Vision Improvement by Correcting Higher-order Aberrations With Phase Plates in Normal Eyes

Comparison of Outcomes after Wavefront-optimized and Topography-guided Transepithelial Photorefractive Keratectomy

PURPOSE

To evaluate the outcomes of wavefront-optimized (WFO) and topography-guided (TG) transepithelial photorefractive keratectomy (transPRK) in the treatment of myopia and myopic astigmatism.

METHODS

Patients who underwent transPRK using the WaveLight EX500 excimer laser for the correction of myopia and myopic astigmatism between January 2022 and March 2023 were divided into groups of WFO transPRK (77 eyes of 36 patients) or TG transPRK (63 eyes of 31 patients) in this retrospective, observational cohort study. The preoperative and postoperative 3-month refractive and visual outcomes of the two groups were analyzed.

RESULTS

In both groups, the uncorrected distance visual acuity was 0.0 logarithm of the minimum angle of resolution or better in 95% of eyes 3 months postoperatively, and the mean manifest refraction spherical equivalent was within ±1.0 diopter in 90% of eyes. No significant differences were observed between the groups in terms of the uncorrected distance visual acuity or astigmatism. A significant induction of higher order aberrations (HOAs) was observed in both groups. However, the induction of total corneal HOAs (p = 0.014) and spherical aberrations (p < 0.001) was significantly lower in the TG group than that in the WFO group.

CONCLUSIONS

WFO and TG transPRK effectively improved the visual and refractive outcomes; however, the induction of total corneal HOAs and spherical aberration was lesser following the TG ablation.

Improving iconic memory through contrast detection training with HOA-corrected vision

Iconic memory and short-term memory are not only crucial for perception and cognition, but also of great importance to mental health. Here, we first showed that both types of memory could be improved by improving limiting processes in visual processing through perceptual learning. Normal adults were trained in a contrast detection task for ten days, with their higher-order aberrations (HOA) corrected in real-time. We found that the training improved not only their contrast sensitivity function (CSF), but also their iconic memory and baseline information maintenance for short-term memory, and the relationship between memory and CSF improvements could be well-predicted by an observer model. These results suggest that training the limiting component of a cognitive task with visual perceptual learning could improve visual cognition. They may also provide an empirical foundation for new therapies to treat people with poor sensory memory.

A feasibility study for objective evaluation of visual acuity based on pattern-reversal visual evoked potentials and other related visual parameters with machine learning algorithm

BACKGROUND

To develop machine learning models for objectively evaluating visual acuity (VA) based on pattern-reversal visual evoked potentials (PRVEPs) and other related visual parameters.

METHODS

Twenty-four volunteers were recruited and forty-eight eyes were divided into four groups of 1.0, 0.8, 0.6, and 0.4 (decimal vision). The relationship between VA, peak time, or amplitude of P100 recorded at 5.7°, 2.6°, 1°, 34', 15', and 7' check sizes were analyzed using repeated-measures analysis of variance. Correlations between VA and P100, contrast sensitivity (CS), refractive error, wavefront aberrations, and visual field were analyzed by rank correlation. Based on meaningful P100 peak time, P100 amplitude, and other related visual parameters, four machine learning algorithms and an ensemble classification algorithm were used to construct objective assessment models for VA. Receiver operating characteristic (ROC) curves were used to compare the efficacy of different models by repeated sampling comparisons and ten-fold cross-validation.

RESULTS

The main effects of P100 peak time and amplitude between different VA and check sizes were statistically significant (all P < 0.05). Except amplitude at 2.6° and 5.7°, VA was negatively correlated with peak time and positively correlated with amplitude. The peak time initially shortened with increasing check size and gradually lengthened after the minimum value was reached at 1°. At the 1° check size, there were statistically significant differences when comparing the peak times between the vision groups with each other (all P < 0.05), and the amplitudes of the vision reduction groups were significantly lower than that of the 1.0 vision group (all P < 0.01). The correlations between peak time, amplitude, and visual acuity were all highest at 1° (r_s = - 0.740, 0.438). VA positively correlated with CS and spherical equivalent (all P < 0.001). There was a negative correlation between VA and coma aberrations (P < 0.05). For different binarization classifications of VA, the classifier models with the best assessment efficacy all had the mean area under the ROC curves (AUC) above 0.95 for 500 replicate samples and above 0.84 for ten-fold cross-validation.

CONCLUSIONS

Machine learning models established by meaning visual parameters related to visual acuity can assist in the objective evaluation of VA.

Binocular accommodative response with extended depth of focus under controlled convergences

Vergence and accommodation can be mismatched under virtual reality viewing conditions, and this mismatch has been thought to be one of the main causes of visual discomfort. The goal of this study was to investigate how optical conditions of the eyes affect accommodative responses to different convergence. Specifically, we hypothesized that extending the depth of focus (DoF) could weaken the control of the screen on accommodation, so that accommodation could be induced by convergence. To test this hypothesis, we extended the DoF using Zernike spherical aberrations (fourth and sixth orders) induced by a binocular adaptive optics (AO) vision simulator. Nine normal subjects between the ages of 21 and 34 (26 ± 5) years were recruited. Three optical conditions were generated: AO condition (aberration-free), monovision condition, and extended depth of focus (EDoF) condition. Binocular accommodative responses, along with binocular visual acuity and stereoacuity, were measured under all three optical conditions with varied binocular vergence levels. At 3 diopters of binocular convergence, the EDoF condition was the most efficient in inducing excessive accommodative response compared with the monovision condition and the AO condition. Visual acuity was impaired with EDoF as compared with the other two conditions. The average stereoscopic thresholds (at 0 vergence) under the EDoF condition were degraded compared with the AO condition but were superior to those of the monovision condition. Therefore, despite some compromise to visual performance, extending the DoF could allow for a more natural vergence–accommodation relationship, providing the potential for alleviating the vergence–accommodation conflict and associated visual fatigue symptoms in virtual reality.

Ocular monochromatic aberrations in a rural Chinese adult population

PURPOSE

To investigate the distribution of monochromatic aberrations in a rural Chinese adult population and the possible effect of aberrations on the development of refractive error.

METHODS

A total of 404 Chinese adults who grew up in rural Yongnian County, Handan City, Northern China, were included. Monochromatic aberrations of left eyes were measured using iTrace Dynamic Laser Refraction at 5.0-mm pupil size without cycloplegia.

RESULTS

Mean age of all participants was 49.9 ± 10.5 years (range, 31 to 86 years). Mean spherical equivalent was 0.22 ± 1.14 diopters (D) (range, -7.06 to +3.62 D). With age, the refraction demonstrated a significant hyperopic shift (r = 0.25, p < 0.01). Oblique trefoil (C3), vertical coma (C3), horizontal coma (C3), and spherical aberration (SA) (C4) significantly differed from zero (-0.065 ± 0.133 μm, -0.043 ± 0.161 μm, +0.070 ± 0.115 μm, and +0.058 ± 0.082 μm, respectively). Total root mean square values of higher-order aberrations (HOAs) were 0.296 ± 0.147 μm, with predominant ones of coma (0.180 ± 0.115 μm), trefoil (0.151 ± 0.116 μm), and SA (0.081 ± 0.060 μm). Root mean square values of total HOAs, coma, trefoil, SA, and third- to seventh-order aberrations significantly increased with age (p < 0.01). Total HOAs, SA, coma, and trefoil were not significantly different between hyperopic, emmetropic, and myopic eyes after adjusting for age (p = 0.26, 0.15, 0.24, and 0.28, respectively). Zernike coefficient of SA showed a significant difference between hyperopic (0.076 ± 0.086), emmetropic (0.056 ± 0.079), and myopic (0.028 ± 0.088) eyes (p = 0.00).

CONCLUSIONS

Ocular refraction in rural Chinese adults showed significantly hyperopic shift with age. Magnitudes of HOAs in rural Chinese adults were similar to that of other populations and significantly increased with age but showed no differences in myopic, emmetropic, and hyperopic adults. The existence of HOAs is not, in itself, sufficient to account for the myopia epidemic in China.

Accounting for the phase, spatial frequency and orientation demands of the task improves metrics based on the visual Strehl ratio

Advances in ophthalmic instrumentation have allowed high order aberrations to be measured in vivo. These measurements describe the distortions to a plane wavefront entering the eye, but not the effect they have on visual performance. One metric for predicting visual performance from a wavefront measurement uses the visual Strehl ratio, calculated in the optical transfer function (OTF) domain (VSOTF) (Thibos et al., 2004). We considered how well such a metric captures empirical measurements of the effects of defocus, coma and secondary astigmatism on letter identification and on reading. We show that predictions using the visual Strehl ratio can be significantly improved by weighting the OTF by the spatial frequency band that mediates letter identification and further improved by considering the orientation of phase and contrast changes imposed by the aberration. We additionally showed that these altered metrics compare well to a cross-correlation-based metric. We suggest a version of the visual Strehl ratio, VScombined, that incorporates primarily those phase disruptions and contrast changes that have been shown independently to affect object recognition processes. This metric compared well to VSOTF for letter identification and was the best predictor of reading performance, having a higher correlation with the data than either the VSOTF or cross-correlation-based metric.

The effect of corneal anterior surface eccentricity on astigmatism after cataract surgery

BACKGROUND AND OBJECTIVE

To evaluate the effect of cornea eccentricity on induced astigmatism after cataract surgery.

PATIENTS AND METHODS

The study included 125 eyes of 87 patients. Preoperative corneal astigmatism, pachymetry, and eccentricity were measured. During cataract surgery, the location of the main incision (2.8-mm clear corneal) was selected to be either superior, superior-nasal, superior-temporal, nasal, or temporal to decrease the preexisting corneal astigmatism. Aspheric intraocular lenses were implanted. Keratometry and manifest refraction were recorded 6 months after surgery. Astigmatism was calculated using vector subtraction software.

RESULTS

Three parameters significantly affected postoperative astigmatism: preoperative amount of corneal astigmatism, eccentricity of anterior cornea, and location of the main incision. The mean surgically induced astigmatism (SIA) was calculated to be: superior = 0.82 diopters (D), superior-nasal = 0.50 D, superior-temporal = 0.63 D, temporal = 0.45 D, and nasal = 0.55 D. Superior incision induced the greatest SIA and temporal incision induced the smallest SIA. The eccentricity of anterior cornea showed significantly positive correlation with the amount of SIA (P < .001). The preoperative corneal cylinder power showed significantly positive correlation with the amount of SIA (P < .001).

CONCLUSION

Postoperative astigmatism was affected by various factors in cataract surgery. The greatest postoperative astigmatism is expected in corneas with high anterior eccentricity, high preoperative corneal astigmatism, and superior location of the main incision.

Visual impact of Zernike and Seidel forms of monochromatic aberrations

PURPOSE

The aim of this study was to examine the impact of different aberrations modes (e.g., coma, astigmatism, spherical aberration [SA]) and different aberration basis functions (Zernike or Seidel) on visual acuity (VA).

METHODS

Computational optics was used to generate retinal images degraded by either the Zernike or Seidel forms of second through fourth-order aberrations for an eye with a 5-mm pupil diameter. High contrast, photopic VA was measured using method of constant stimuli for letters displayed on a computer-controlled, linearized, quasimonochromatic (lambda = 556 nm) display.

RESULTS

Minimum angle of resolution (MAR) varied linearly with the magnitude (root mean square error) of all modes of aberration. The impact of individual Zernike lower- and higher-order aberrations (HOAs) varied significantly with mode, e.g., arc minutes of MAR per micrometer of root mean square slopes varied from 7 (spherical defocus) to 0.5 (quadrafoil). Seidel forms of these aberrations always had a smaller visual impact. Notably, Seidel SA had 1/17th the impact of Zernike SA with the same wavefront variance, and about 1/4th the impact of Zernike SA with matching levels of r wavefront error. With lower-order components removed, HOAs near the center of the Zernike pyramid do not have a large visual impact.

CONCLUSIONS

The majority of the visual impact of high levels of fourth-order Zernike aberrations can be attributed to the second-order terms within these polynomials. Therefore, the impact of SA can be minimized by balancing it with a defocus term that flattens the central wavefront (paraxial focus) or maximizes the area of the pupil with a flat wavefront. Over this wide range of aberration types and levels, image quality metrics based on the Point Spread Function (PSF) and Optical Transfer Function (OTF) can predict VA as reliably as VA measures can predict retests of VA, and, thus, such metrics may become valuable predictors of both VA and, via optimization, refractions.

Determinants of contrast sensitivity for the tumbling E and Landolt C

PURPOSE

To compare the object spatial frequencies that underlie contrast sensitivity for the tumbling E and Landolt C across a range of optotype sizes and under conditions biased toward the magnocellular (MC) and parvocellular (PC) pathways.

METHODS

Contrast thresholds of two visually normal observers were measured using tumbling E optotypes that were either low-pass filtered or high-pass filtered with a two-dimensional Gaussian filter. Optotypes were presented using steady-pedestal and pulsed-pedestal paradigms to target the MC and PC pathways, respectively. Object frequencies essential for orientation judgments of the tumbling E were derived from plots of log contrast threshold vs. log filter cutoff frequency, and results were compared with those obtained previously for the Landolt C under identical testing conditions.

RESULTS

The object frequency used to judge the orientation of the tumbling E increased systematically with increasing target angular subtense, and the effect of target size differed depending on whether performance was mediated by the inferred MC or PC pathway. The overall pattern of results was similar for the tumbling E and Landolt C, but there was generally less dependence of object frequency on target angular subtense for the tumbling E.

CONCLUSIONS

The tumbling E and Landolt C are not equivalent in terms of the object frequencies that mediate orientation judgments. However, both optotypes show scale-dependent changes in object frequency, particularly under test conditions that favor the PC pathway. The scale dependence of these broadband optotypes can pose a challenge in interpreting test results using these targets. A potential solution is to use spatially filtered optotypes with limited, known object frequency content.

Effects of monochromatic aberration on visual acuity using adaptive optics

PURPOSE

To investigate the effect on visual acuity of correcting specific Zernike aberrations.

METHODS

Visual acuity was tested for 12 young subjects using a Freiburg Acuity Test procedure under conditions with wavefront aberrations corrected by an adaptive optics system. The adaptive optics system consists of a Hartmann-Shack wavefront sensor, a deformable mirror, relevant optical channels and closed loop control system. Five aberration correction paradigms were used to correct different Zernike terms.

RESULTS

With the second order aberration fully corrected, a significant improvement in visual acuity was observed (0.056 logMAR, t = 2.79, p = 0.018). Further correction of third order Zernike aberrations resulted in an additional improvement of 0.041 logMAR (t = 2.63, p = 0.023). But an additional correction of spherical aberration did not produce a significant increase in visual acuity (t = 1.10, NS). Full correction of aberrations achieved the best visual acuity with an improvement of 0.022 logMAR (t = 2.46, p = 0.032). The visual acuity was found to increase with the root mean square values of the residual aberrations with a linear relationship (r = 0.63, p < 0.0001).

CONCLUSIONS

Correction of monochromatic wavefront aberrations improves visual acuity for normal eyes and the improvement of visual acuity is proportional to the change of root mean square of wavefront aberrations.

Higher-Order Wavefront Aberrations for Populations of Young Emmetropes and Myopes

Purpose

To test the hypothesis that human eyes have a central tendency to be free of higher-order aberrations by analyzing wavefront aberrations for two young populations of respectively emmetropic and myopic subjects.

Methods

Both right and left eyes of 75 emmetropes and 196 myopes were measured for corneal wavefront aberration using a Humphrey corneal topographer and for the whole eye wavefront aberration using a WASCA wavefront sensor without pupil dilation. 35 Zernike aberration coefficients over a 6.0 mm pupil diameter were derived, and statistics of the higher-order terms (3^rd to 5^th orders) were tested.

Results

When signed Zernike aberrations of the right and left eyes were averaged together for the emmetropes, three higher-order modes (j=6, 12 and 13) were significantly different from zero in both the cornea and the whole eye (P<0.0005), and three additional terms (j=14, 15 and 17) were statistically non-zero for the whole eye. As the signs of y-axis asymmetrical terms in the left eye were flipped, three more terms in either the cornea (j=8, 18 and 19) or the whole eye (j=8, 10 and 20) became statistically non-zero. For the myopes, 8 corneal terms and 5 whole-eye terms were statistically non-zero when the two eyes were averaged together. As the signs flipped, the majority of the Zernike aberration terms were statistically different from zero.

Conclusions

Human eyes have systematical higher order aberrations in population, and factors that cause bilateral symmetry of wavefront aberrations between the right and left eyes made important contribution to the systematical aberrations.

Uncorrected wavefront error and visual performance during RGP wear in keratoconus

PURPOSE

To examine the relationship between uncorrected residual wavefront error and visual performance (VP) in rigid gas permeable (RGP) contact lens-wearing keratoconic eyes.

METHODS

Seven eyes from six subjects (six moderate, one severe) were studied (mean +/- SD age: 42.71 +/- 11.38 years). Significant corneal scarring was an exclusion criterion. Measurements were taken with RGP lenses in place. After pupil dilation, the VP measures of high contrast logMAR visual acuity (VA) and Pelli-Robson contrast sensitivity (PRCS) were measured through a 5-mm artificial pupil. Wavefront error was measured using a Shack-Hartmann wavefront sensor and calculated over 5 mm. For both VP and wavefront error, comparisons were made to previously collected normal values by calculating the interval encompassing 95% of normals, then reporting how many of the seven keratoconic eyes fell outside the normal interval. Additionally, second to sixth order aberrations were processed into four previously reported image quality metrics: root mean square of the wavefront (RMSw), root mean square of the slope (RMSs), average blur strength (Bave) and diameter containing 50% light energy (D50) and regressed against VP measures.

RESULTS

Five of seven keratoconic eyes fell outside the normal interval (-0.23 to 0.09) for VA and two of seven fell outside the normal interval (1.59 to 2.03) for PRCS. Five of seven keratoconic eyes fell outside the normal interval (0.07 to 0.35 microm) for total higher order RMS. Linear regressions demonstrated relationships between both VA and PRCS and the image quality metrics RMSw, D50, RMSs, and Bave with R values for VA = 0.30, 0.30, 0.47, 0.62, and PRCS = 0.21, 0.15, 0.45, 0.75 respectively.

CONCLUSIONS

VP in RGP-wearing keratoconic eyes is reduced and higher order wavefront aberrations are elevated compared to normals. Metrics of retinal image quality demonstrate a relationship between keratoconic VP and residual wavefront aberrations. This relationship suggests developing corrections that more completely correct aberrations may improve visual performance in keratoconus.

Measuring contrast sensitivity in normal subjects with OPTEC® 6500: influence of age and glare

BACKGROUND

The purpose of this study was to develop age-related curves for contrast sensitivity (CS) in normal subjects under day and night conditions with and without glare.

METHODS

Sixty-one healthy eyes from 61 subjects were measured with the OPTEC(R) 6500 P under day and night conditions (luminance levels: 85 cd/m(2) and 3.0 cd/m(2) with and without glare; spatial frequencies: 1.5, 3, 6, 12 and 18 cycles/degree). A reliability analysis with five repeated measurements of six persons on 4 days was performed to examine the repeatability. The influence of age on contrast sensitivity, forward and backward scatter was examined by means of linear regression.

RESULTS

Contrast sensitivity was significantly reduced under night conditions with glare, whereas glare had less influence under daylight illumination. Mean reliability coefficients are 0.87 (day), 0.77 (day with glare), 0.69 (night) and 0.81 (night with glare), which suggests sufficient retest reliability of the device. Regression analyses showed a highly significant influence of age, but the variance of the measurement values is not explained by age alone. The coefficients of determination for the regression of area under the log contrast sensitivity function (AULCSF) on age are 0.33 (photopic), 0.34 (photopic with glare), 0.29 (mesopic) and 0.36 (mesopic with glare, p < 0.0001 in all cases).

CONCLUSION

A significant relationship between age, CS and scatter was confirmed in our study. The results provide baseline values for the examination of patients with different diseases in which contrast sensitivity is impaired (such as glaucoma, cataracts and amblyopia) and might be useful in studies of roadworthiness or in investigation of the impact of intraocular lenses.

Characterizing the wave aberration in eyes with keratoconus or penetrating keratoplasty using a high-dynamic range wavefront sensor

PURPOSE

The purpose of this study was to characterize aberrations in 2 populations of eyes, namely those with keratoconus (KC) and those having undergone penetrating keratoplasty (PK), using a large-dynamic range Shack-Hartmann wavefront sensor.

DESIGN

Prospective comparative case series.

PARTICIPANTS

Twenty-one people with ocular pathologic features (either KC or PK) were recruited for this study. A previously compiled population of 190 people with no pathologic features other than refractive error was used as a means for comparison.

METHODS

Thirty-three abnormal eyes (19 with KC and 14 PK) were measured using a high-dynamic range wavefront sensor, and Zernike coefficients were computed over a 6-mm pupil. The data then were used to characterize the populations by themselves, as well as to compare them with the population of normal eyes.

MAIN OUTCOME MEASURES

Root mean square (RMS) higher-order aberration (HOA), percent of higher-order or total aberration variance, and magnitude of individual Zernike modes (in micrometers). Visual benefit of correcting higher-order aberrations was used when comparing pathologic and normal populations.

RESULTS

The keratoconic eyes exhibited 2.24 microm of HOA RMS on average. Vertical coma accounted for 53+/-32% (mean+/-standard deviation [SD]) of the HOA variance and was the most dominant higher-order aberration. The PK subjects had an average higher-order RMS of 2.25 microm, and trefoil dominated in this population with an average HOA variance contribution of 38+/-23% (mean+/-SD). The KC and PK higher-order aberrations represented 16+/-20% and 16+/-13% (mean+/-SD) of the total aberration variance, whereas the ratio was only 1+/-1% in the normal population. A visual benefit calculation on 15 KC eyes and 14 PK eyes yielded a result of 4.4+/-2.0 and 6.0+/-1.5 (mean+/-SD), respectively, whereas the normal population had a visual benefit of only 2.1+/-0.4.

CONCLUSIONS

Eyes with KC and PK have higher-order aberrations that are approximately 5.5 times more than what is typical in normal eyes. Vertical coma is the dominant higher-order aberration in people with KC, whereas PK eyes are dominated by trefoil, spherical aberration, and coma. Correcting these aberrations may provide substantial improvements in vision beyond what is possible with conventional correction methods.

Theoretical Comparison of Aberration-correcting Customized and Aspheric Intraocular Lenses

PURPOSE

To assess the performance and optical limitations of standard, aspheric, and wavefront-customized intraocular lenses (IOLs) using clinically verified pseudophakic eye models.

METHODS

White light pseudophakic eye models were constructed from physical measurements performed on 46 individual cataract patients and subsequently verified using the clinically measured contrast sensitivity function (CSF) and wavefront aberration of pseudophakic patients implanted with two different types of IOLs. These models are then used to design IOLs that correct the astigmatism and higher order aberrations of each individual eye model's cornea and to investigate how this correction would affect visual benefit, subjective tolerance to lens misalignment (tilt, decentration, and rotation), and depth of field.

RESULTS

Physiological eye models and clinical outcomes show similar levels of higher order aberration and contrast improvement. Customized correction of ocular wavefront aberrations with an IOL results in contrast improvements on the order of 200% over the control and the Tecnis IOLs. The customized lenses can be, on average, decentered by as much as 0.8 mm, tilted > 10 degrees , and rotated as much as 15 degrees before their polychromatic modulation transfer function at 8 cycles/degree is less than that of the Tecnis or spherical control lens. Correction of wavefront aberration results in a narrower through focus curve but better out of focus performance for +/- 0.50 diopters.

CONCLUSIONS

The use of realistic eye models that include higher order aberrations and chromatic aberrations are important when determining the impact of new IOL designs. Customized IOLs show the potential to improve visual performance.

Large-dynamic-range Shack-Hartmann wavefront sensor for highly aberrated eyes

A conventional Shack-Hartmann wavefront sensor has a limitation that increasing the dynamic range usually requires sacrificing measurement sensitivity. The prototype large-dynamic-range Shack-Hartmann wavefront sensor presented resolves this problem by using a translatable plate with subapertures placed in conjugate with the lenslet array. Each subaperture is the same size as a lenslet and they are arranged so that they overlap every other lenslet position. Three translations of the plate are required to acquire four images to complete one measurement. This method increases the dynamic range by a factor of two with no subsequent change in measurement sensitivity and sampling resolution of the aberration. The feasibility of the sensor was demonstrated by measuring the higher order aberrations of a custom-made phase plate and human eyes with and without the plate.

The placido wavefront sensor and preliminary measurement on a mechanical eye

PURPOSE

The hardware and software of a novel wavefront sensor was developed (The sensor presented here is patent pending.). It has the same principal of the Hartmann-Shack (HS) and other sensors that are based on slope information for recovery of wavefront surface, but a different symmetry, and does not use individual microlenses. This polar symmetry might offer differences during practical measurements that may add value to current and well-established "gold standard" techniques.

METHODS

The sensor consists of a set of concentric "half-donut" surfaces (longitudinally sectioned toroids) molded on an acrylic surface with a CCD located at the focal plane. When illuminated with a plane wavefront, it focuses a symmetric pattern of concentric discs on the CCD plane; for a distorted wavefront, a nonsymmetric disc pattern is formed (similar to images of a placido-based videokeratographer). From detection of shift in the radial direction, radial slopes are computed for a maximum of 2880 points, and the traditional least-squares procedure is used to fit these partial derivatives to a set of 15 conventional OSA-VSIA Zernike polynomials. Theoretical computations for several synthetic surfaces containing low-order aberration (LOA) and high-order aberration (HOA) were implemented for both the HS and the new sensor.

RESULTS

Root mean square error (RMSE) in microns when theoretical data was taken as control, for HS sensor and new sensor, was 0.02 and 0.00003 for LOA (defocus, astigmatism) and 0.07 and 0.06 for HOA (coma, spherical, and higher terms), respectively. After this, practical preliminary measurements on a mechanical eye with a 5-mm pupil and 10 different defocus aberrations ranging from -5 D to 5 D, in steps of 1 D, were compared between sensors. RMSE for difference in measurements for HS and new sensor for sphere, cylinder, and axis, was 0.13 D, 0.07 D, and 11. Measurements were taken only on defocus aberrations. Qualitative images for astigmatism are shown.

DISCUSSION

Although practical in vivo tests were not conducted in this first study, we also discuss certain possible alignment differences that may arise as a result of the different symmetry of the new sensor. To take any conclusive assumption regarding the accuracy and/or precision of this new sensor, when compared with other well-established sensors, statistically significant in vivo measurements will need to be conducted.

Design and fabrication of a freeform phase plate for high-order ocular aberration correction

In recent years it has become possible to measure and in some instances to correct the high-order aberrations of human eyes. We have investigated the correction of wavefront error of human eyes by using phase plates designed to compensate for that error. The wavefront aberrations of the four eyes of two subjects were experimentally determined, and compensating phase plates were machined with an ultraprecision diamond-turning machine equipped with four independent axes. A slow-tool servo freeform trajectory was developed for the machine tool path. The machined phase-correction plates were measured and compared with the original design values to validate the process. The position of the phase-plate relative to the pupil is discussed. The practical utility of this mode of aberration correction was investigated with visual acuity testing. The results are consistent with the potential benefit of aberration correction but also underscore the critical positioning requirements of this mode of aberration correction. This process is described in detail from optical measurements, through machining process design and development, to final results.

Measurement of wave-front aberration in soft contact lenses by use of a Shack-Hartmann wave-front sensor

Lower- and higher-order wave-front aberrations of soft contact lenses were accurately measured with a Shack-Hartmann wave-front sensor. The soft contact lenses were placed in a wet cell filled with lens solution to prevent surface deformation and desiccation during measurements. Aberration measurements of conventional toric and multifocal soft contact lenses and a customized soft contact lens have proved that this method is reliable. A Shack-Hartmann wave-front sensor can be used to assess optical quality of both conventional and customized soft contact lenses and to assist in enhancing lens quality control.