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Chromatic aberration: A new metric in IOL performance Joseph J.K. Ma, MD, FRCSC Figure 3: The effect of chromatic aberration is visible around the dark edges of the lower photograph (especially on the right). The images show only a part of the photo from the corner of the original image to emphasize the effect of aberration. Source: Stan Zurek What chromatic aberration is and why it matters for your pseudophakic patients C hromatic aberration is im- portant in the performance of any optical system. As white light passes through a lens, each component wavelength is refracted differently, depending on the lens material's index of refraction for that wavelength. The property of the lens that describes this is known as dispersion. Longer wavelengths bend less and shorter wavelengths bend more, resulting in a dispersion of light, as one sees in a prism. Essentially, CA is the fail- ure of the different wavelengths of light to converge on a single point. In three dimensions, this happens both along the optical path (axial CA) and within the plane of focus (transverse CA). Every refractive material has some degree of CA. However, depending on the optical properties of the material, that dispersion of light into its compo- nent colors can be narrower or wider. The greater the dispersion, the less sharp the image can be, even when it is in maximal focus. Photography provides a great way to understand this. Photographers use a number of tricks to avoid or reduce CA as they capture and process images. In the enlarged corner of a photograph, you can see how a photo without CA com- pares to the same image with CA (Figure 3). In the lower image, there is some blur and a color fringe of light, representing transverse CA. CA and IOLs In the eye, axial CA results in some wavelengths of light focusing beyond the retina and some in front of the retina, reducing image quality. Ophthalmolo- gists take advantage of axial chromatic aberration when we use the duochrome red/green test. The difference in focus be- tween red and green light is what makes this test useful in refining the refraction. Although we rarely think of it this way, axial CA is also responsible for myopic shift at night. Our eyes use more blue-vi- olet light at night, and that light focuses further in front of the fovea, inducing night myopia. Following cataract surgery, the IOL that is implanted can either reduce or increase CA, and this can be influenced by both material and design features. The most important contribution is from the IOL material. Each implant has a fixed level of CA that is intrinsic to the lens material itself. The CA and dispersion properties of lenses currently marketed today vary considerably (Figure 4). If the material has higher chromatic aberration and light dispersion (indi- cated by a lower Abbe number), there will be some blur or reduction in image quality even when perfect emmetropia is achieved, similarly to how a photo- graph can be in focus but still have color fringing along the edges of objects in the photo. Secondly, the design of an IOL made of any given material can, to a degree, be manipulated to correct for chromatic aberration through a particular use of diffractive optics. For most ophthalmol- ogists, "diffraction" is synonymous with reduced image quality and decreased contrast sensitivity because of their association of diffraction as an accepted tradeoff for multifocality. This is not a property of diffractive optics. In fact, dif- fraction results in the opposite dispersion of light from refraction, which is what allows a strategically designed diffraction pattern to minimize CA. In photogra- phy, diffractive optics have been used to improve optical image quality. Canon, for example, has used a multilayer dif- fractive element to decrease chromatic aberration in a high-end telephoto lens since 2001. A new IOL that was recently launched in Europe, the Tecnis Symfony, also uses a diffractive mechanism to correct for CA to improve contrast. 4

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