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Adam J. Winkeler, Larry N. Thibos, Jayoung Nam, Carolyn Begley; Observation and Investigation of Tear Film Derived Caustics Cast Onto the Iris Surface. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4260.
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The purpose of this study was to investigate the formation and properties of optical caustics cast onto the iris surface through areas of tear film disruption or instability.
A slit-lamp microscope modified to emit near infrared light was used to observe the irises and tear films of five subjects. After anesthetic instillation, subjects were asked to refrain from blinking while images were recorded using a high-resolution CCD camera attached to the slit-lamp. Independent rotations of the slit-lamp observation and illumination systems about the cornea’s center of rotation provided parallax clues for the source of caustics observed on the iris. Lateral displacement of caustics relative to similarly shaped areas of overlying tear film disruption was measured as a function of rotation of the illumination system. Triangulation of these measurements along with the illumination system rotation angles were used to calculate the depth of the caustic patterns relative to the cornea.
Caustics appeared as patches of low spatial-frequency light and dark patterns underneath patches of similarly-shaped, disrupted tear film observed by retro-illumination. Lateral displacement of these posterior caustics relative to the anterior tear film was observed upon rotation of the illumination system. Conversely, rotation of the observation system resulted in displacement of the tear film disruption relative to the caustics. The depth of the caustics was calculated to be 3.19 ± 1.07 mm (Mean ± SD), which agrees with anatomical separation of the cornea and iris, thus confirming the caustics exist on the iris surface. Caustics were also observed on the iris underlying identification markings on contact lenses.
The observed low spatial-frequency patches are caustics cast onto the iris through the propagation of converging and diverging light derived from local differences in tear film thickness. Caustics formed on the iris surface provide a non-invasive method to study tear film disruption over regions of the cornea or a contact lens not accessible by retro-illumination from the fundus. Retro-illumination of the tear film by light reflected from the iris allows visualization of fine details, whereas caustics formed on the iris show the low-pass filtering effects of light propagation.
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