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S. S. Chin, K. M. Hampson, E. A. H. Mallen; Binocular Correlation Between Aberration Dynamics. Invest. Ophthalmol. Vis. Sci. 2008;49(13):981.
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The dynamic correlation between ocular aberrations beyond defocus is currently unknown. We measured the aberrations simultaneously in both eyes by using a binocular Shack-Hartmann sensor.
The binocular Shack-Hartmann sensor consists of a single laser source and one camera for system simplicity and low cost. The sampling frequency is 20.65 Hz. Hot mirrors are used to permit open-view observation. The target is a Maltese cross presented on an LCD monitor, subtending 11.32 min of arc at the retina. Measurements were carried out on six observers, aged between 23 and 33 years. Their refractive errors ranged between plano to -7.00 D of sphere and less than -1 D of astigmatism. Each measurement run lasted 24 s and five repeated measurements were taken. Coherence function analysis was used to investigate the correlation between each aberration of the two eyes in the frequency domain. Since the break-up of tears in the two eyes is likely to be asymmetrical, a lowering of the coherence values would be expected. To study the effect of tear film break-up, the experiment was repeated with dry front surface scleral contact lenses. Due to the preservation of tear film between the lens and the cornea, the blink reflex could be avoided and tear film break-up eliminated. Zernike coefficients up to and including 8th radial order were calculated (excluding tip and tilt).
Results were subject-dependent. The coherence values were different for each resolvable frequency component of each aberration. In general, the coherence values were fairly low for each aberration (typically < 0.1). Averaging the coherence values across frequecies and subjects showed that the correlation is highest for defocus (Z4), followed by Z29 and Z26. The removal of tear break-up by means of scleral contact lenses resulted in insignificant changes in the coherence values.
Dynamic correlation of ocular aberrations was studied in detail with a binocular Shack-Hartmann sensor. Coherence function analysis shows that the inter-ocular correlation between aberrations is dependent upon the subject, aberration modes and frequency components. Generally the coherence values are low suggesting poor correlation between the aberration dynamics of the two eyes. Uneven tear film break-up does not account for this low coherence.
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