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Howard C. Howland, Anupama Kumar, Evguenia Ivakhnitskaia, Reshmitha Radhakrishnan, Robert L. Toha; Measurement Of Contact Lens Movement From Variations In Direction Of Apparent Gaze Using 1st Purkinje Images. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6528.
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© ARVO (1962-2015); The Authors (2016-present)
To measure contact lens movements under natural viewing conditions using 1st Purkinje images.
A PowerRefractor, which uses the location of the first Purkinje image relative to the pupil, was employed to track the direction of gaze in subjects, with and without contact lenses, viewing targets on a tangent screen at 1 m distance. In order to calibrate individual subject’s eye movements, these were recorded while the subjects fixated Maltese crosses positioned at known angles relative to the recording instrument. Continuous recordings of eye movements over several seconds, sampled at 30 Hz, were averaged, and the angular variances about the average fixation points were calculated and compared for the same subjects with and without contact lenses. Autocorrelations were performed on individual data vectors, and the comparison t-tests were corrected to reflect the number of independent data points. Optical analysis showed that the motion of the first Purkinje image is a function both of the direction of the gaze and the slippage of the lens. Assuming constant fixation accuracy, the actual motion of the contact lens may be calculated from the optical parameters of the contact lens, and the recorded rms angular variation of the gaze may be using a proportionality constant obtained from ray tracing.
Preliminary experiments with two subjects showed that the recorded variations in Purkinje images with contact lenses exceeded those without contact lenses by 0.56° + 0.19°. Assuming that the fixation accuracy remained the same with or without contact lenses, this implies rms movements of the contact lenses on the order of a millimeter.
A method for measuring the rms movement of a contact lens in a fixating eye has been described. Preliminary results show these movements to be on the order of a millimeter. The optical methods used here may be employed to measure the power of contact lenses on fixating eyes.
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