Eyeworld

EW NEWS & OPINION 18 June 2015 by Daniel H. Chang, MD Seeing a sharper world through rose (and blue) colored glasses Abbe numbers, often used to quantify CA, are calculated with spe- cific wavelengths of red, yellow and blue light. Higher Abbe numbers indicate lower dispersion of colors and lower CA. The phakic eye has a baseline level of chromatic aberration (pri- marily resulting from the dispersive properties of the cornea and crys- talline lens) that induces about 1.25 D of chromatic refractive difference between red and blue light. This means that when white light is focused on the retina, the red wave- lengths are actually 0.25 D hyper- opic (behind the retina) and blue wavelengths are 1.0 D myopic (in front of the retina). An eye's CA can be improved with cataract surgery by using an IOL with an Abbe num- ber greater than that of the natural lens (47). With a tighter focus of all colored wavelengths, patients could actually experience the best vision of their lives. Using a high Abbe number IOL may be more important than reducing spherical aberration (SA) because CA is a material-dependent property that benefits every eye. SA is a surface curvature property, so the ability of an IOL to reduce SA depends on the actual shape or amount of SA present in the eye, the size of the pupil, and the centration of the lens. Chromatic refraction Most manufacturers do not publish the chromatic aberration properties of their IOLs. Ophthalmic diagnostic devices, which typically use mono- chromatic (usually infrared) light, are not able to measure CA either. However, by performing a simple chromatic refraction, it is possible to characterize the impact of CA on pseudophakic vision. When performing a chromatic refraction, one needs to approxi- mate a single wavelength of light, and then compare that to another single wavelength as far away on the spectrum as possible. If the wave- lengths are too close together on the spectrum—say, red and green—there Chromatic refraction demonstrates that wavelength-dependent aberrations may play a bigger role in pseudophakic visual quality than most surgeons realize W hite light is comprised of different colors (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 refracts or bends independently. The classic example is a prism, which splits white light into its component colors, creating a beautiful rainbow of dispersed light. In the human eye, dispersion should be minimized so that all colors of light focus to- gether to provide sharp, high quality vision. With basic monofocal IOLs, patients rarely complain about the quality of their vision. However, as we increase the complexity of the optical system with toricity and multifocality, and as we increase ex- pectations from cataract surgery, any degradation in visual quality could become problematic. We should therefore be aware of the potential effects of chromatic aberration on visual quality. Chromatic aberration Chromatic aberration (CA) is defo- cus caused by the wavelength-de- pendent nature of a lens material's refractive index. By convention, when referring to the refractive index of a given material (e.g., a cornea, spectacle lens, or IOL), 589- nm yellow light is used. For that same material, longer wavelengths propagate through the material faster (the refractive index is lower), and shorter wavelengths propagate through slower (the refractive index is higher). Therefore, by Snell's law, longer wavelengths bend less and shorter wavelengths bend more. The result is a color defocus. When white light is focused on the retina, red light induces 0.25 D of hyperopia and blue light induces 1.0 D of myopia. The red and blue wavelengths are focused behind and in front of the retina, respectively. As light passes through a lens, the different wavelength components of white light are bent differently, causing dispersion or chromatic aberration. Abbe numbers are calculated using the index of refraction for specific red (656 nm), yellow (589 nm), and blue (486 nm) wavelengths. continued on page 20

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