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L.S. Kwok, T. Pham, M.T. Coroneo; Off-Axis Secondary Image Formation Due to Peripheral Light Focusing as a Source of Pseudophakic Dysphotopsia . Invest. Ophthalmol. Vis. Sci. 2003;44(13):271.
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Purpose: To quantify the influence of pupil size and intraocular lens (IOL) design on unwanted photic effects (dysphotopsia) due to secondary image formation from off-axis light focusing. Methods: The refractive optics of the pseudophakic human eye were simulated with an anatomically-based computer model. Non-sequential optical ray-tracing (OptiCAD) was performed with an IOL of anterior radius 14.23 mm; posterior radius 25 mm; 0.72 mm centre thickness; 5.5, 6, 6.5 mm dia; and refractive index of 1.5597. Peripheral light focusing due to off-axis light was calculated for light rays incident on the temporal cornea at angles of 0° to 120° to the sagittal plane. Pupil diameters of 3, 5 and 7 mm were used. Results: Focused light was able to strike the nasal retina over a large range of angles up to 90º incidence (frontal plane). The maximum intensity achieved was 1.4-1.5 log units over incident intensity. For a bandwidth threshold of 1 log unit intensity, the angle bandwidth increased with pupil size, being 38º at 3 mm to 50º at 7 mm. The greatest area of secondary light formation occurred around 80º incidence. IOL diameter made little difference. A smaller 20º bandwidth of incident light was able to focus on the nasal edge of the IOL optic with an intensity of 0.4 log units over incident intensity. Conclusions: Off-axis light can form focal areas on the nasal retina and IOL optic edge. These secondary foci have sufficient intensity and area to become noticeable under certain viewing conditions. Their occurrence over the physiological range of pupil sizes may explain the unwanted photic effects experienced by pseudophakic patients. Our results indicate that current IOL design principles need revision to minimize secondary image formation by peripheral light focusing.
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