Progress in the spectacle correction of presbyopia. Part 1: Design and development of progressive lenses.

Most of the commercial advances in the spectacle correction of presbyopia continue to occur in progressive lens design, which has been the focus of intense research and development over the past 60 years by major spectacle lens manufacturers. While progressive lens design and manufacturing techniques have advanced at a steady pace, recent progress in 'free-form' lens surfacing has opened up many exciting possibilities that will in all likelihood bring about a paradigm shift in the current model of progressive lens fabrication and distribution. The first installment of this two-part series will review the fundamental optical principles and early development work associated with progressive lenses.