![]() These measurements were corrected for the OTF of the measurement window and extrapolated to the cutoff frequency of the display. The display MTF was determined empirically by displaying a series of high contrast, sinusoidal gratings and then measuring the displayed contrast and phase using SpectraScan PR714 (Photo Research) radiometer configured with a 2-pixel wide, rectangular measurement window oriented parallel to the gratings. Monochromatic aberrometry measurements (Wavefront Sciences, COAS) across the central 2.5 mm pupil confirmed that the subjects’ OTFs were within a couple of percent of diffraction limit, and thus during the experiment subjects used a 2.5 mm artificial pupil and their optics were represented in the pre-compensation process by the diffraction limited OTF of a 2.5 mm pupil. The display non-linearity and its two-dimensional MTF as well as the subject’s OTF were measured and pre-compensated in the image calculations. 5, 30Īll optical computations employed a 5 mm pupil. Partial implementation of the Burton and Haig method has been used previously to examine the impact of HOAs on VA. In order to employ this method accurately, the optical degradation of the display monitor and the subject’s eye must be pre-compensated within the blur calculations by deconvolving the computed image by the display MTF and the observer’s eye OTF. This method employs computationally blurred stimuli, which generate retinal images in the experimental subject’s eye degraded only by diffraction and the specific monochromatic HOAs included in the computational model 28, 29. We studied the visual impact of monochromatic HOAs on vision using the approach first described in detail by Burton and Haig 27. The Seidel convention for defining monochromatic aberrations, defines SA as a 4 0r 4, and thus lacks the r 2 term that dominates the central region of Zernike SA ( Figure 1). It is clearly possible, therefore, that the observed effect of Zernike SA on VA is due not to the r 4 component of the polynomial (the component that makes it “spherical aberration” and is primarily responsible for the WFE at the pupil margins), but rather by the r 2 term generating spherical defocus-like wavefronts in the pupil center. These results suggest that VA is determined primarily by the WFE in the central portion of the pupil, and optimum VA is achieved by flattening the central wavefront. This structure of individual Zernike modes becomes important because experimental studies have shown that, in eyes with SA, best VA (subjective) refractions for circular pupils are dominated by the central optics 24 – 26. For example, the “spherical aberration” (SA) Zernike polynomial includes both r 4 and r 2 terms and defines the following wavefront error (WFE)Ĭonsequently, the WFE in the central 50% of the pupil is dominated not by the r 4 term (which is approximately zero in the central region of the pupil), but by the opposite sign r 2 term ( Figure 1). 23Īlthough individual Zernike polynomials (modes) are considered as individual aberrations, they contain multiple terms. Studies using computationally blurred letter charts 5, or deformable mirrors 8, 21, 22 have shown that individual Zernike modes closer to the center of the Zernike pyramid (lower meridional frequencies) have more impact than those near the edge (higher meridional frequencies). It appears, however, that not all HOAs have the same ability to degrade vision. Conversely, correcting HOAs using deformable mirrors 7, 19, custom lathed optical corrections 20, and aspheric IOLs 11 can improve VA. Increased levels of Higher Order Aberrations (HOAs) introduced by multifocal (aberrated) Intraocular Lenses (IOLs) 12, multifocal contact lenses 13, aberrated custom contact lenses 14, corneal disease 15, and refractive surgery 16 – 18 can all lead to reduced visual acuity. Best Spectacle Corrected VA in studies of post-refractive surgery, intraocular implants, and keratoconic eyes. Accordingly, visual acuity continues to be employed clinically as a surrogate measure of image quality when assessing the impact of higher order aberrations (e.g. In addition to the well-documented impact of lower order aberrations on VA, more recent studies have shown that higher order aberrations can also degrade VA 5 – 8. This surrogate test of retinal image quality forms the basis of subjective refractions for the simple reason that the VA is relatively easy to measure and it varies in direct proportion to the level of defocus, modulated only by pupil size (MAR = Blur/4, where blur = blur circle diameter = defocus (D) × pupil diameter) 1 – 4. In the clinical environment, visual acuity (VA) testing is the gold standard, albeit indirect method for assessing optical quality of the retinal image.
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