Aspheric Lens Design Success
contact lens case reports
Aspheric Lens Design Success
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
In the world of multifocal GP lenses, there are basically three types of simultaneous vision lenses. In the United States, the most used designs incorporate anterior and/or posterior aspheric optics to generate the distance and near corrections. The success of the aspheric designs is often predicated on two factors — lens centration and pupil size.
Today, we address the centration issue by incorporating larger overall diameters, similar to those used in orthokeratology. However, one of the more overlooked anatomical features is pupil diameter, which can significantly influence the visual outcome. Because these lenses work on the mechanism of simultaneous vision, the only clear optics being presented to the retina are those that take place in the center 5.0mm of the lens.
Figure 1 illustrates three pupil diameters and the influence of this anatomical feature on the potential near add powers. Another obstacle is that pupil diameters decrease with age, a mechanism called senile miosis.
Figure 1. The role of pupil diameter in aspheric multifocal lenses.
New GP Materials
It's often stated that aspheric GP lenses perform best at near when minor (upward) translation occurs. Unfortunately, an aspheric lens that decenters induces a significant amount of radial astigmatism and vertical coma. Together, these have a departmental effect on the quality of the near optics.
For years we've known that we could enhance the effective add power of aspheric lenses through higher-index GP materials. Two manufacturers, Contamac and Paragon Vision Sciences, have responded with new high-index GP materials specifically designed for GP multifocals. The Contamac HR material has a refractive index of 1.505 and is currently FDA approved. The Paragon HDS HI material has a refractive index of 1.544 and is currently pending FDA approval.
Figure 2 shows the (in-air) add powers associated with a posterior aspheric design manufactured in a traditional GP index material (yellow line) and a higher-index material (green line). On eye, the effective add power will be proportionally less due to the index of refraction of the tears. Figure 3 illustrates a similar increase in add effect when anterior aspheric lens designs are manufactured in the higher-index material. CLS
Figure 2. Increased add power associated with high-index posterior aspheric lens designs.
Figure 3. Increased add power associated with high-index anterior aspheric lens designs.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.
Contact Lens Spectrum, Issue: January 2009