Mutlifocal concept affords asymmetric PSF

A prototype two-zone, customized contact lens using this idea was designed for Dr Edwin J. Sarver and resulted in decreased chromatic aberration, spherical aberration and spherical equivalent that were verified objectively and subjectively.

The hope is that using this research a multifocal IOL can be designed to reduce perceived glare, halos, night vision disturbances and loss of contrast sensitivity that can occur in current multifocal IOLs. Dr Sarver described this work at the annual meeting of the American Society of Cataract and Refractive Surgery.

He and co-author Dr Donald R. Sanders, Centre for Clinical Research, Elmhurst, Illinois, Chicago, used a refractive multifocal optic that provides customized spherical aberration and astigmatic correction as needed. For IOLs and contact lenses, the design includes a reduction in chromatic aberration. They did this using an asymmetric PSF specifically designed to increase perceived contrast through binocular suppression.

Dr Sarver described the whys and hows of the asymmetric PSF. He showed an optical ray trace for a bifocal surface for distance and near zones that illustrated details of rays about the near and distance foci. He reported that stray light is directed to the upper portion of the image plane for both the distance and near foci and it is fairly uniformly distributed in the upper plane. In contrast, a standard concentric-zone multifocal optic distributes the stray light symmetrically about the foci. These facts also were demonstrated using PSF.

"By rotating the asymmetric PSF optic, we can control how stray light is distributed around the objects in the image plane," Dr Sarver said. "By adjusting the stray light directions in the left and right eye of a patient, we can allow for a perceived increase in the image contrast through binocular suppression."

In an example of image simulation of the asymmetric PSF, he presented two different views for the eyes.

"This allows a reduction of aberration by binocular suppression," he commented. In another example of image simulation of the symmetric PSF, both eyes had the same view, and therefore, there was no reduction in aberrations (Figure 1).