What role does chromatic aberration play in IOLs?
White light is comprised of different wavelengths of visible light, ranging from red (700 nm) to violet (400 nm). As white light passes through an optical system, each of its component wavelengths bends independently.
By Snell’s law, faster-moving longer wavelengths bend less than slower-moving shorter wavelengths, dispersing the various colors to different focal points along the optical axis.
This dispersion is referred to as longitudinal chromatic aberration (LCA). The impact of LCA is thought to be greater than the impact of all higher-order aberrations1, but its effect on IOLs or on pseudophakic vision has not been well characterized.
The Abbe number can be used to quantify LCA in optical materials. Based on Abbe number calculations, the phakic eye has a baseline LCA of about 1.25 D of chromatic refractive difference between red (650 nm to 700 nm) and blue (450 nm) light.2 In pseudophakic eyes, the IOL implanted can potentially increase or decrease the eye’s LCA relative to the phakic state, depending on the properties of the IOL.
Unfortuntaely, Abbe numbers are not made publicly available by IOL manufacturers. Additionally, the effects of chromophores and diffractive patterns on LCA have not been published. A study presented at the 2016 Association for Research in Vision and Ophthalmology (ARVO) meeting attempted to further characterize LCA in IOLs, through in vitro optical bench and in vivo clinical testing.3
To explore the amount of ocular LCA of different IOLs in vitro, five intraocular lenses made from two different hydrophobic acrylic materials were placed in an ACE model eye on an optical bench. The ACE (Average Cornea Eye) aphakic eye model simulates a human cornea with average spherical aberration (SA) and LCA.4
Through-focus modulation, transfer-function (MTF) measurements were performed using a 3.0-mm aperture. Measurements were performed at five different wavelengths: 450 nm, 500 nm, 550 nm, 600 nm, and 650 nm. LCA was expressed as the difference in optical power for the different wavelengths.
LCA of the aphakic eye model was 1.04 D (Figure 1). The first hydrophobic acrylic material had LCA of 1.30 D (Tecnis, Johnson & Jonson Vision Care/Abbott Medical Optics), and the second hydrophobic material had LCA of 1.77 D (Acrysof, Alcon Laboratories).
The presence of a chromophore did not significantly affect LCA. Multifocal diffractive rings did not affect the LCA at the far focal point, but an achromatic diffractive ring pattern significantly reduced LCA as measured in vitro.
This study demonstrated that in a physiologically representative eye model the chromatic aberration of IOLs can be measured and varies widely for different IOL materials.
However, bench testing is insufficient to fully understand optical quality because it is such a pure measure. It is difficult to accurately model the imperfections of the human eye—and impossible to model all the complex factors that go into the human visual system, including higher order aberrations, ocular surface, and retinal factors; emotions, moods, and fatigue; and the ability of the brain to compensate for aberrations, interpret visual stimuli, and interpolate visual information.