Aspheric apodized diffractive optic design has quality-of-vision benefits, according to bench studies

January 1, 2009

An eye model was used to evaluate image quality associated with different available multifocal IOLs. The overall results show benefits of an aspheric apodized diffractive design.

Key Points

Berlin-The findings from a series of studies demonstrate differences in optical performance of available multifocal IOL designs, and comparisons between implants show superiority of an aspheric apodized diffractive design for providing better image quality and reducing glare and halo in low photopic and mesopic conditions, according to research presented by Jim Schwiegerling, PhD, at the annual meeting of the European Society of Cataract and Refractive Surgeons.

Dr. Schwiegerling, associate professor, Department of Ophthalmology and Vision Sciences, University of Arizona, Tucson, in collaboration with Junoh Choi, MS, used a digital imaging system and a custom eye model to evaluate the optical performance of 3 D- and 4 D-add apodized diffractive IOLs (AcrySof ReSTOR, Alcon Laboratories), two full-aperture diffractive IOLs (Tecnis, Advanced Medical Optics [AMO]; Acri.LISA, Carl Zeiss Meditec), and a zonal refractive IOL (ReZoom, AMO).

"Our results show overall superiority for multifocal IOLs with diffractive optics compared with a zonal refractive design," Dr. Schwiegerling said. "However, our testing also indicates that apodizing the diffractive lens has benefits for further suppressing the glare and halos that can occur with simultaneous vision with multifocal optics.

Artificial cornea

The custom eye model uses an artificial cornea created to reproduce population averages for chromatic and spherical aberration and mimics the imaging properties of the human eye. A digital camera then captures images through the eye model. Through-focus modulation transfer function (MTF) measurements were made by analyzing the blur of a narrow slit image captured through the "eye" on the camera.

At 50 cycles/mm at a 3-mm aperture, the MTF was similar for the spherical and two aspheric versions of the apodized diffractive IOLs. At a 6-mm aperture, a benefit was found for the aspheric design of the apodized diffractive IOL.

"The similarity among the [apodized diffractive] lenses at the smaller aperture was expected because the aspheric optic design affects the periphery of the lens and would have an impact on image quality only at a larger pupil size," Dr. Schwiegerling said.

The MTFs with the other multifocal designs illustrate various design philosophies, he said. The zonal refractive lens has limited near performance for small pupil diameters because most of the near zones are blocked under these conditions. For large pupils, the incident light is split equally between distance and near vision. The foldable model of the full-aperture diffractive lens equally splits performance between the distance and near vision for both the 3- and 6-mm pupils, whereas the bifocal model of the full-aperture diffractive lens biases distance vision in a 2:1 ratio for both pupil sizes. Both versions of the apodized diffractive lens equally split distance and near performance for small pupils but progressively bias distance vision as the pupil size increases.

Night driving

Additional testing used the digital imaging system to capture images of an Early Treatment Diabetic Retinopathy Study letter chart and oblique point source (50-µm diameter pinhole target at 18° off the optical axis), and a portable version of the system was used to capture an actual night-driving scene "seen" through the eye model using a 6-mm aperture. The results from these studies demonstrated the ability of the apodized diffractive optic to suppress glare and halos relative to the other multifocal lenses.

The captured images with the letter charts demonstrated that the aspheric 3 D- and 4 D-add apodized diffractive IOLs were associated with the best contrast relative to the other multifocal lenses. Comparing the two aspheric apodized diffractive IOLs, the 3 D-add model provided slightly improved contrast for distant objects relative to the 4 D-add version.

In the testing with the oblique point source, the full-aperture diffractive IOLs were associated with the appearance of arcs around bright light sources, whereas a continuous flare was seen around the lights in testing in which the multizonal refractive lens was placed in the eye model. These artifacts were suppressed by the apodized diffractive IOLs.

These findings were confirmed by the testing in the night-driving scenes, where clarity of letters on street signs was best with the apodized diffractive IOLs and artifacts around street lights were least.

"Since the optical performance of the apodized diffractive lens shifts toward distance vision under dark conditions when the pupil is enlarged, artifacts that appear under night-driving conditions are reduced. These artifacts remain for the other optic designs because the lenses continue to balance performance between distance and near," Dr. Schwiegerling said. "The continuous change in power from distance and near is reproduced as a continuous flare, with the full-aperture diffractive optics and the inclusion of both distance and near zones within the lens aperture contributing to the light artifacts under large-pupil conditions."