Ross Franklin

Automatic estimation of the corneal limbus in videokeratoscopy

An algorithm for estimating the corneal limbus from videokeratoscopic images is proposed. After the image is transformed to a polar grid, a novel edge-detection procedure, suitable for the detection of the soft edge produced by the limbus, is used to locate the limbus. Outliers due to the eyelids, eyelashes, and videokeratoscopic rings are removed by taking advantage of the approximate circularity of the cornea. An ellipse which minimizes the sum of the squared algebraic errors is fitted to the remaining edge points. Comparisons between the proposed algorithm, a manual computer-based technique and an algorithm which uses conventional edge-detection techniques demonstrate the accuracy of the proposed algorithm.

Combining central and peripheral videokeratoscope maps to investigate total corneal topography.

Purpose. To extend the area of standard corneal topography maps, one central map is combined with six peripheral maps after correlating them in a custom written computer program. Methods. The point corresponding to the vertex normal of the central map is found in each of the peripheral maps. Data from the peripheral maps can then be added on to the edges of the central map to create a topography map that extends from limbus to limbus horizontally and vertically. Results. The average size of the combined maps from 15 subjects was 11.3 ± 0.3 mm horizontally and 10.3 ± 0.3 mm vertically, compared to 9.2 ± 0.4 mm horizontally and 7.5 ± 0.7 mm vertically for the standard single maps. These values represent an increase in surface area of approximately 70%. Conclusions. The topography of the entire cornea can be represented by combining multiple measurements from a Placido videokeratoscope. Conic fits based on central topography data are a poor representation of the total corneal shape.

Optical considerations in the contact lens correction of infant aphakia.

Background. Correction of infant aphakia with conventional soft and rigid designs induces significant amounts of positive spherical aberration. Methods. Different levels of positive spherical aberration were induced by rigid lenses positioned close to the eye. Visual acuity and depth of focus were measured for twelve young adult subjects. Results. Positive spherical aberration acts to produce an increase in relative depth of focus, but because of the reduction of best visual acuity, there is a concurrent decrease in absolute depth of focus. As the spherical aberration increases, there is a progressive loss of best-corrected visual acuity. Conclusions. The correction of spherical aberration in contact lenses for infant aphakia provides the best theoretical optical conditions for normal ocular growth and visual development. We provide examples of the optimal aspheric surfaces to minimize spherical aberration in contact lenses for infant aphakes.