Parametric Modeling of Free-Form Surfaces for Progressive Addition Lens

By the mid-40s, the amplitude of accommodation has declined sufficiently so that most of us can no longer accommodate clearly and comfortably for close tasks. This condition, called presbyopia, is related to the growth of the lens. Nowadays, due to the development of the free-form technologies, the progressive addition lenses (PAL) are the best solution for the presbyopia. To obtain the necessary optical properties, a continuous change in the curvature along a line (called corridor) must be provided on a surface of the lens (progressive surface). Method: In this work a method for the parametric design and analysis of progressive addition lens, based on discrete shape modeling, is proposed. Both the optical (e.g. power and addition) and the geometrical (e.g. inset, corridor length, amplitude of the distance and near vision area) parameters have been taken into account. Result: Different parameters are assessed in the design stage of PAL. The method developed for the analysis of surface optical properties, especially with regard to the astigmatic surface power, has proved an essential tool for the analysis of results. A key role in the resultant optical properties of the designed PAL is covered by the distribution of the curves in the intermediate area and by the curvature equation along the corridor. Discussion & Conclusion: The free-form technologies, recently introduced in the manufacturing process of ophthalmic lenses, allow the production of high performance and custom PALs. Surface power and astigmatic surface power show similar behaviour to other commercial progressive additional lens but, in addition, the designer can define the distribution of astigmatism in the intermediate region. Compared to the methods proposed in literature, this method shows more opportunities in the design parameters definition and allows highly customized lens, designed on the main habits of the wearer. Moreover the exchange data formats for the CNC manufacturing process were described.