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Thomas W. Raasch; Optimizing Subjective Refraction. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2822.
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Subjective refraction plays a key role in most clinical environments, and in many evaluations of ophthalmic therapies and visual performance. The purpose of this study is to investigate potential improvements in subjective refraction, in terms of efficiency, precision, and accuracy, resulting from the ability to independently manipulate dioptric powers in M, J0, and J45. We have developed a clinical refractor that can directly control these low order refractive components along orthogonal axes. This enables the implementation of a wider range of psychophysical procedures, such as the method of adjustment and adaptive maximum-likelihood strategies.
Adaptive procedures are evaluated with this instrument, and through calculation of retinal images created by various combinations of pupil size, low order defocus, and higher order aberrations. The procedure implemented here is a two-dimensional (J0/J45) adaptive maximum-likelihood strategy that converges on the optimum astigmatic correction to maximize subjective clarity of focus. The subject is presented with a set of two-alternative forced-choice trials, similar to the task in a conventional subjective refraction. The procedure terminates either after a fixed number of trials, or when the area of the 95% confidence ellipse declines to a predetermined level.
The adaptive procedure enables subjects to reach an astigmatic endpoint, within ±1/8D, in 16-18 trials. The precision of the measurement, in terms of test-retest differences, varies across subjects, but declines as spherical defocus, and/or higher-order aberration level increases.
Clinical refractors that are capable of flexibly manipulating spherocylindrical powers in three-dimensional dioptric power space enable the implementation of psychophysical procedures that can improve the efficiency, precision, and accuracy of subjective refraction. Adaptive maximum-likelihood strategies may become important components of automated, computerized clinical refraction equipment.
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