| Abstract |
Visual performance is defined by how well a visual task of interest can be performed by a given individual. Therefore, the purpose of a conventional, ophthalmic refraction of the eye is to determine that combination of spherical and cylindrical lenses which optimizes visual acuity. This study is concerned with the problem of objectively determining the best correction of an eye from measurements of wavefront aberrations. The wavefront aberration is a function that characterizes the image-forming properties of any optical system (Born & Wolf, 1985). It is defined as the optical deviation of the wavefront along a certain ray from the perfect spherical wavefront. A Hartmann-Shack wavefront sensor was used to measure the monochromatic wave aberrations of the subjects. We determined the accuracy of two groups of objective methods for predicting the results of conventional, sphero-cylindrical refraction from wavefront aberrations in a population of 81 eyes. All metrics used in this study are mathematical functions that have as input the normalized Zernike expansion coefficients and as output a single value. The second group of metrics, as opposed to the first on, included the Stiles-Crawford apodization filter in the calculations. We investigate whether the predictions from these methods were improved. Accuracy for predicting defocus varied from 0.28D to 0.4D across methods. Objective determined defocus was significantly correlated with the subjective sphere. The metric mVSMTF, which included the Contrast Sensitivity Function in its calculations, was one of the most precise methods and exhibited the optimal performance. It is important to mention that we introduced a new metric, called weightedMTF, which proved good predictor of optical quality. This metric was inspired by mVSMTF but it differs significantly as it includes the impact of higher spatial frequencies. Inclusion of Stiles-Crawford improved the predictions from all metrics. All metrics discussed were calculated both for monochromatic and polychromatic light. The link between visual performance and optical quality of the eye enjoys a renewed interest. However, visual perception is highly subjective and involves many aspects of image quality. Therefore, it may be unrealistic to suppose that one single metric will capture all aspects of image quality. Instead, multivariate metrics may be needed to adequately predict the visual effects caused by optical defects of the eye and to prescribe the ideal correction.
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Υπολογισμός του διαθλαστικού σφάλματος με χρήση αναλυτών μετώπου κύματος
