Effect of Progressive Lens Shape on Space Perception

Presented at the 2012 meeting of the European Academy of Optometry and Optics, this study used virtual reality simulation to compare the degree of swim perceived in dynamic vision by two lens designs: one a traditional progressive lens, and the other a revolutionary new approach to progressive lens geometry.

Introduction

The increase in power from the top to the bottom of a progressive lens creates increasing amounts of prism which induces increasing degrees of image deviation1. This distortion becomes most noticeable and most disturbing during dynamic vision, where it produces the well-known “swim effect.”2-4 While optical designers have long worked to minimize “swim,” it has been impossible with prior technology to do so without also reducing visual field size.

One can quantify the “swim effect:” The difference in horizontal displacement of a vertical line seen through the upper and the lower portions of the lens is a value we can call Δx. This value is a function of the top-to-bottom change in power and the shape of the lens. By dividing Δx by the maximum power variation ΔP, we can derive an objective criterion of “swim,” Δd, which is the end-to-end normalized deformation. To minimize “swim,” Δd must be close to 0, as it is in a single vision lens.

Challenged to reduce “swim” without decreasing field size, Essilor scientists explored every aspect of the lens design process. The result was a revolutionary new design technology called Nanoptix Technology™, which conceptualizes the lens as composed of tiny optical elements, each of which can be individually corrected to bring Δd close to zero, while respecting the progressive gradient. This study used virtual reality simulation to provide proof-of-concept that this revolutionary lens geometry could produce clinically meaningful results.

Methods

The effects of any given curve on the image of a physical object (eg, a grid) can be mapped (Figure 1) and then modeled in a virtual reality environment (Figure 2).5 This study used a virtual reality simulator to compare the effects on dynamic vision of two lens designs: a classic progressive lens design and a design with the new geometry designed to reduce “swim.”

Using the virtual reality simulator, 10 subjects were shown two images of a grid and asked to compare the images in dynamic vision. Presented in random order, one was a grid as it would be seen through a standard progressive design and the other was a grid as it would appear through the new lens geometry. In each comparison, subjects were asked to state a preference. Each subject made at least four comparisons. Scoring was based on the total number of times each image was chosen as preferred (rather than on individual subject preference).

Results

Comparing the grids in dynamic vision, the grid produced by the optimized geometry was preferred in 62% of the comparisons. The grid produced by the standard progressive design was preferred in 23% (Figure 3). There was no preference in 15% of the comparisons.

Conclusion

In this study, Essilor vision scientists demonstrated that “swim” could be quantified, and that an innovative lens geometry could provide a clinically meaningful reduction in “swim.” Study subjects clearly preferred the virtual reality image provided by the revolutionary new progressive lens geometry to that provided by ordinary progressive lens optics.