No abstract

The results identify another type of sunlight hazard: the peripheral focusing of obliquely incident light. UVR from albedo (reflected ambient light) is capable of establishing PLF in the anterior segment, but this can be shielded by UV-blocking soft contact lenses. Sunglasses may be unable to shield oblique rays, unless side protection is incorporated. Contact lenses can offer UVR protection against all angles of incidence, including the peak-response angle. They can also protect the eye in settings in which the wearing of sunglasses is not feasible or convenient.

Our aim was to examine secondary image formation in the anterior segment caused by peripheral light focusing (PLF) in the human cornea, and in particular the crystalline lens. Non-sequential ray-tracing (OptiCAD) was applied to an anatomically based human eye model, which incorporates a gradient index crystalline lens. For analysis of the limbal effect, we varied the incident angle from 100 to 122 degrees, while for the crystalline lens effect, the incident angle was varied from 60 to 90 degrees. The corneal shapes studied included central radii from 7.4 to 8.2 mm with a range of shape factors. In each case, we computed the peak and average intensities, and the area of exposure at the limbus or lens periphery. The computation was repeated with a previous model eye for comparison. For the limbal effect, a peak intensity gain of x22.5 was found at an incident angle of 104 degrees which compares well with previous results. The average intensity gain at this angle was x7.5 over an area of 0.23 mm2. Steeper corneal curvature produced a greater PLF effect. For the crystalline lens effect, maximum UVA (365 nm) intensity gain peaked at x8.6 at 84 degrees with average intensity gain of x2.3. The area of UVA exposure peaked at 4.7 mm2 at 70 degrees. A relatively wide range (30 degrees ) of incident angles produced peak PLF gains of x3 or more in the lens. Significant focusing of light is directed to the nasal limbus, and to a lesser extent to the crystalline lens over a broad range of incident angles. PLF in the nasal cornea is reduced by an order of magnitude when a UV-blocking soft contact lens is used. The concentration levels and intraocular sites of PLF action on UV and visible light suggest a new mechanism of phakic dysphotopsia and lens phototoxicity.

Off-axis light incident at the temporal cornea formed intense foci in the nasal edge of the IOL. Multiple secondary images can be formed. These spots constitute glare sources and may be of sufficient intensity to disturb vision. These findings indicate that off-axis light may be an important contributor to some disability glare problems and that IOL edge design should be reevaluated to minimize dysphotopsia associated with peripheral focusing effects.