correcting astigmatism therefore have symbols in their peripheral part, which has no optical function, enabling the clinician to verify correct orientation of the lens on the eye. These symbols include angular markings for assessing the positioning error and where applicable comparing it to a tolerance with respect to the optimal position of the lens, assuming that this tolerance is known. 
     The invention is directed to a novel method of producing angular tolerance markings specific to each lens and allowing for the fact that the acceptable angular tolerance depends on the cylinder of the lens. In addition to correcting astigmatism, the lens can incorporate any other type of correction. One objective of the invention is to determine directly whether the angular position of the lens is within the tolerable range or not. Another objective of the invention is to save clinicians time and facilitate their work. The invention can be used on any type of lens for correcting astigmatism, the lens itself constituting another aspect of the invention. 
     SUMMARY OF THE INVENTION 
     With the above aims in view, the invention proposes a method of producing tolerance markings on a contact lens for correcting astigmatism which has a given cylinder, wherein the tolerance markings comprise at least two straight line segments in the peripheral part of the contact lens with an angle 2α T  between them depending on the cylinder of the contact lens. 
     In one particular embodiment of the invention, an angle α T  equal to half the angle between the two straight line segments of the tolerance markings satisfies the following equation: ##EQU1## where C is the cylinder expressed in diopters and P T  is a threshold of reduced visual acuity to be complied with, expressed as a percentage and having a value in the range from 10% to 50%. 
     The law of evolution giving the degradation threshold P T  as a function of the cylinder C to be corrected is preferably represented between two envelope curves defined by the following two equations: ##EQU2## 
     In a more specific embodiment of the invention, the law of evolution of the degradation threshold P T  as a function of the cylinder C to be corrected satisfies the following equation: ##EQU3## 
     The invention is also directed to a contact lens for correcting astigmatism having an optically active central area, at least one surface of which is not circumferentially symmetrical about its optical axis, and a peripheral area in which there are tolerance markings formed by the method as defined hereinabove. 
     More generally, the invention is directed to a series of lenses each carrying tolerance markings including at least two straight line segments, wherein the angle of the straight line segments varies with the cylinder C to be corrected from one lens to another. 
     The description and drawing of a preferred embodiment of the invention given hereinafter explain more clearly the aims and advantages of the invention. The description is clearly given by way of example and is not limiting on the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the limiting angular error as a function of the cylinder to be corrected for various fixed degradation thresholds. 
     FIG. 2 shows the preferred law of evolution of the degradation threshold as a function of the cylinder to be corrected and two envelope curves for that law. 
     FIG. 3 shows the limiting angular error as a function of the cylinder to be corrected in the case of a variable degradation threshold and two envelope curves for that error. 
     FIG. 4 is a front view of a lens with tolerance markings. 
     FIG. 5 shows variant markings. 
     FIG. 6 shows lenses with variant markings. 
     FIG. 7 shows various arrangements of the markings at the periphery of a contact lens. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A conventional contact lens for correcting astigmatism has an optically active central area, at least one face of which is not circumferentially symmetrical about its optical axis, and a peripheral area in which angular positioning symbols may be placed. 
     Incorrect angular orientation in the plane perpendicular to the optical axis when positioning the contact lens on the eye leads to imperfect correction of astigmatism, as already mentioned, manifested in residual astigmatism which reduces visual acuity. The residual astigmatism cylinder, known as the residual cylinder, is proportional to the astigmatism cylinder to be corrected and to the orientation error. The orientation error (referred to hereinafter as the angular error) degrades visual acuity in proportion to the degree of astigmatism to be corrected. The tolerance to be respected in the orientation of the lens therefore becomes more severe as the cylinder to be corrected increases. 
     The residual cylinder R can theoretically be expressed as a function of the cylinder C to be corrected and the angular error α by the following equation (see W. A. Douthwaite, Contact Lens optics and design, 2 nd  ed. Butterworth-Heinemann, Oxford 1995): 
     
         R=2 C sin (α) 
    
     The reduced visual acuity due to the residual cylinder R, expressed in the form of a positive reduction percentage P, can be modeled in the following form (see C. Fauquier, T. Bonnin, C. Miege, E. Roland, Influence of combined power error and astigmatism on visual acuity, in Vision Science and its Applications, OSA Technical Digest Series 1995, SaE6(1995)): 
     
         P=44.3 R-R.sup.2 
    
     where R is in diopters. 
     It is feasible to tolerate a degraded visual acuity limited to a degradation threshold P T  and it is then possible to determine the limiting angular error α T  of the lens according to the cylinder C to be corrected using an equation derived from the previous two equations: ##EQU4## where C is expressed in diopters and P T  is expressed as a percentage. 
     The degradation threshold P T  takes a fixed value, for example in the range from 10% to 50%, if it is considered independently of the cylinder to be corrected. FIG. 1 shows the dependence of the limiting angular error α T  as a function of the cylinder C to be corrected for respective degradation threshold P T  values of 11%, 21%, 37% and 50%. The limiting angular error α T  is very small, in the order of a few degrees, for large cylinders (5 diopters and more). A limiting angular error α T  this small is very difficult to comply with and in practice a slightly greater angular error is tolerated for severe astigmatism. 
     It is therefore possible to use a value of the degradation threshold P T  increasing as a function of the cylinder C to be corrected. In the embodiment of the invention described here by way of non-limiting example, the mathematical law giving the degradation threshold P T  as a function of the cylinder C to be corrected is represented between two envelope curves defined by the following two equations: ##EQU5## 
     FIG. 2 shows the two envelope curves 1, 2. 
     The law of evolution of the degradation threshold P T  as a function of the cylinder C to be corrected preferably satisfies the following equation: ##EQU6## 
     This law is represented by the curve 3 between the two envelope curves 1, 2 in FIG. 2. 
     The limiting angular error α T  of the lens is then written: ##EQU7## where C is expressed in diopters. 
     The curve 4 in FIG. 3 shows this function. The angular error envelope curves are obtained in exactly the same fashion by substituting in the equation expressing the limiting angular error α T  expressions for the evolution threshold P T  corresponding to the two degradation threshold envelope curves 1, 2. The curves 5, 6 in FIG. 3 are the angular error envelope curves. 
     The limiting angular error α T  can then be determined for each value of the cylinder C to be corrected. 
     Angular tolerance markings comprising two straight line segments separated by an angle of 2α T  are formed in the peripheral area of the lens around an axis corresponding to the theoretical horizontal or vertical axis of the lens. Two symmetrical markings are preferably disposed at diametrally opposite places on the lens. FIG. 4 shows one such arrangement also including two dots showing the theoretical horizontal axis of the lens. 
     Markings compatible with conventional types of lens are formed on the lens in a manner known to the skilled person, by milling, etching, stamping, excimer laser, coloration or any other conventional method. 
     Many variants of angular tolerance markings can be used that preserve the same characteristic angle, possibly with a middle straight line segment between two other segments or with circular arcs linking the segments. FIG. 5 shows examples of such markings and FIG. 6 shows complete lenses carrying variants of the angular tolerance markings. 
     Similarly, many variants of the disposition of the markings in the peripheral area of the lens can be produced by the same method, and FIG. 7 shows a series of such dispositions. 
     The present invention is not limited to the details of the embodiments described hereinabove by way of example. In contrast, it encompasses all modifications that may suggest themselves to the skilled person.