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| |  ARVO: New Method Developed for Measuring Crystalline Lenses and Intraocular Lenses By Mike Fillon FORT LAUDERDALE, FL -- April 29, 2004 -- Researchers developed an imaging system for in vivo measurement of curvatures, radii, tilt and decentration of both crystalline lenses and intraocular lenses (IOLs). Based on Purkinje imaging technology, lead researcher, Patricia Rosales, PhD, Instituto de Optica (CSIC), Madrid, Spain, said studies show the system to be rapid, systematic and reliable. She reported the results here on April 26th in a poster presentation at the Association for Research in Vision and Ophthalmology Annual Meeting. Dr. Rosales said the internal aberrations of the eye depend on the crystalline lens (or IOL) geometry, its position and refractive index. "Tilt and decentration data on individual eyes are important, to assess the sources of optical quality degradation in the eye." She explained how the Purkinje-Image concept works: Light rays striking the eye produce four reflections, called Purkinje images, from the front and rear surfaces of the cornea and lens. The first (virtual) and the fourth (real) Purkinje images lie in the same focusing plane. These two images move similarly under translation but differentially under rotation. The change in their separation is used to determine eye rotation free of translation error. This new system consists of illuminating channels for right eye/left eye with one double infrared (IR) light emitting diode (LED) at an angle of approximately 20 degrees, and collimated light from a single IR LED at approximately 14 degrees. An imaging channel with an IR-enhanced charged coupled device camera with a telecentric lens, collinear with the line of sight; a fixation channel with a stimuli displayed on a minidisplay; and a Badel system to correct refractive errors and change vergence. For the study, the researchers recruited five moderately myopic subjects with an average age of 29.6 years. Dr. Rosales and colleagues measured anterior and posterior crystalline lens surface radius of curvature using two methods, tilt and decentration of the crystalline lens in vivo in a set of normal eyes, and changes of the crystalline lens radii of curvature, tilt and decentration with accommodation. The estimations were validated through simulations with the Zemax optical design program, and control experiments in eyes with IOLs of known specifications, or measured though alternative methods. These simulations found repeatedly that this new method to calculate the radii of curvature provided better estimates (approximately 8%) than the equivalent mirror method, and that lens tilt and decentration estimates were not very sensitive with differences of 0.06 degrees and 0.05 respectively, to the lens model used -- spherical versus aspheric surfaces. Also, the anterior lens radii measured experimentally in vivo ranged from 7.5 to 11.6 mm, and the posterior lens radii from -4.9 to -6.8 mm across the eyes. Lens tilt ranged from 2.2 to -14 degrees, and lens decentration from -0.03 to +0.29 mm. Experimental variability was 1,3 degrees and 0.06 mm for lens tilt and decentration respectively.
[Presentation title: "A purkinje imaging system for in vivo measurement of curvatures, tilt and decentration of the crystalline lens." Poster B-533]
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