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E. Dalimier, J. C. Dainty, J. L. Barbur; Effects of Higher-Order Aberrations on Visual Performance as a Function of Light Level. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1512.
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Adaptive Optics (AO) techniques have recently been used to investigate the effects of higher-order (HO) aberrations on contrast sensitivity. The purpose of this study was to measure functional visual performance with and without correction of HO aberrations over a range of ambient illumination in order to establish how HO aberration correction may benefit functional vision.
Seven young and healthy subjects were tested with a visual stimulator that incorporated an AO system. The main features of the latter were a Shack-Hartmann wavefront sensor, a Badal optometer and cylindrical lenses for the pre-correction of refractive errors, and a 35 element bimorph mirror for the dynamic correction of higher-order aberrations. The system demonstrated good correction of ocular higher-order aberrations (i.e., less than 0.1 microns remaining rms wavefront error for a 6mm pupil, after static pre-correction of refractive errors). Contrast acuity measurements (ASEM, 74, 551-559, 2003) in narrow band illumination were carried out through the system with either a CRT monitor or a digital projector. Calibrated neutral density filters were also used to set the appropriate light levels.
Measurements of functional visual performance with a fixed 6 mm pupil throughout photopic, mesopic and scotopic regimes showed a decrease of AO benefit when light level was decreased. Furthermore, measurements over a range of pupil sizes at each light level made it possible to quantify the typical benefit that can be expected from correction of HO aberrations as a function of light level (i.e., 0.3 to 1000 trolands). Numerical analysis was performed as an attempt to explain the measured values of AO benefit, in relation to the luminance dependant neural sensitivity.
The results show that at low retinal illuminances, neural factors limit the effect that increased higher-order aberrations can have on visual performance. Measurements also provide an estimate of the AO benefit for functional vision expected under natural viewing conditions from scotopic to photopic light levels.
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