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Michael Simpson; Parameters affecting vignetting and negative dysphotopsia in the far peripheral visual field of the pseuodophakic eye. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2970.
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As visual angle increases, light becomes increasingly vignetted (blocked) at the edge of an intraocular lens (IOL) for large visual angles, and light that bypasses the IOL completely may also create a second displaced image. These imaging characteristics are likely to be the source of occasional reports of dark shadows in the temporal visual field (negative dysphotopsia), and calculations have been made to estimate the visual angles at which this occurs.
The Zemax raytrace software was used to extend the work reported in J. Opt. Soc. Am. A v31, pp. 2642-2649 (2014), to evaluate visual angles for a range of eye characteristics. The same basic eye was used (Figure 1), but the pupil diameter and ocular distances were varied. The main parameter of interest is simply the percentage of light rays passing through the iris that also pass through the posterior surface of the IOL. A routine was written in Zemax to determine this for collimated light with different input visual angles.
Representative plots are given in Fig. 2 for the eye described in Fig. 1. The onset of vignetting varies with various parameters, but the visual angle at which focused light is truncated by the IOL can be as low as 50 degrees. By 75-80 degrees of input visual angle, about half the light is no longer focused by the IOL for any pupil diameter for this example. Vignetting is lower for smaller iris-to-IOL separations, calculated separately, but even when this is 0.5 mm about half the energy is obstructed for a visual angle of 85 degrees for larger pupils.
The IOL is very much smaller than the natural crystalline lens, and the visual angle at which about 50% of the light is no longer focused by the IOL in a model eye may be a useful parameter for evaluating “dark shadows”. Clinical data for the relevant parameters are rarely reported and difficult to measure, but the visual angle for the strongest shadow, along with the physical characteristics of the eye, could be used to create individual eye models for comparison to clinical date. Any decentrations and tilts of the IOL and the iris should also be included, since these would increase the effect for smaller pupils.
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