Gildas Marin

Aberration generation by contact lenses with aspheric and asymmetric surfaces

PURPOSE We explored the potential of aberration correction in the human eye by using a new generation of soft contact lenses with aspheric and asymmetric surfaces. METHODS Soft contact lens samples were designed with one asymmetrical surface (front) and one spherical (back) to produce predetermined amounts of desired pure defocus, astigmatism, trefoil, coma, and spherical aberration. Contact lens wavefront aberrations were measured ex vivo using a Fizeau-Tolanski interferometer and compared with the in vivo wavefronts obtained by subtracting the aberrations of the eye with and without the contact lenses. These second set of measurements were obtained using a Shack-Hartmann sensor. RESULTS We found that an aberration-free contact lens sample induced in the eye a small amount of residual aberration. We obtained a good match between the ex vivo and in vivo wavefront measurements for most of the samples of the contact lenses. CONCLUSIONS The aberrations generated by soft contact lenses on the eye were predictable. Rotations and translations of the contact lenses with respect to correct position on the eye were, however, the main limitation for precise correction of the ocular aberrations.

Simulated Optical Performance of Custom Wavefront Soft Contact Lenses for Keratoconus

Purpose. Outstanding improvements in vision can theoretically be expected using contact lenses that correct monochromatic aberrations of the eye. Imperfections in such correction inherent to contact lenses are lens flexure, translation, rotation, and tear layer effects. The effects of pupil size and accommodation on ocular aberration may cause further difficulties. The purpose of this study was to evaluate whether nonaxisymmetric soft contact lenses could efficiently compensate for higher-order aberrations induced by keratoconus and to what extent rotation and translation of the lens would degrade this perfect correction.. Methods. Height topography data of nine moderate to severe keratoconus corneas were obtained using the Maastricht Shape Topographer. Three-dimensional ray tracing was applied to each elevation topography to calculate aberrations in the form of a phase error mapping. The effect of a nonaxisymmetric soft contact lens tailored to the corneal aberrations was simulated by adding an opposite phase error mapping that would theoretically compensate all corneal-induced optical aberrations of the keratoconus eyes. Translation (0.25, 0.5, 0.75, and 1.0 mm) and rotation (2.5°, 5.0°, 7.5°, and 10°) mismatches were introduced. The modulation transfer function (MTF) of each eye with each displaced correction and with various pupil sizes (3, 5, and 7 mm) was deduced from the residual phase error mapping. A single performance criterion (mtfA) was calculated as the area under the MTF over a limited spatial frequency range (5 to 15 periods per degree). Finally, the ratio (RmtfA) of corrected mtfA over uncorrected mtfA provided an estimate of the global enhancement in contrast sensitivity with the customized lens. Results. The contrast improvement ratios RmtfA with perfectly located lenses were for an average pupil size of 4.5 mm between 6.5 and 200. For small translation errors (0.25 mm), RmtfA ranged between 2 and 7. The largest lens translation tested (1 mm) often resulted in poorer performance than without correction (RmtfA <1). More than threefold improvements were achieved with any of the angular errors experimented. RmtfA values showed significant variations for pupil diameters between 3 and 7 mm.. Conclusions. Three-dimensional aberration-customized soft contact lenses may drastically improve visual performance in patients with keratoconus. However, such lenses should be well positioned on the cornea. In particular, translation errors should not exceed 0.5 mm. Angular errors appeared to be less critical. It is further questioned whether the visual system is able to adapt to variations in optical performance of the correction in situ due to lens positioning and pupil size.

Visual acuity under combined astigmatism and coma: Optical and neural adaptation effects

Previous studies suggest that certain combinations of coma and astigmatism improve optical quality over astigmatism alone. We tested these theoretical predictions on 20 patients. Visual acuity (VA) was measured under best spherical correction for different conditions: low- and higher order aberrations corrected, in the presence of 0.5 D of induced astigmatism, and adding different amounts of coma to 0.5 D of astigmatism. Measurements were performed for different relative angles between coma and astigmatism and for selected conditions, also through-focus. Adding coma (0.23 μm for 6-mm pupil) to astigmatism resulted in a clear increase of VA in 6 subjects, consistently with theoretical optical predictions, while VA decreased when coma was added to astigmatism in 7 subjects. In addition, in the presence of astigmatism only, VA decreased more than 10% with respect to all aberrations corrected in 13 subjects, while VA was practically insensitive to the addition of astigmatism in 4 subjects. The effects were related to the presence of natural astigmatism and whether this was habitually corrected or uncorrected. The fact that the expected performance occurs mainly in eyes with no natural astigmatism suggests relevant neural adaptation effects in eyes normally exposed to astigmatic blur.

Combining coma with astigmatism can improve retinal image over astigmatism alone

We demonstrate that certain combinations of non-rotationally symmetric aberrations (coma and astigmatism) can improve retinal image quality over the condition with the same amount of astigmatism alone. Simulations of the retinal image quality in terms of Strehl Ratio, and measurements of Visual Acuity under controlled aberrations with adaptive optics were performed, varying defocus, astigmatism and coma. Astigmatism ranged between 0 and 1.5D. Defocus ranged typically between -1 and 1D. The amount of coma producing best retinal image quality (for a given relative angle between astigmatism and coma) was computed and the amount was found to be different from zero in all cases (except for 0D of astigmatism). For example, for a 6mm pupil, in the presence of 0.5D of astigmatism, a value of coma of 0.23mum produced (for best focus) a peak improvement in Strehl Ratio by a factor of 1.7, over having 0.5D of astigmatism alone. The improvement holds over a range of >1.5D of defocus and peak improvements were found for amounts of coma ranging from 0.15mum to 0.35mum. We measured VA under corrected high order aberrations, astigmatism alone (0.5D) and astigmatism in combination with coma (0.23mum), with and without adaptive optics correction of all the other aberrations, in two subjects. We found that the combination of coma with astigmatism improved decimal VA by a factor of 1.28 (28%) and 1.47 (47%) in both subjects, over VA with astigmatism alone when all the rest of aberrations were corrected. Nevertheless, in the presence of typical normal levels of HOA the effect of the coma/astigmatism interaction is considerably diminished.

Influence of spherical aberration, stimulus spatial frequency, and pupil apodisation on subjective refractions.

To test competing hypotheses (Stiles Crawford pupil apodising or superior imaging of high spatial frequencies by the central pupil) for the pupil size independence of subjective refractions in the presence of primary spherical aberration.

Astigmatism Impact on Visual Performance: Meridional and Adaptational Effects

Purpose Astigmatic subjects are adapted to their astigmatism and perceptually recalibrate upon its correction. However, the extent to which prior adaptation to astigmatism affects visual performance, whether this effect is axis dependent, and the time scale of potential changes in visual performance after astigmatism correction are not known. Moreover, the effect of possible positive interactions of aberrations (astigmatism and coma) might be altered after recalibration to correction of astigmatism. Methods Visual acuity (VA) was measured in 25 subjects (astigmats and non-astigmats, corrected and uncorrected) under induction of astigmatism and combinations of astigmatism and coma while controlling subject aberrations. Astigmatism (1.00 diopter) was induced at three different orientations, the natural axis, the perpendicular orientation, and 45 degrees for astigmats and at 0, 90, and 45 degrees for non-astigmats. Experiments were also performed, adding coma (0.41 &mgr;m at a relative angle of 45 degrees) to the same mentioned astigmatism. Fourteen different conditions were measured using an 8-Alternative Forced Choice procedure with Tumbling E letters and a QUEST algorithm. Longitudinal measurements were performed up to 6 months. Uncorrected astigmats were provided with proper astigmatic correction after the first session. Results In non-astigmats, inducing astigmatism at 90 degrees, produced a statistically lower reduction in VA than at 0 or 45 degrees, whereas in astigmats, the lower decrease in VA occurred for astigmatism induced at the natural axis. Six months of astigmatic correction did not reduce the insensitivity to astigmatic induction along the natural axis. Differences after orientation of astigmatism were also found when adding coma to astigmatism. Conclusions The impact of astigmatism on VA is greatly dependent on the orientation of the induced astigmatism, even in non-astigmats. Previous experience to astigmatism plays a significant role on VA, with a strong bias toward the natural axis. In contrast to perceived isotropy, the correction of astigmatism does not shift the bias in VA from the natural axis of astigmatism.

Evaluation of a global algorithm for wavefront reconstruction for Shack-Hartmann wave-front sensors and thick fundus reflectors.

Conventional aberration analysis by a Shack–Hartmann aberrometer is based on the implicit assumption that an injected probe beam reflects from a single fundus layer. In fact, the biological fundus is a thick reflector and therefore conventional analysis may produce errors of unknown magnitude. We developed a novel computational method to investigate this potential failure of conventional analysis.

Impact of astigmatism and high-order aberrations on subjective best focus.

We studied the role of native astigmatism and ocular aberrations on best-focus setting and its shift upon induction of astigmatism in 42 subjects (emmetropes, myopes, hyperopes, with-the-rule [WTR] and against-the-rule [ATR] myopic astigmats). Stimuli were presented in a custom-developed adaptive optics simulator, allowing correction for native aberrations and astigmatism induction (+1 D; 6-mm pupil). Best-focus search consisted on randomized-step interleaved staircase method. Each subject searched best focus for four different images, and four different conditions (with/without aberration correction, with/without astigmatism induction). The presence of aberrations induced a significant shift in subjective best focus (0.4 D; p < 0.01), significantly correlated (p = 0.005) with the best-focus shift predicted from optical simulations. The induction of astigmatism produced a statistically significant shift of the best-focus setting in all groups under natural aberrations (p = 0.001), and in emmetropes and in WTR astigmats under corrected aberrations (p < 0.0001). Best-focus shift upon induced astigmatism was significantly different across groups, both for natural aberrations and AO-correction (p < 0.0001). Best focus shifted in opposite directions in WTR and ATR astigmats upon induction of astigmatism, symmetrically with respect to the best-focus shift in nonastigmatic myopes. The shifts are consistent with a bias towards vertical and horizontal retinal blur in WTR and ATR astigmats, respectively, indicating adaptation to native astigmatism.

Compared distortion effects between real and virtual ophthalmic lenses with a simulator

In this study we have compared the subjective effect of distortions simulated ophthalmic lenses in a virtual lens simulator to the equivalent real ophthalmic test lenses, in static monocular and dynamic, monocular and binocular conditions, taking care of matching as best as possible virtual and real conditions. Though visual perception was found to be similar in static condition, distortions were judged to be exaggerated by the virtual lenses in dynamic conditions.