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Abstract

A method for computing a spectacle lens, which includes a region (distance portion) designed for viewing at large distances and in particular 'to infinity', a region (near portion) designed for viewing at short distances and in particular 'reading distances,' and a progressive zone disposed between the distance portion and the near portion, in which the power of the spectacle lens increases, from a value at a distance reference point located in the distance portion to a value at the near reference point located in the near portion along a curve (principal line) veering towards the nose, by an amount designated as addition power. The method includes the steps of stipulating as initial parameters a course of a projection x.sub.0 (y) of the principal line on an x,y plane, and also properties of the spectacle lens along the principal line whilst taking into account spherical, cylindrical, and possibly also prismatic prescription values and the addition power, as well as an interpupillary distance, and computing with these stipulations at least one strip of second order on a progressive surface of the spectacle lens, stipulating an object-distance function Al(y) which describes a change of object distance with a movement, in particular a lowering, of a glance, determining on each horizontal meridian of the progressive spectacle lens a point of penetration of a principal ray through the progressive surface, for which point a distance of a point of intersection of this principal ray with a plane which bisects the interpupillary distance is equal to the object distance given by the object-distance function Al(y), computing for the entirety of these points of penetration lying on the principal viewing line a course of the projection x'.sub.0 (y) on the x,y plane, equating the course x.sub.0 (y) to x'.sub.0 (y) and checking the coincidence, subsequently iteratively repeating the steps a. to e. until the projection x.sub.0 (y) of the principal line is equal (within given limits) to the course of the principal viewing line projection x'.sub.0 (y) used for the computation of the respective surface.

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