In a known manner, an ophthalmic eyeglass which is intended for a presbyopic wearer must produce appropriate ophthalmic corrections for several visual distances. For a long time multifocal or progressive eyeglasses which have different optical powers between distinct parts of these eyeglasses have been provided for this purpose. Such an eyeglass provides the wearer with an ophthalmic correction which is suited to the distance of a viewed object, if the direction of view of the wearer passes through the eyeglass in a part of the latter which has been provided for this visual distance. A wearer who has been newly fitted with such eyeglass must then get used to orienting his head and his eyes depending on the distance from the viewed object, in order to use the appropriate part of the eyeglass.
Nevertheless, such an orientation of the head and eyes may not be spontaneous, or may even be impossible in certain circumstances. In fact, the multifocal or progressive eyeglasses which are currently available are suitable for allowing the wearer to read by looking downwards. They are therefore suitable for reading a book which is placed on a table or resting on the knees in a sitting position. Thus, the ophthalmic correction which is produced in the lower part of these lenses is suitable for near vision, i.e. for a vision distance of approximately 0.4 m (meter). It is therefore unsuitable for viewing obstacles which are situated on the ground in front of the wearer, at a distance from the wearer's eyes which is greater than 0.5 m. Now, such a situation is that of approaching a staircase, for example. The image of the first steps of the staircase which is formed on the wearer's retina is then not clear, and the wearer can be the victim of an accident caused by this poor visual perception. Another example of a situation for which current multifocal or progressive eyeglasses are not suitable is reading a page of script which is attached to a wall, at eye level. In fact, the wearer is then compelled to read the document through the far vision zone of his spectacle eyeglasses.
The document EP 0 367 878 describes an eyeglass with concentric zones which have several focal distance values at any point on its surface. Such an eyeglass can simultaneously provide two ophthalmic correction values, for example for far vision and for near vision, irrespective of the direction of view through the eyeglass. But, because part of the optical power is contributed by light diffraction effect, such eyeglass has significant chromatism. The effective optical power of the eyeglass cannot then be matched precisely for all the colours with respect to the spherical defect which is determined for an ametropic subject. Moreover, irrespective of the distance of the object which is viewed by the wearer of such eyeglass, and irrespective of the direction of his view through the eyeglass, part of the light which enters via the pupil converges according to a focal distance which is not appropriate. This light part does not then participate in the clear image which is produced on the wearer's retina according to whichever of the focal distances is appropriate. This results in a loss of contrast which affects the wearer's vision in all circumstances.
USPTO Backgrounds
USPTO Backgrounds