For several years, the person skilled in the art has envisaged developing diffractive lenses (see, for example, European patent EP-A-0 064 812, U.S. Pat. No. 4,637,697, or French patent application number FR-88 06699, filed May 19, 1988).
Diffractive contact lenses in relief, as shown in accompanying FIG. 1, are generally in the form of a component 10 having two optical surfaces 14 and 16, one of which has a zoned Fresnel grating, concentric with the axis 12 of the lens and constituted by a series of annular structures 17.sub.1, 17.sub.2, 17.sub.3, . . . in relief.
The structures 1m generally all have the same profile. In addition they occupy the same area. The outer radii of the various structures therefore obey a geometrical progression of the type: .sqroot.K. r where r designates the outside radius of the central structure, and K designates successive integers.
The focal lengths fn of the diffractive component made in this way are given by: fn=r.sup.2 /2n.lambda., in which
r=the outside radius of the central structure; PA0 n=the order of diffraction; and PA0 .lambda.: the wavelength under consideration.
Order of diffraction n=+1 may be obtained using sampled kinoform type structures in relief, i.e. structures each having a profile built up from M levels, at phase differences of 2 .pi./M, where M is greater than 2.
Orders of diffraction n=+1 and n=-1 may be obtained simultaneously with structures in relief of the square wave function type generating a phase difference of .pi..
The performance of a diffractive contact lens in relief and expressed in terms of diffraction efficiency depends firstly on the product: depth of relief times the difference between the indices on either side of the optical surface in relief. This product is generally called the "relief optical depth".
The relief optical depth lies generally between 1 and 11/2 times the mean utilization wavelength (about 0.5 .mu.).
In order to reduce the accuracy constraints on making the component (i.e. to increase the depth of the relief as much as possible) it quickly appears to the person skilled in the art that the optical surface in relief needs to be placed adjacent to the cornea of the eye since the index difference n.sub.1 -n.sub.3 is less than n.sub.1 -n.sub.2, where n.sub.1 is the index of the material forming the component, n.sub.2 is the index of air, and n.sub.3 is the index of tear liquid.
Reference can usefully be made to the above-mentioned prior documents in order to obtain an understanding of the structure and the function of diffractive contact lenses.
It is observed that such lenses have not, in practice, become the subject of large-scale industrial manufacture.
This seems to be due to the fact that in spite of their theoretically attractive properties, diffractive contact lenses in relief nevertheless suffer from unacceptable drawbacks.
Firstly, since it is almost essential for the sharp-edged optical surface 16 in relief to be placed adjacent to the cornea of the eye, as described above, wearing diffractive contact lenses in relief quickly leads to irritation.
In addition, the operation of diffractive contact lenses in relief varies widely as a function of the hydration conditions of the surface in relief.
In addition, the presence of set backs on the inside face of the lenses encourages rapid deposition of proteins and fats contained in tear liquid, thereby clogging the inside faces of the lenses and reducing diffraction efficiency.
The object of the present invention is to eliminate the above-mentioned drawbacks.