Carmen Canovas

Hybrid adaptive-optics visual simulator

We have developed a hybrid adaptive-optics visual simulator (HAOVS), combining two different phase-manipulation technologies: an optically addressed liquid-crystal phase modulator, relatively slow but capable of producing abrupt or discontinuous phase profiles; and a membrane deformable mirror, restricted to smooth profiles but with a temporal response allowing compensation of the eyes aberration fluctuations. As proof of concept, a phase element structured as discontinuous radial sectors was objectively tested as a function of defocus, and a correction loop was closed in a real eye. To further illustrate the capabilities of the device for visual simulation, we recorded extended images of different stimuli through the system by means of an external camera replacing the subjects eye. The HAOVS is specially intended as a tool for developing new ophthalmic optics elements, where it opens the possibility to explore designs with irregularities and/or discontinuities.

Comparison of Hartmann analysis methods

Analysis of Hartmann-Shack wavefront sensors for the eye is traditionally performed by locating and centroiding the sensor spots. These centroids provide the gradient, which is integrated to yield the ocular aberration. Fourier methods can replace the centroid stage, and Fourier integration can replace the direct integration. The two--demodulation and integration--can be combined to directly retrieve the wavefront, all in the Fourier domain. Now we applied this full Fourier analysis to circular apertures and real images. We performed a comparison between it and previous methods of convolution, interpolation, and Fourier demodulation. We also compared it with a centroid method, which yields the Zernike coefficients of the wavefront. The best performance was achieved for ocular pupils with a small boundary slope or far from the boundary and acceptable results for images missing part of the pupil. The other Fourier analysis methods had much higher tolerance to noncentrosymmetric apertures.

A wavelength tunable wavefront sensor for the human eye

We have designed and assembled an instrument for objective measurement of the eyes wave aberrations for different wavelengths with no modifications in the measurement path. The system consists of a Hartmann-Shack wave-front sensor and a Xe-white-light lamp in combination with a set of interference filters used to sequentially select the measurement wavelength. To show the capabilities of the system and its reliability for measuring at different wavelengths, the ocular aberrations were measured in three subjects at 440, 488, 532, 633 and 694 nm, basically covering the whole visible spectrum. Even for the shortest wavelengths, the illumination level was always several orders of magnitude below the safety limits. The longitudinal chromatic aberration estimates and the wavelength dependence of coma and spherical aberration, as examples of higher-order aberration terms, were compared to the predictions of a chromatic eye model, with good agreement. To our knowledge, this is the first report of a device to objectively determine the spectral fluctuations in the ocular wavefront.

Binocular visual acuity for the correction of spherical aberration in polychromatic and monochromatic light.

Correction of spherical (SA) and longitudinal chromatic aberrations (LCA) significantly improves monocular visual acuity (VA). In this work, the visual effect of SA correction in polychromatic and monochromatic light on binocular visual performance is investigated. A liquid crystal based binocular adaptive optics visual analyzer capable of operating in polychromatic light is employed in this study. Binocular VA improves when SA is corrected and LCA effects are reduced separately and in combination, resulting in the highest value for SA correction in monochromatic light. However, the binocular summation ratio is highest for the baseline condition of uncorrected SA in polychromatic light. Although SA correction in monochromatic light has a greater impact monocularly than binocularly, bilateral correction of both SA and LCA may further improve binocular spatial visual acuity which may support the use of aspheric-achromatic ophthalmic devices, in particular, intraocular lenses (IOLs).

Customized eye models for determining optimized intraocular lenses power.

We have developed a new optical procedure to determine the optimum power of intraocular lenses (IOLs) for cataract surgery. The procedure is based on personalized eye models, where biometric data of anterior corneal shape and eye axial length are used. A polychromatic exact ray-tracing through the surfaces defining the eye model is performed for each possible IOL power and the area under the radial MTF is used as a metric. The IOL power chosen by the procedure maximizes this parameter. The IOL power for 19 normal eyes has been determined and compared with standard regression-based predictions. The impact of the anterior corneal monochromatic aberrations and the eye’s chromatic aberration on the power predictions has been studied, being significant for those eyes with severe monochromatic aberrations, such as post-LASIK cataract patients, and for specific IOLs with low Abbe numbers.

Night Myopia Studied with an Adaptive Optics Visual Analyzer

Purpose Eyes with distant objects in focus in daylight are thought to become myopic in dim light. This phenomenon, often called “night myopia” has been studied extensively for several decades. However, despite its general acceptance, its magnitude and causes are still controversial. A series of experiments were performed to understand night myopia in greater detail. Methods We used an adaptive optics instrument operating in invisible infrared light to elucidate the actual magnitude of night myopia and its main causes. The experimental setup allowed the manipulation of the eyes aberrations (and particularly spherical aberration) as well as the use of monochromatic and polychromatic stimuli. Eight subjects with normal vision monocularly determined their best focus position subjectively for a Maltese cross stimulus at different levels of luminance, from the baseline condition of 20 cd/m2 to the lowest luminance of 22×10−6 cd/m2. While subjects performed the focusing tasks, their eyes defocus and aberrations were continuously measured with the 1050-nm Hartmann-Shack sensor incorporated in the adaptive optics instrument. The experiment was repeated for a variety of controlled conditions incorporating specific aberrations of the eye and chromatic content of the stimuli. Results We found large inter-subject variability and an average of −0.8 D myopic shift for low light conditions. The main cause responsible for night myopia was the accommodation shift occurring at low light levels. Other factors, traditionally suggested to explain night myopia, such as chromatic and spherical aberrations, have a much smaller effect in this mechanism. Conclusions An adaptive optics visual analyzer was applied to study the phenomenon of night myopia. We found that the defocus shift occurring in dim light is mainly due to accommodation errors.

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.

Preclinical metrics to predict through-focus visual acuity for pseudophakic patients

This study compares the clinical through-focus visual acuity (VA) in patients implanted with different intraocular lens (IOL) to optical bench testing of the same IOLs to evaluate the suitability of optical metrics of predicting clinical VA. Modulation transfer function and phase transfer function for different spatial frequencies and US Air Force pictures were measured using an optical bench for two monofocal IOLs, three multifocal IOLs and an extended range of vision IOL. Four preclinical metrics were calculated and compared to the clinical through-focus VA collected in three different clinical studies (243 patients in total). All metrics were well correlated (R2≥0.89) with clinical data and may be suitable for predicting through-focus VA in pseudophakic eyes.

Laboratory-Measured MTF of IOLs and Clinical Performance

We read with interest the article by Esteve-Taboada et al.1 The authors measured the modulation transfer function (MTF) of three intraocular lenses (IOLs) using a corneal eye model with zero spherical aberration using monochromatic light. This means that corneal higher order aberrations and chromatic aberration were not taken into account for the evaluation, even though it has been shown that spherical aberration and chromatic aberration have a significant effect on visual acuity and contrast sensitivity. The exclusion of spherical aberration in the measurement set-up follows the general guidelines of the International Organization of Standardization (ISO) for IOLs (ISO 11979-2:1999). The purpose of ISO standards, in general, is to ensure quality, safety, and efficiency, as well as to facilitate international trade.2 For this particular part of the standard, the primary purpose is to ensure manufacturing quality, and not in vivo performance.3 The current ISO standard provides the reader with a warning by stating that when it comes to MTF measurements “No inference should be made to performance in real eyes” (ISO 119792:2014, Clause C.2). In a previous commentary on a similar study by Artigas et al.,4 Norrby warned that the aberration-free ISO model eye is not valid for assessing aspherical lenses.3 The three lens designs in the study of Esteve-Taboada et al. are refractive/diffractive designs, which are known to affect chromatic aberrations. The TECNIS Symfony IOL is designed to correct chromatic aberration5 for all distances, whereas the trifocal lenses are expected to influence chromatic aberration for near and/or intermediate vision.6 Excluding chromatic aberration and spherical aberration in the measurement set-up has a significant influence on the measured values. To illustrate this effect, Figure 1 shows MTF measurements7 similar to those of Esteve-Taboada et al., but now using a model eye that includes corneal spherical aberration of 0.27 micrometers, and that also includes chromatic aberration by using white light and having suitable dispersive properties of the cornea and fluid medium in which the IOL is immersed. As such, the model eye represents the spherical aberration and chromatic aberration that is also found in an average human eye. The difference in MTF obtained in this eye model and the results obtained by Esteve-Taboada et al. is obvious, and cannot be explained by the slight difference in pupil size in both studies. The difference can only be explained by the fact that the measurement conditions for MTF in the chromatic eye model were more representative for the clinical situation. The authors provide far-reaching statements concerning the implications of their findings toward clinical behavior, even though the understanding of the correlation with clinical outcomes is still limited for metrics based on measurements in a model eye. It must be understood that whatever correlation there may be, it will depend greatly on how the lenses were measured. An optimal correlation can only be achieved if the measurement conditions for MTF are representative for the clinical situation.

Effect of the equivalent refractive index on intraocular lens power prediction with ray tracing after myopic laser in situ keratomileusis

Purpose To determine the impact of the equivalent refractive index (ERI) on intraocular lens (IOL) power prediction for eyes with previous myopic laser in situ keratomileusis (LASIK) using custom ray tracing. Setting AMO B.V., Groningen, the Netherlands, and the Department of Ophthalmology, Baylor College of Medicine, Houston, Texas, USA. Design Retrospective data analysis. Methods The ERI was calculated individually from the post‐LASIK total corneal power. Two methods to account for the posterior corneal surface were tested; that is, calculation from pre‐LASIK data or from post‐LASIK data only. Four IOL power predictions were generated using a computer‐based ray‐tracing technique, including individual ERI results from both calculation methods, a mean ERI over the whole population, and the ERI for normal patients. For each patient, IOL power results calculated from the four predictions as well as those obtained with the Haigis‐L were compared with the optimum IOL power calculated after cataract surgery. Results The study evaluated 25 patients. The mean and range of ERI values determined using post‐LASIK data were similar to those determined from pre‐LASIK data. Introducing individual or an average ERI in the ray‐tracing IOL power calculation procedure resulted in mean IOL power errors that were not significantly different from zero. The ray‐tracing procedure that includes an average ERI gave a greater percentage of eyes with an IOL power prediction error within ±0.5 diopter than the Haigis‐L (84% versus 52%). Conclusion For IOL power determination in post‐LASIK patients, custom ray tracing including a modified ERI was an accurate procedure that exceeded the current standards for normal eyes. Financial Disclosure Dr. Canovas, Mr. van der Mooren, Dr. Piers, and Dr. Artal hold a provisional patent application on the ray‐tracing procedure. Dr. Canovas, Mr. van der Mooren, Dr. Rosén, and Dr. Piers are employees of AMO Groningen B.V. Dr. Koch is a consultant to Abbott Medical Optics, Inc., and Alcon Laboratories, Inc. No other author has a financial or proprietary interest in any material or method mentioned.