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M. Nowakowski, A. Goncharov, C. Dainty; Study of Optical Effect of Tear Film Variation After Blinking in Young Healthy Eyes. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1561.
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Ocular aberrations have recently been studied using improved aberrometers based on the Hartmann-Shack wavefront sensor that provides much higher accuracy than other techniques. The purpose of this study was to measure the changes of ocular aberrations on the visual axis after consecutive blinking, which is directly related to tear film variation from blink to blink. The optical effect of the tear film on vision is of great interest for eye modeling, corneal topography and eye aberrometry in general.
Five young and healthy subjects were studied with an ocular aberrometer that incorporates the Hartmann-Shack wavefront sensor, pupil-tracking system, Badal adjustment stage, fixation target and laser-diode probing beam. The aberrometer was used to measure ocular aberrations in the plane on visual axis. Each measurement consists of 20 frames within 2 sec, which are sufficient to find standard deviation (SD) after a single blink. A series of measurements taken after several blinks provide SD for multiple blinks. We compare the SD for single and multiple blinks for individual aberration groups to find the general trend of tear film variation and its contribution to ocular aberrations.
The data was collected for 5 young eyes without any corneal abnormalities. Numerical analysis of each single Zernike term (up to 4th order) was performed to study the variations of its amplitude after blinking. We found that there is a large variation in amplitude within our set of Zernike terms due to blinking. The standard deviation for some aberrations after multiple blinks was greater than for other groups.
The experimental data shows that not all aberration terms are affected by the tear film fluctuations. We observed that the terms with horizontal axis of symmetry (in the pupil) undergo a significant change in amplitude after blinking. These terms include horizontal/vertical astigmatism Z(2,2), vertical coma Z(3,-1), quadratic astigmatism Z(4,2). In addition, the defocus term and spherical aberration also showed similar fluctuations in amplitude. We found that for 6mm pupil, the RMS wavefront error reaches its minimum value within 4-5sec after blink, which corresponds to the most regular distribution of the tear film.
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