Abstract
Purpose :
Intraocular lenses (IOLs) are widely used following cataract surgery, but there is no consensus yet about why some patients see bothersome peripheral dark shadows (negative dysphotopsia). Recent evaluations have shown that the focused image has a limited extent, due to the small IOL diameter causing “vignetting” (Fig 1). This can lead to darkness at about 80°-90°of visual angle, though light bypassing the IOL provides illumination more peripherally. Improved optical modeling is described here, using a structured object, rather than point or extended light sources used previously.
Methods :
Simulated images of a text chart were created using a raytrace model of an eye, with a simple high refractive index IOL that has a textured edge and no haptics. The images correspond to angles subtended at the 2nd nodal point, which approximately relates the retinal location to the input visual angle of a phakic eye. The point spread function (PSF) varies strongly with radial location, and a simplified image was created using just the horizontal PSF values for all vertical points. Rays were added to the image, after weighting by the object luminance, total transmittance, and a cosine normalization with angle to increase peripheral intensity.
Results :
Images are given in Fig 2. The object was moved sideways between calculations with a 2.5 mm pupil, and an increasing pupil diameter was also used.
Conclusions :
The simulated images are only very approximately correct, because the actual retinal scaling is unknown, and there are limitations due to plotting angular locations as a rectangular image. However, the images illustrate characteristics that have already been inferred from raytracing. (a) With a 2.5 mm pupil there is a prominent dark shadow. (b) Light missing the IOL experiences lower power and forms a larger image, and also comes from a lower visual angle (the “D” images illustrate this). (c) A small increase in pupil diameter causes the shadow to fade. The limit of the focused image that causes the shadow should be a feature of most IOLs, yet relatively few patients are bothered. The shadow location is fixed with respect to the eye, but unlike the blind spot, the retina is sensitive, and the overall illumination varies as the pupil diameter and ambient scene change. Measuring shadow details, and comparing them to simulations, should confirm the primary cause, and provide a basis for developing improved solutions.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.