C54866: Progressive power lenses - part 5

This article looks at the sixth and seventh generation of progressive lens designs.

Sixth generation progressive designs

In 2006, Essilor’s continuing research to perfect the Varilux lens led to a new design which retained all the best features of previous generations and applied the very latest technology to both the design and method of manufacture of the lens. This enabled the design to address the comfort of the field of vision and to provide a significant improvement in the overall effect of the lens in wear.

Ophthalmic lens design is normally concerned only with foveal vision, where the visual acuity is at its maximum and the resolution is, therefore, highest. The criterion for a best form spectacle lens is that it should produce a good quality image on the fovea. Since the pupil diameter is small, it is usual to consider the effect of the lens only upon single rays of light which meet the lens at a point. At night time, however, the pupil may increase its diameter several times and the new analysis method employed for the Varilux Physio design assumed a finite pupil diameter of 6mm. At this diameter, the light emanating from the lens is not just a single ray but, instead, a narrow pencil of light within which, for an ideal spherical lens, the emergent wave front should be spherical (figure 1). For distance vision, the wave front aberration in the refracted pencil is defined as any departure from a spherical surface concentric with the second principal focus of the lens. Owing to the peculiar nature of a progressive surface, the refracted pencil can only be spherical over a small area of the lens, but any deformation can be modified by adjustment of the concave surface of the lens. This is the principle of Essilor’s Wavefront Management System, employed in their sixth generation design, which set out to provide the optimum acuity for distance vision.