The technical portion of the profession of an optician consists in mounting a pair of ophthalmic lenses in a frame selected by a wearer. Such mounting comprises three main operations:
acquiring the shape of the bezel of each of the two rims of the eyeglass frame selected by the future wearer, i.e. acquiring the shape of the grooves that run around the insides of the rims of the frame;
centering each lens, which consists in determining the position that the lens is to occupy on the frame in order to be appropriately centered in front of the pupil of the wearer's eye so as to perform properly the optical function for which it is intended; and
shaping each lens, which consists in machining or cutting its outline to the desired shape, given the shape of the bezel and the defined centering parameters, and then beveling at the end of such machining, which beveling consists in making an engagement ridge on the edge face of the lens for the purpose of holding the lens in the bezel of the frame.
In the context of the present invention, attention is given mainly to the first operation of acquiring the shapes of the bezels in the rims of the selected eyeglass frame, and to the third operation of machining the edge face of the lens.
In practice, the optician begins by feeling the bottoms of the grooves in the two rims of the selected eyeglass frame so as to determine accurately the coordinates of a plurality of points that characterize the shape of a longitudinal curve traced by the bezel of each rim. Knowing the shape of this curve enables the optician to deduce the approximate shape that needs to be presented by the outline of the lens that is to be machined so as to enable it to be inserted in the corresponding rim of the frame.
In order to improve the accuracy with which the lens is engaged in the corresponding rim of the frame, and as explained in patent EP 0 819 967 in the name of the Applicant, the optician may perform an additional acquisition operation of feeling the transverse shape of a section of the bezel so as to discover the shape of the engagement between the engagement ridge of the ophthalmic lens and the bezel in the eyeglass frame. Acquiring this shape makes it possible to calculate the engagement depth of the engagement ridge in the bezel, thus making it possible to machine the outlines of each lens with increased accuracy so as take account of the engagement depth.
In the context of lenses that are strongly curved, the Applicant has found that inaccuracy remains when calculating engagement, which inaccuracy gives rise to difficulties when mounting lenses in the frame, and possibly making mounting impossible due to interference between the bevel and the margins of the bezel.
Furthermore, in frames having rims that present zones that are highly skewed, i.e. very twisted, the bezel presents a considerable angle of inclination relative to the edge face of the lens, such that an unsightly gap can appear between the rim of the frame and the edge face of the ophthalmic lens.