Eloy A. Villegas

Optical Quality of the Eye in Subjects with Normal and Excellent Visual Acuity

PURPOSE To study the relationship between visual acuity (VA) and the eyes optical quality in subjects with normal and excellent spatial vision. VA ranged from decimal values of 1.0 (20/20) to 2.0 (20/10) when defocus and astigmatism were carefully corrected. METHODS In 60 eyes of young subjects, visual and optical performance with the natural pupil were measured. A forced-choice procedure was used to measure tumbling-E high-contrast VA (HCVA) and low-contrast VA (LCVA). Wavefront aberration (WA) was measured using a Hartmann-Shack sensor. The associated point-spread function (PSF) and modulation transfer function (MTF) were also estimated. High-order aberrations (HOA) and several image quality parameters were represented as a function of VA. Subjects were classified into three groups according to their VA, and average optical parameters were calculated. RESULTS Coma and trefoil vary between 0 and 0.5 mum, and spherical aberration ranges from -0.40 mum to +0.45 mum, with an average value of approximately zero. LCVA is not correlated with any of the aberration terms. Coma and spherical aberration are not correlated with HCVA. However, eyes with trefoil equal to or higher than 0.25 mum have an HCVA less than 1.5. The average optical quality in eyes with HCVA greater than 1.4 is slightly better than in eyes with normal VA. However, some eyes had relatively poor image quality and excellent VA. CONCLUSIONS No significant correlations were found between VA measurements and the optical quality of the eye in young subjects with normal or excellent spatial vision. Some subjects with normal degrees of aberrations attained excellent VA.

Spatially resolved wavefront aberrations of ophthalmic progressive-power lenses in normal viewing conditions

Purpose. To measure the wavefront aberration at different locations in progressive-power lenses (PPL’s) isolated and in situ (PPL’s plus eye). Methods. A Hartmann-Shack wavefront sensor was used to measure progressive-power lenses and human eyes either independently or in combination. In each selected zone, the lens was placed and tilted accordingly to simulate natural viewing conditions. We measured 21 relevant locations across an isolated PPL (plano lens of power addition of 2 D). In six of the locations, the wavefront aberration of the eye plus PPL were obtained in two ways: (1) by direct measurement of the system and (2) by adding the individual wavefront aberrations of the eye and the lens for each appropriate zone. In every case, we obtained the wavefront aberration as Zernike polynomials expansions, the root mean square error, the point-spread function, and the Strehl ratio. Results. Along the corridor of the PPL, third-order coma and trefoil, and astigmatism were the dominant aberrations. In areas of the PPL outside the corridor, astigmatism increased, whereas other aberrations remained similar to the lens center. Small differences were found between the direct and calculated methods used to obtain the wavefront aberration of the eye with the lens, and the possible sources of errors were discussed. In some lenses zones, the aberrations of the lens may be compensated by the particular aberrations of the eye, yielding improved optical performance over that present in the lens alone. Conclusions. We designed and built a wavefront sensor to perform spatially resolved aberration measurements in ophthalmic lenses, in particular in PPL’s, either isolated or in combination with the eye. The aberrations appearing in the PPL were compared with those in normal aged eyes.

Minimum amount of astigmatism that should be corrected

Purpose To evaluate how small amounts of astigmatism affect visual acuity and the minimum astigmatism values that should be corrected to achieve maximum visual performance. Setting Optics Laboratory, University of Murcia, Murcia, Spain. Design Case series. Methods A wavefront sensor was used to measure astigmatism and higher‐order aberrations (HOAs) in normal young eyes with astigmatism ranging from 0.0 to 0.5 diopter (D). Astigmatism was corrected for natural pupil diameters using a purpose‐designed cross‐cylinder device. Visual acuity was measured for high‐contrast and low‐contrast stimuli at best subjective focus with the natural and corrected astigmatism. From the aberrations, optical image‐quality metrics were calculated for 3 conditions: natural astigmatism, corrected astigmatism, and astigmatism only (with all HOAs removed). Results The study evaluated 54 eyes. There was no significant correlation between the amount of astigmatism and visual acuity. The correction of astigmatism improved visual acuity for only high‐contrast letters from 0.3 D, but with a high variability between subjects. Low‐contrast visual acuity changed randomly as astigmatism was corrected. The correction of astigmatism increased the mean image‐quality values; however, there was no significant correlation with visual performance. The deterioration in image quality given by astigmatism higher than 0.3 D was limited by HOAs. Conclusions In most subjects, astigmatism less than 0.5 D did not degrade visual acuity. This suggests that under clinical conditions, the visual benefit of precise correction of astigmatism less than 0.5 D would be limited. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

Correlation between Optical and Psychophysical Parameters as a Function of Defocus

Purpose. To evaluate how ocular optical image quality and psychophysical estimates of visual performance compare to each other as a function of defocus. Methods. We measured the optical modulation transfer function using a double-pass apparatus and psychophysical estimates of visual performance: contrast sensitivity function (CSF) and visual acuity. Both sets of data were obtained under the same optical conditions. Results. We measured optical and psychophysical parameters as a function of defocus. We studied the correlation between optical parameters (Strehl ratio and the logarithm of the volume in the double-pass image [log_Vol D-P]) and psychophysical parameters (the area under the fitted CSF represented in a logarithmic scale with the spatial frequency in linear scale [Area CSF-log_lin] and visual acuity) for different values of defocus. Conclusions. Strehl ratio is well correlated with the psychophysical estimates of the visual performance for moderate amount of defocus (within 1 D), whereas the other parameter (log_Vol D-P) is well correlated for larger ranges of defocus (within 2 D) and for different pupil diameters. These results suggest that optical measurements could be used for clinical testing of ophthalmic optics.

Comparison of aberrations in different types of progressive power lenses.

Recently, computer numerically controlled machines have permitted the manufacture of progressive power lenses (PPLs) with different designs. However, the possible differences in optical performance among lens designs are not yet well established. In this work, the spatially resolved aberrations, at 20 relevant locations, of three PPLs with different designs were measured with a Hartmann–Shack wavefront sensor. The wavefront aberration (WA), its root mean square error (RMS) and the point‐spread function were obtained. Spatially resolved plots are shown for all aberrations, astigmatism alone, and for higher order aberrations. The average RMS of all zones is also compared, and the standard deviation is used as a parameter to evaluate the level of hard‐soft design. We find differences in the spatial distribution of the aberrations but not in the global RMS, indicating that current PPLs are rather similar to a waterbed, with the aberrations being the water: they can be moved but they cannot be eliminated.

An analytical model describing aberrations in the progression corridor of progressive addition lenses.

Purpose. In the progression corridor of a typical progressive addition lens (PAL) with an addition of 2.5 D, the power changes by roughly 1/8 D/mm. This renders a power difference of some 0.5 D across a typical pupil diameter of 4 mm. Contrary to this fact, PALs do work well in the progression zone. To explain why, we apply a simple model to derive wavefront characteristics in the progression zone and compare it with recent experimental data. Methods. We consider a simple analytic function to describe the progression zone of a PAL, which has been introduced by Alvarez and other authors. They considered the power change and astigmatism, which are second-order wavefront aberrations. We include third-order aberrations and compare them with spatially resolved wavefront data from Hartmann-Shack-sensor measurements. Results. The higher-order aberrations coma and trefoil are the dominant aberrations besides astigmatism as given by experimental data. According to our model, the third-order aberrations in the transition zone are strongly coupled to the power change and the cubic power of the pupil radius. Their overall contribution according to experimental data is nicely reproduced by our model. The numeric contribution of higher-order aberrations is small and, for practical purposes, the wavefront can be described locally by the second-order components of sphere and astigmatism only. Conclusions. We propose a simple analytical model to understand the optics in the progression corridor and nearby zones of a PAL. Our model confirms that for typical pupil sizes, all higher-order aberrations, including the dominant modes of coma and trefoil, are small enough to render an undisturbed vision in the progression zone. Therefore, higher-order aberrations have a minimal impact on the optical performance of these lenses.

Visual acuity and optical parameters in progressive-power lenses.

Purposes. The purposes of this study are to explore the effect of astigmatism and high-order aberrations of progressive-power lenses (PPLs) on visual acuity (VA) and to find a good optical metric for evaluating visual performance of PPLs. Methods. A Hartmann-Shack (HS) wavefront sensor was used to measure PPLs and human eyes either independently or in combination. An additional channel permits the measurement of VA under the same optical conditions. Measurements were taken in six relevant locations of a PPL and in three eyes of different normal subjects. In every case, we obtained the wavefront aberration as Zernike polynomials expansions, the root mean square (RMS) error, and two metrics on point spread function (PSF): Strehl ratio and the common logarithm of the volume under the PSF normalized to one (Log_Vol_PSF). Results. Aberration coupling of the PPL with the eye tends to equalize the retinal image quality between central and peripheral zones of the progressive lenses. In the corridor of the PPL, the combination of small amounts of coma, trefoil, and astigmatism (total RMS 0.1 &mgr;m) does not significantly affect VA. The continuous increase of astigmatism from corridor to outside zones reduces moderately the quality of vision. The highest correlations between optical metrics and VA were found for Log_Vol_PSF of the entire system eye plus PPL. Conclusions. Ocular aberrations reduce optical quality difference between corridor and peripheral zones of PPLs. In the same way, VA through the corridor is similar to that of eyes without a lens and it decreases slowly toward peripheral locations. VA through PPLs is well predicted by the logarithm of metrics directly related with image spread (Log_Vol_PSF or equivalent) of the complete system of the eye with the lens.

Effect of corneal aberrations on intraocular lens power calculations.

PURPOSE: To use ray tracing to determine the influence of corneal aberrations on the prediction of the optimum intraocular lens (IOL) power for implantation in normal eyes and eyes with previous laser in situ keratomileusis (LASIK). SETTING: Hospital Universitario Virgen de la Arrixaca, Murcia, Spain. DESIGN: Case series. METHODS: The optimum IOL power was calculated by ray tracing using a patient‐customized eye model in cataract surgery cases. The calculation can be performed with or without inclusion of the patients corneal aberrations. Standard predictions were also generated using current state‐of‐the‐art IOL power calculation techniques. The results for all predictions were compared with the optimum IOL power after cataract surgery. RESULTS: For patients without previous LASIK (n = 18), the standard approaches and the ray‐tracing procedure gave a similar mean absolute residual error and variance. The incorporation of corneal aberrations did not improve the accuracy of the ray‐tracing prediction in these cases. For post‐LASIK patients (n = 10), the ray‐tracing prediction incorporating corneal aberrations generated the most accurate results. The difference between the prediction with and without considering corneal aberrations correlated with the amount of corneal spherical aberration (r2 = 0.82), resulting in a difference of up to 3.00 diopters in IOL power in some cases. CONCLUSIONS: Ray tracing using patient‐customized eye models was a robust procedure for IOL power calculation. The incorporation of corneal aberrations is crucial in post‐LASIK eyes, primarily because of the elevated corneal spherical aberration. Financial Disclosure: Mrs. Canovas and Dr. Artal hold a provisional patent application on the ray‐tracing procedure. Mrs. Canovas is an employee of Abbott Medical Optics Groningen B.V. No other author has a financial or proprietary interest in any material or method mentioned.

Refractive accuracy with light-adjustable intraocular lenses

Purpose To evaluate efficacy, predictability, and stability of refractive treatments using light‐adjustable intraocular lenses (IOLs). Setting University Hospital Virgen de la Arrixaca, Murcia, Spain. Design Prospective nonrandomized clinical trial. Methods Eyes with a light‐adjustable IOL (LAL) were treated with spatial intensity profiles to correct refractive errors. The effective changes in refraction in the light‐adjustable IOL after every treatment were estimated by subtracting those in the whole eye and the cornea, which were measured with a Hartmann‐Shack sensor and a corneal topographer, respectively. The refractive changes in the whole eye and light‐adjustable IOL, manifest refraction, and visual acuity were obtained after every light treatment and at the 3‐, 6‐, and 12‐month follow‐ups. Results The study enrolled 53 eyes (49 patients). Each tested light spatial pattern (5 spherical; 3 astigmatic) produced a different refractive change (P<.01). The combination of 2 light adjustments induced a maximum change in spherical power of the light‐adjustable IOL of between −1.98 diopters (D) and +2.30 D and in astigmatism of up to −2.68 D with axis errors below 9 degrees. Intersubject variability (standard deviation) ranged between 0.10 D and 0.40 D. The 2 required lock‐in procedures induced a small myopic shift (range +0.01 to +0.57 D) that depended on previous adjustments. Conclusions Light‐adjustable IOL implantation achieved accurate refractive outcomes (around emmetropia) with good uncorrected distance visual acuity, which remained stable over time. Further refinements in nomograms and in the treatment’s protocol would improve the predictability of refractive and visual outcomes with these IOLs. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

Optical Measurement of Straylight in Eyes With Cataract

PURPOSE To measure straylight in a cohort of patients with cataract using a novel optical instrument and to correlate optical straylight values with clinical grade of cataracts and psychophysical straylight values. METHODS Measurements were performed on 53 eyes of 44 patients with cataract admitted to the ophthalmology service of the university hospital in Murcia, Spain, and 9 young volunteers with no known ophthalmic pathology. Lens opacities were classified according to the Lens Opacities Classification System Ill (LOCS III) under slit-lamp examination. Intraocular straylight was additionally assessed psychophysically using the C-Quant straylight meter (Oculus Optikgeräte GmbH, Wetzlar, Germany). RESULTS Optical measurements of the logarithm of the straylight parameter ranged from 1.01 to 2.01 (mean: 1.43 ± 0.244) in patients with cataract and 0.80 to 1.08 (mean: 0.92 ± 0.104) in healthy young volunteers. Straylight differed by a statistically significant amount among different LOCS III groups (P < .05). Moreover, the optically measured straylight parameter was positively correlated to the psychophysically estimated value (r = 0.803, P < .05). CONCLUSIONS A new compact optical instrument suitable for clinical measurements of straylight in the human eye has been developed. Optically measured straylight values were highly correlated to those that were obtained psychophysically. Optical measurement of straylight can be used for the objective classification of cataract opacities based on their optical impact. [J Refract Surg. 2016;32(12):846-850.].