Scoring of progressive power lenses by means of user power maps

A progressive power lens (PPL) has a relatively complex power distribution, with regions for far, intermediate, and near vision, and also regions affected by aberrations, mainly astigmatism and defocus. Scoring techniques (Sheedy, Optom Vis Sci 2004;81:350-61) have been proposed to mark and classify PPLs according to their optical performance at each region. The objective of this study is to show that although scoring is useful to compare different lens designs, its outcome is highly dependent on the way power is defined and/or computed.

To demonstrate this, the progressive surfaces of many different current designs have been measured with a profiler. From these measurements and by means of exact ray tracing techniques, we obtain both the power maps that we would measure with a focimeter or a typical lens mapper and the power maps that the user perceives at the actual position of use. The scoring technique from Sheedy has then been applied to both types of maps.

It is shown that the parameters that define the characteristics of a PPL when the power map is measured with a standard mapper are not significantly associated with the actual performance of the lens when it is fitted. In a similar way, it is shown that two lenses that have been optimized according to the same target power distribution may get completely different marks depending on the definition of power that has been used in the optimization process. We also propose a graphical method to easily grasp the overall PPL performance by simultaneously presenting four-dimensional information from the scoring technique.

Although in general Sheedy's test is an important tool to compare different PPL designs, it cannot be used to compare those designs optimized for improving user power from classical front side designs when power has been measured for both types of designs with a standard focimeter.