US Patent 7144110 Double progressive spectacle lens

A double progressive spectacle lens having a prefabricated progressive surface and a second progressive surface for correcting a spherical ametropia or a cylindrical ametropia, in which the surface properties of the progressive surface in the vicinity of the principal line of sight are asymmetric, the asymmetry being determined by the symmetry factor SA, which is determined in relation to the level A by the ratio of the smaller to the larger horizontal distance between the principal line of sight and the location at which the surface astigmatism in the horizontal section reaches the value A [dpt].