The effect of chromatic dispersion on pseudophakic optical performance.

AIM

Monochromatic and chromatic aberrations limit the visual performance of pseudophakic eyes. Chromatic aberration is caused by the chromatic dispersion of optical materials which can be characterised by their Abbe numbers. This study examines how chromatic dispersion affects pseudophakic optical performance at different wavelengths and spatial frequencies.

METHODS

Abbe numbers were measured for acrylic and silicone intraocular lenses (IOLs). A schematic eye model based on cataract population data was used to compute monochromatic and photopic polychromatic modulation transfer functions (MTFs) for pseudophakic eyes with aspheric IOLs. IOL Abbe numbers were varied without changing other eye model parameters to determine how chromatic dispersion affects pseudophakic MTF and chromatic difference of refraction. Additional calculations were performed for (1) acrylic or silicone materials and (2) high-pass optical filters blocking either UV radiation or UV radiation and short wavelength visible light.

RESULTS

Shorter wavelengths account for approximately two thirds of pseudophakic chromatic difference of refraction or longitudinal chromatic aberration. Increasing Abbe number (reducing chromatic dispersion) decreases total chromatic difference of refraction and increases photopic polychromatic MTF. For a specific spatial frequency, there is an effective pseudophakic depth of wavelength over which a particular MTF level is achieved or exceeded. Depth of wavelength narrows with decreasing Abbe number or increasing spatial frequency. Blue-blocking IOL chromophores improve photopic MTF performance by less than 1.5%.

CONCLUSIONS

Most pseudophakic longitudinal chromatic aberration arises from the chromatic dispersion of IOLs rather than the cornea and other ocular media. Increasing the Abbe number of optic materials improves overall pseudophakic optical performance. Optical transmission of medium and high spatial frequency modulation information has a spectrum similar to photopic luminous efficiency, accounting for the inability of blue-blocking chromophores to improve photopic pseudophakic contrast sensitivity significantly and demonstrating the excellent mutual adaptation of modulation transfer by the eye's optics and management of that data by the retina and brain.