New Objective Refraction Metric Based on Sphere Fitting to the Wavefront

Purpose. To develop an objective refraction formula based on the ocular wavefront error (WFE) expressed in terms of Zernike coefficients and pupil radius, which would be an accurate predictor of subjective spherical equivalent (SE) for different pupil sizes. Methods. A sphere is fitted to the ocular wavefront at the center and at a variable distance, t. The optimal fitting distance, topt, is obtained empirically from a dataset of 308 eyes as a function of objective refraction pupil radius, r0, and used to define the formula of a new wavefront refraction metric (MTR). The metric is tested in another, independent dataset of 200 eyes. Results. For pupil radii r0≤2 mm, the new metric predicts the equivalent sphere with similar accuracy (2 mm, the mean error of traditional metrics can increase beyond 0.25D, and the MTR remains accurate. The proposed metric allows clinicians to obtain an accurate clinical spherical equivalent value without rescaling/refitting of the wavefront coefficients. It has the potential to be developed into a metric which will be able to predict full spherocylindrical refraction for the desired illumination conditions and corresponding pupil size

Intracapsular accommodation mechanism in terms of lens curvature gradient

The intracapsular accommodation mechanism (IAM) may be understood as an increase in the lens equivalent refractive index as the eye accommodates. Our goal was to evaluate the existence of an IAM by analysing observed changes in the inner curvature gradient of the lens. To this end, we fitted a gradient index and curvature lens model to published experimental data on external and nucleus geometry changes during accommodation. For each case analysed, we computed the refractive power and equivalent index for each accommodative state using a ray transfer matrix. All data sets showed an increase in the effective refractive index, indicating a positive IAM, which was stronger for older lenses. These results suggest a strong dependence of the lens equivalent refractive index on the inner curvature gradient

Depth-of-field after orthokeratology: a theoretical study

Trabajo presentado al 8th European Meeting on Visual and Physiological Optics, celebrado en Antwerp (Bélgica) del 22 al 24 de agosto de 2016.We evaluated the possible effect of orthokeratology on accommodative response. Using optical modelling we computed the negative half of the depth-of-field (DoFi) for the range of target vengeances from –1.00 D to –3.00 D, of two eye models designed to mimic the levels of primary and secondary spherical aberration found in 24 patients before and after undergoing orthokeratology (ortho-k). Five trained observers were subjected to a resolution task to identify the negative threshold of the depth-of-field of these model eyes by viewing a set of computed images representative of the model eyes trough focus retinal image quality for five target vengeances (TV), from –1.00 to – 3.00 D. The differences in the DoFi estimated by the five observers were maximum for a –3.00 D TV (0.21 D), with the post ortho-k model presenting a higher DoFi compared to the pre ortho-k model. Differences were consistent for all five observers and all TV’s. In conclusion, the increase in spherical aberration after ortho-k seems to contribute to a small increase in the DoFi. Although small, the benefits might be sufficient to improve retinal image quality in eyes with high accommodative lag.Peer reviewe

Objective Amplitude of Accommodation Computed from Optical Quality Metrics Applied to Wavefront Outcomes

AbstractPurposeWe studied the accuracy and precision of 32 objective wavefront methods for finding the amplitude of accommodation obtained in 180 eyes.MethodsOcular accommodation was stimulated with 0.5 D steps in target vergence spanning the full range of accommodation for each subject. Subjective monocular amplitude of accommodation was measured using two clinical methods, using negative lenses and with a custom Badal optometer.ResultsBoth subjective methods gave similar results. Results obtained from the Badal optometer where used to test the accuracy of the objective methods. All objective methods showed lower amplitude of accommodation that the subjective ones by an amount that varied from 0.2 to 1.1 D depending on the method. The precision in this prediction also varied between subjects, with an average standard error of the mean of 0.1 D that decreased with age.ConclusionsDepth of field increases subjective of amplitude of accommodation overestimating the objective amplitude obtained with all the metrics used. The change in the negative direction of spherical aberration during accommodation increases the amplitude of accommodation by an amount that varies with age

Morphology, topography, and optics of the orthokeratology cornea

The goal of this work was to objectively characterize the external morphology, topography, and optics of the cornea after orthokeratology (ortho-k). A number of 24 patients between the ages of 17 and 30 years (median ¼ 24 years) were fitted with Corneal Refractive Therapy® contact lenses to correct myopia between −2.00 and −5.00 diopters (D) (median ¼ −3.41 D). A classification algorithm was applied to conduct an automatic segmentation based on the mean local curvature. As a result, three zones (optical zone, transition zone, and peripheral zone) were delimited. Topographical analysis was provided through global and zonal fit to a general ellipsoid. Ray trace on partially customized eye models provided wave aberrations and retinal image quality. Monozone topographic description of the ortho-k cornea loses accuracy when compared with zonal description. Primary (C0 4) and secondary (C0 6) spherical aberration (SA) coefficients for a 5-mm pupil increased 3.68 and 19 times, respectively, after the treatments. The OZ area showed a strong correlation with C0 4 (r ¼ −0.49, p < 0.05) and a very strong correlation with C0 6 (r ¼ 0.78, p < 0.01). The OZ, as well as the TZ, areas did not correlate with baseline refraction. The increase in the eye’s positive SA after ortho-k is the major factor responsible for the decreased retinal optical quality of the unaccommodated eyeThis work was funded in part by European Fund for Regional Development (FEDER) through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT). FCT provided financial support in the framework of projects PTDC/SAU-BEB/098391/2008, PTDC/SAU-BEB/ 098392/2008 and the Strategic Project PEST-C/FIS/UI607/ 2011. The authors have no proprietary interest in the methods and devices described in this manuscript

Refractive error sensing in natural multifocal eyes

Trabajo presentado al 6th EOS Topical Meeting on Visual and Physiological Optics celebrado en Dublin (Irlanda) del 20 al 22 de agosto de 2012.Several examples of eyes with multifocal wavefronts are analyzed by comparing subjective and aberrometric refraction. These multifo-cal eyes are analyzed using a generalized refractive error sensing method. Our results suggest an important effect on the refraction caused by inhomogeneous pupil transmittance (SCE or vignetting.). [Introduction]: In some eyes subjective refraction may differ more than 1 D from objective refraction obtained by aberrometry or autorrefracto-metry. Usually these eyes present wavefront errors larger than ave-rage (in particular coma and/or spherical aberration). Here, our goal was to assess the hypothesis that these important discrepancies in these highly aberrated eyes might be explained by their multifocal properties. In normal eyes, the prevalence of these cases is not high. We identified 8 out of 178 eyes (~5%) from a previous study. However the prevalence in post-surgical eyes is expected to be much higher. The analysis was based on refractive error sensing, which was generalized to account for multifocal wavefronts, as well as to include the effect of possible irregular and/or non-uniform transmission pupils (Stiles-Crawford effect, etc.).[Discussion]: The study was focused on the above mentioned 8 eyes, which sho-wed differences greater than 1 D between subjective and objective equivalent sphere, Se. Se was obtained subjectively by standard subjective refraction and also by a more precise custom Badal system. For the objective refraction we used retinoscopy, autorefracto-meter (Canon T1000) and aberrometer (irx3, Imagine Eyes). In what follows we present one example of bifocal eye, which showed the maximum discrepancy of nearly 2.5 D between subjective and ob-jective refraction. This case is especially interesting, since there are two different possible explanations for such a large difference. The left panel of Fig. 1 shows the distribution of equivalent sphere across a 4.9 mm pupil diameter, showing values ranging from -5 to -9 D. The right panel shows the frequency histogram of that distribution under three hypothetical situations. Red line corresponds to the histogram of the plain aberrometric measure which implicitly assumes a perfect circular pupil with uniform transmission. This histogram shows two peaks separated by more than 2.5 D, which is indicative of a bifocal wavefront. Nevertheless, the peak on the right is domi-nant as it shows Se = -5.8 D. The blue histogram simulates the effect of pupil vignetting by the upper eye lid. This simulation was inclu-ded, since this phenomenon was actually observed in this subject. For that purpose we assumed that the area of the pupil above the das-hed line was blocked by the eye lid. The resulting (blue) histogram is totally different, now showing a single peak, which is much closer to the value of the subjective refraction (Se=-8.2 D). The green histogram represents a second possible explanation. It was computed by weighting the data points according to the Stiles-Crawford effect (SCE). Here we used standard nominal values, ex-cept for the fact that we shifted the SCE peak downwards (its positi-on is indicated by the cross on the left panel.). As expected, the decentered SCE yields an intermediate situation: The two peaks remain but their relative heights changed so that now the dominant peak is the left one (Se =-8.2 D). The other cases analyzed so far exhibit two or even three peaks, which means that there are two or three possible refractions. One of these peaks is always close to the subjective refraction. Their relative heights depend on the position of the SCE peak.[Conclusions]: A small percentage of normal eyes show bifocal wavefronts, with power differences greater than 1 D (or even 2.5 D in some cases), which are typically associated to high values of HOA. These bifocal eyes often show important discrepancies between subjective and objective refraction. Our results suggest that these discrepancies may appear because aberrometric methods ignore the inhomogene-ous effective pupil transmittance. The generalization of refractive error sensing proposed here seems suitable to obtain the refraction in these multifocal eyes. It may be especially relevant for postsurgi-cal eyes, which often present high levels of HOA, and hence may show multifocal wavefronts.Supported by grant FIS2011-22496 (Spain).Peer reviewe

Computing retinal contour from optical biometry

Purpose. To describe a new methodology that derives horizontal posterior retinal contours from partial coherence interferometry (PCI) and ray tracing using the corneal topography. Methods. Corneal topography and PCI for seven horizontal visual field eccentricities correspondent to the central 60 degrees of the posterior pole were obtained in 55 myopic eyes. A semicustomized eye model based on the subject’s corneal topography and the Navarro eye model was generated using Zemax-EE software. The model was used to compute the optical path length in the seven directionswhere PCImeasurementswere obtained.Vitreous chamber depth was computed using the PCI values obtained at each of those directions. Matlab software was developed to fit the best conic curve to the set of points previously obtained. We tested the limit in the accuracy of the methodology when the actual cornea of the subject is not used and for two different lens geometries. Results. A standard eye model can induce an error in the retina sagitta estimation of the order of hundreds of micrometers in comparison with the semicustomized eye model. However, the use of a different lens model leads to an error of the order of tens of micrometers. The apical radius and conic constant of the average fit were j11.91 mm and j0.15, respectively. In general, a nasal-temporal asymmetry in the retina contour was found, showing mean larger values of vitreous chamber depth in the nasal side of the eye. Conclusions. The use of a semicustomized eye model, together with optical path length measured by PCI for different angles, can be used to predict the retinal contour within tenths of micrometers. This methodology can be useful in studies trying to understand the effect of peripheral retinal location on myopia progression as well as modeling the optics of the human eye for a wide field.This study has been funded by FEDER through the COMPTETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of projects PTDC/SAU-BEB/098391/2008, PTDC/SAU-BEB/098392/2008, and the Strategic Project PEST-C/FIS/UI607/2011. NL was also supported by a Fundacion Seneca de la Region de Murcia grant 15312/PI/10. RN was supported by the Spanish Ministry of Economia y Competitividad and European Union grant FIS2011-22496 and by the Government of Aragon group E99. We thank Larry N. Thibos for his comments on the manuscript

Predicted Torque Model in Low-Frequency-Assisted Boring (LFAB) Operations

A low-frequency-assisted boring operation is a key cutting process in the aircraft manufacturing sector when drilling deep holes to avoid chip clogging based on chip breakage and, consequently, to reduce the temperature level in the cutting process. This paper proposes a predicted force model based on a commercial control-supported chip breaking function without external vibration devices in the boring operations. The model was fitted by conventional boring measurements and was validated by vibration boring experiments with different ranges of amplitude and frequency. The average prediction error is around 10%. The use of a commercial function makes the model more attractive for the industry because there is no need for intrusive vibration sensors. The low-frequency assisted boring (LFAB) operations foster the chip breakage. Finally, the model is generic and can be used for different cutting materials and conditions. Roughness is improved by 33% when vibration conditions are optimal, considered as a vibration amplitude of half the feed per tooth. This paper presents, as a novelty, the analysis of low-frequency vibration parameters in boring processes and their effect on chip formation and internal hole roughness. This has a practical significance for the definition of a methodology based on the torque model for the selection of conditions on other hole-making processes, cutting parameters and/or materials.This research was funded by the European Union’s Horizon 2020 research and innovation program under grant agreement no. 723698 (ForZDM)

Opto-mechanical artificial eye with accommodative ability.

The purpose of this study was to describe the design and characterization of a new opto-mechanical artificial eye (OMAE) with accommodative ability. The OMAE design is based on a second-pass configuration where a small source of light is used at the artificial retina plane. A lens whose focal length can be changed electronically was used to add the accommodation capability. The changes in the OMAE's aberrations with the lens focal length, which effectively changes the accommodative state of the OMAE, were measured with a commercial aberrometer. Changes in power and aberrations with room temperature were also measured. The OMAE's higher-order aberrations (HOAs) were similar to the ones of the human eye, including the rate at which fourth-order spherical aberration decreased with accommodation. The OMAE design proposed here is simple, and it can be implemented in an optical system to mimic the optics of the human eye

Effect of phenylephrine on static and dynamic accommodation.

PURPOSE: We tested the hypothesis that changes in accommodation after instillation of Phenylephrine Hydrochloride (PHCl) observed in some studies could be caused by changes in optics. METHODS: We performed two experiments to test the effects of PHCl on static and on dynamic accommodation in 8 and 6 subjects, respectively. Objective wavefront measurements were recorded of the static accommodation response to a stimulus at different distances or dynamic accommodation response to a sinusoidally moving stimulus (between 1 and 3 D of accommodative demand at 0.2Hz). The responses were characterized using two methods: one that takes into account the mydriatic optical effects on the accommodation produced by higher-order aberrations of the eye and another that takes into account only power changes paraxially due to the action of the ciliary muscle and regardless of the pupil size. RESULTS: When mydriatic optical effects were taken into account, differences in responses before and after PHCl instillation were 0.51±0.53 D, and 0.12±0.15, for static and dynamic accommodation, respectively, and were statistically significant (p0.313). CONCLUSIONS: The mydriatic effect of the PHCl causes optical changes in the eye that can reduce the objective and subjective measurement of accommodation