Analysis of keratoscopic images for detecting fixational eye movements and ocular surface deformation

摘要

A sequence of videokeratoscopic images was registered using commercially available instrument E300 at a rate of 50rnfps. During the 20 seconds measurement, subject's head was fixed strongly. Acquired images were analyzed forrndetecting fixational eye movements and corneal surface deformation. For this purpose two rings were extracted fromrneach frame and the ellipses were fitted to them, using least square method. The time series of the ellipses geometricalrnparameters were considered: minor and major axes length as well as the ellipses center and the orientation. Thernfrequency spectra of mentioned parameters were obtained by application of the Fast Fourier Transform. The longitudinalrnposition of the corneal apex was controlled, thanks to the cone side viewer installed inside the videokeratoscope.rnThe average amplitude of the variation of the ellipse's axes length is around 20μm and of the orientation of the ellipsernaround 0,1 rad. In the signals frequency characteristics, appear the peak corresponding to the heart rate. No clearrnrelationship was found between the variations of the fitted ellipse parameters and the longitudinal position of the cornealrnapex.rnThe fixational eye movements were examined using two different methods. One of them consists of calculating therncorrelation function between the first and successive frame of the sequence and searching its maximum. The other isrnbased on tracking the center of the ellipse fitted to particular ring of the videokeratoscopic image. The accuracy of thernsecond method found to be higher.rnSimple methods proposed in this work can extend the application of videokeratoscopic measurements.