Patent ID: 7318642

Claim:
A method for designing a progressive addition lens, comprising the steps of: a.) designing a first progressive addition surface having a channel, and one or more areas of unwanted astigmatism on either side of the channel, each area of unwanted astigmatism comprising a maximum unwanted astigmatism; b.) expressing the first surface as a plurality of sag values; and c.) modifying each of the one or more areas of unwanted astigmatism to provide a second progressive addition surface in which the maximum unwanted astigmatism of each of the areas is reduced and substantially constant throughout the area; wherein step c.) further comprises; (i) specifying a surface u(x,y) as a plurality of sag values at points on an x–y grid; (ii) defining an area which area comprises a point ( X m, Y m ) of maximum unwanted astigmatism; (iii) approximating the sag values for the area using a toric surface; (iv) calculating principal curvatures at the point of maximum unwanted astigmatism; and (v) adjusting the sag values until the maximum unwanted astigmatism shifts to another surface location that is within the area; and (vi) repeating steps (iii) through (v) until the maximum unwanted astigmatism within the area is reduced and is substantially constant throughout the area.