Effect of multifocal spectacle lenses on accommodative errors over time: Possible implications for myopia control.

The study purpose was to improve understanding of how multifocal spectacle lenses affect accommodative errors and whether this changes over time. Fifty-two myopes aged 18 to 27 years were allocated randomly to one of two progressive addition lens (PAL) types with 1.50 D additions and different horizontal power gradients across the near-periphery boundary. Lags of accommodation were determined with a Grand Seiko WAM-5500 autorefractor and a COAS-HD aberrometer for several near distances with the distance correction and the near PAL correction. For the COAS-HD the neural sharpness (NS) metric was used. Measures were repeated at three-month intervals over 12 months. At the final visit, lags to booster addition powers of 0.25, 0.50, and 0.75 D were measured. Except at baseline, both PALs' data were combined for analysis. For the Grand Seiko autorefractor, both PALs reduced accommodative lag at baseline compared with SVLs (p < 0.05 and p < 0.01 at all distances for PAL 1 and PAL 2, respectively). For the COAS-HD, at baseline PAL 1 reduced accommodative lag at all near distances (p < 0.02), but PAL 2 only at 40 cm (p < 0.02). Lags measured with COAS-HD were greater for shorter target distances with PALs. After 12 months' wear, the PALs no longer reduced accommodative lags significantly, except at 40 cm distance, but 0.50 D and 0.75 D booster adds decreased the lags to those measured at baseline or less. In conclusion, for PALs to reduce accommodative lag effectively, addition power should be tailored to typical working distances and after the first year of wear should be boosted by at least 0.50 D to maintain efficacy.

Central and peripheral choroidal thickness and eye length changes during accommodation.

PURPOSE: Eye length increases during accommodation, both on-axis and in the periphery. The aim of this study was to determine whether the peripheral choroid thins with accommodation and to determine the relationship with eye length changes measured at the same location. METHODS: Subjects included 53 young adults in good ocular and general health, with 19 emmetropes and 34 myopes. Measurements from the right eye were made for 0 D and 6 D accommodation stimuli for ±30° horizontal visual field/retinal locations in 10° steps. Valid eye length and choroidal thickness measurements were obtained for 37 and 47 participants, respectively, and both measures were taken for 31 participants. 2.5% phenylephrine was instilled to dilate the pupils. Participants turned their eyes, without head movement, to fixate targets and to make the target 'as clear as possible' during measurements. Correction was made for the influence of lens thickness changing at different peripheral angles. Choroidal thickness was measured with a spectral-domain-Optical Coherence Tomographer. For peripheral images, the internal cross target on the capture screen was moved from the centre to 17.25° nasal/temporal positions. RESULTS: In accordance with previous literature, eye length increased with accommodation. The greatest change (mean ± SD) of 41 ± 17 µm occurred at the centre, with a mean change across the locations of 33 µm. There were no significant differences between emmetropes and myopes. Choroidal thickness decreased with accommodation, with changes being about two-thirds of those occurring for eye length. The greatest change of -30 ± 1 µm occurred at the centre, with a mean change of -21 µm. Greater choroidal thinning occurred for myopes than for emmetropes (23 ± 11 vs. 17 ± 8 µm, p = 0.02). CONCLUSIONS: With accommodation, eye length increased and the choroid thinned, at both central and peripheral positions. Choroidal thinning accounted for approximately 60% of the eye length increase across the horizontal ±30°.

Accommodation lags are higher in myopia than in emmetropia: Measurement methods and metrics matter.

PURPOSE: To determine whether accommodative errors in emmetropes and myopes are systematically different, and the effect of using different instruments and metrics. METHODS: Seventy-six adults aged 18-27 years comprising 24 emmetropes (spherical equivalent refraction of the dominant eye +0.04 ± 0.03 D) and 52 myopes (-2.73 ± 0.22 D) were included. Accommodation responses were measured with a Grand Seiko WAM-5500 and a Hartmann-Shack Complete Ophthalmic Analysis System aberrometer, using pupil plane (Zernike and Seidel refraction) and retinal image plane (neural sharpness-NS; and visual Strehl ratio for modulation transfer function-VSMTF) metrics at 40, 33 and 25 cm. Accommodation stimuli were presented to the corrected dominant eye, and responses, referenced to the corneal plane, were determined in the fellow eye. Linear mixed-effects models were used to determine influence of the refractive group, the measurement method, accommodation stimulus, age, race, parental myopia, gender and binocular measures of heterophoria, accommodative convergence/accommodation and convergence accommodation/convergence ratios. RESULTS: Lags of accommodation were affected significantly by the measurement method (p < 0.001), the refractive group (p = 0.003), near heterophoria (p = 0.002) and accommodative stimulus (p < 0.05), with significant interactions between some of these variables. Overall, emmetropes had smaller lags of accommodation than myopes with respective means ± standard errors of 0.31 ± 0.08 D and 0.61 ± 0.06 D (p = 0.003). Lags were largest for the Grand Seiko and Zernike defocus, intermediate for NS and VSMTF, and least for Seidel defocus. CONCLUSIONS: The mean lag of accommodation in emmetropes is approximately equal to the previously reported depth of focus. Myopes had larger (double) lags than emmetropes. Differences between methods and instruments could be as great as 0.50 D, and this must be considered when comparing studies and outcomes. Accommodative lag increased with the accommodation stimulus, but only for methods using a fixed small pupil diameter.

Ciliary Muscle Dimension Changes With Accommodation Vary in Myopia and Emmetropia.

Purpose: The purpose of this study was to determine whether accommodation-induced changes in ciliary muscle dimensions vary between emmetropes and myopes, and the effect of the image analysis method. Methods: Seventy adults aged 18 to 27 years consisted of 25 people with emmetropia (spherical equivalent refraction [SER] +0.21 ± 0.36 diopters [D]) and 45 people with myopia (-2.84 ± 1.72 D). There were 23 people with low myopia (>-3 D) and 22 people with moderate myopia (-3 to -6 D). Right eye ciliary muscles were imaged (Visante OCT; Carl Zeiss Meditec) at 0 D and 6 D demands. Measures included ciliary muscle length (CML), ciliary muscle curved length (CMLarc), maximum ciliary muscle thickness (CMTmax), CMT1, CMT2, and CMT3 (fixed distances 1-3 mm from the scleral spur), CM25, CM50, and CM75 (proportional distances 25%-75%). Linear mixed model analysis determined effects of refractive groups, race, and demand on dimensions. Significance was set at P < 0.05. Results: Myopic eyes had greater CML and CMLarc nasally than emmetropic eyes. Myopic eyes had thicker muscles than emmetropic eyes at nasal positions, except CM25 and CMT3, and at CM75 temporally. During accommodation and only nasally, CML reduced in emmetropic and myopic eyes, and CMLarc reduced in myopic eyes only. During accommodation, both nasally and temporally, muscles thickened anteriorly (CMT1 and CM25) and thinned posteriorly (CMT3 and CM75) except for temporal CM75. Moderate myopic eyes had greater temporal CMLarc than low myopic eyes, and the moderate myopes had thicker muscles both nasally and temporally using fixed and proportional distances. Conclusions: People with myopia had longer and thicker ciliary muscles than people with emmetropia. During accommodation, the anterior muscle thickened and the curved nasal muscle length shortened, more in myopic than in emmetropic eyes. The fixed distance method is recommended for repeat measures in the same individual. The proportional distance method is recommended for comparisons between refractive groups.

The use of autorefractors using the image-size principle in determining on-axis and off-axis refraction. Part 1: Analysis of optical principles of autorefractors.

PURPOSE: To study the optical principles and properties of autorefractors that use the image-size principle in which the size of the reimaged retinal image determines refraction. METHODS: The retinal illumination and reimaging of the retinal image were described, as were variations in the basic system. Imaging was determined for systems in which the light source is either diverging or converging as it passes into the eye. Equations were determined to describe the dependence of refraction on the heights and angles of incoming and outgoing beams, and refraction error was determined when eye position was not correct. RESULTS: The fundamental refraction equation is DE=±(α+θ)/h1 where DE is refraction, h1 is the beam height entering the eye, and θ and α are the angles of the incoming and outgoing beams, respectively. The negative sign outside the brackets applies if the beam focuses before entering the eye, while the positive sign applies if the beam focuses after entering the eye. When light is diverging as it reaches the anterior eye, hyperopia produces greater retinal image sizes than myopia. The opposite is the case when light is converging as it reaches the anterior eye. The effect of incorrect ocular longitudinal position on the measured refraction was determined; this produced errors identical to those for vertex errors with ophthalmic lenses. CONCLUSION: For image-size principle autorefractors, simple equations describe the dependence of measured refraction on the height and angle of the instrument beam as it enters the eye and the angle of the light, reflected back from the retina, after it exits the eye. Further work will investigate the validity of such instruments for determining peripheral refraction.

The use of autorefractors using the image-size principle in determining on-axis and off-axis refraction. Part 2: Theoretical study of peripheral refraction with the Grand Seiko AutoRef/Keratometer WAM-5500.

PURPOSE: To determine, through simulations, the likely validity of Grand-Seiko autorefractors with annular targets in peripheral refraction. METHODS: Using a physical model eye, the distance inside the eye to which the Grand Seiko AutoRef/Keratometer WAM-5500 beam was converging and the effective size of its outer diameter at the cornea were determined. Grand-Seiko refraction was calculated from Rx  = (θ + α)/h1 , where θ is the angle of the ingoing radiation beam, h1 is the height of the beam at the anterior cornea and α is the angle of the beam emerging from the eye following reflection at the retina. Two eye models were used: a Navarro schematic eye and a Navarro schematic eye with a contact lens having a highly positive aspheric front surface. RESULTS: The instrument beam was determined to be converging towards the eye to a distance of 24.4 mm behind the corneal vertex, with a 2.46 mm effective size outer diameter of the beam at the anterior cornea. The Grand-Seiko refractions provided accurate estimates of peripheral refraction for the model eyes. The results were closer to Zernike refractions than to Zernike paraxial refraction. Spherical aberration influenced refraction by up to 0.5 D, and peripheral coma had limited influence. CONCLUSION: Grand-Seiko autorefractors in current use, and having a circular annulus with an ingoing effective outer diameter at the front of the eye of about 2.4 mm, are likely to give valid peripheral refractions.

Defocused contrast sensitivity function in peripheral vision.

PURPOSE: Human peripheral detection performance is affected by optical factors such as defocus and higher order aberrations. From optical theory, we would expect defocus to produce local depressions (notches) in the contrast sensitivity function (CSF). However, such notches have not been observed in peripheral vision, and it is unknown whether human peripheral vision can detect local depressions (notches) in the CSF, such as those produced by monochromatic defocus when all monochromatic ocular aberrations are corrected. The purpose of the study was to identify such notches. METHODS: Participants were three adult emmetropes. Following full adaptive optics correction, on-axis and 20° nasal visual field detection CSFs in monochromatic light were measured for the right eye with a 7 mm diameter pupil, both without and with ±2 D defocus, and with separate determinations for horizontal and vertical gratings. Defocused CSFs were compared with predictions based on theoretical modulation transfer functions. RESULTS: Notches in the monochromatic defocused CSFs were identified for peripheral vision at optically predicted spatial frequencies with other monochromatic ocular aberrations corrected, provided that there was adequate spatial frequency sampling. The spatial frequencies of notches were similar to those predicted from optical theory, but their depths (0.3 to 0.9 log unit) were smaller than predicted. CONCLUSION: With fine spatial frequency sampling, notches were identified in defocused monochromatic CSFs when all other monochromatic ocular aberrations were corrected, both on-axis and at 20° eccentricity. Unless recognised as such, notches may contribute to noise in through-focus detection measurements of peripheral visual performance.

Peripheral detection acuity for interference fringes and screen-based Gabor gratings.

This study compared on-axis and peripheral detection acuities measured with interference fringes, that bypass eye optics, and with screen-based Gabor gratings combined with an adaptive optics system. Gabor gratings are sinusoidal gratings incorporated with a Gaussian envelope that attenuate spatial frequency broadening that occur at the window edge. The magnitude of the attenuation was varied. Peripheral detection acuities were always higher for interference fringes than for Gabor gratings. Less attenuated Gabors (with sharper edges) had higher acuities than more attenuated Gabors (with less sharp edges). Theoretical investigations indicated that the spatial frequency broadening occurring due to the sharp edge of the less attenuated Gabors contribute little to high detection grating acuity in the periphery, but that the lower attenuation provides a greater number of visible cycles which is of more benefit to detection than is the case on-axis.

Subjective measurement of the Stiles-Crawford effect with different field sizes.

The Stiles-Crawford effect of the first kind (SCE) is the phenomenon in which light entering the eye near the center of the pupil appears brighter than light entering near the edge. Previous investigations have found an increase in the directionality (steepness) of the effect as the testing location moves from the center of the visual field to parafoveal positions, but the effect of central field size has not been considered. The influence of field size on the SCE was investigated using a uniaxial Maxwellian system in which stimulus presentation was controlled by an active-matrix liquid crystal display. SCE directionality increased as field size increased from 0.5° to 4.7° diameter, although this was noted in four mild myopes and not in two emmetropes. The change with field size was supported by a geometric optics absorption model.

Subjective measurement of the Stiles-Crawford effect of the first kind with variation in accommodation.

PURPOSE: To measure the Stiles-Crawford effect of the first kind (SCE-I), corresponding to central vision, with innovative technology to evaluate changes in the directionality and photoreceptor alignment with accommodation. METHODS: A uniaxial Maxwellian system (spot size in pupil 0.5 mm diameter) was employed, incorporating a spatial light modulator to flicker at 2 Hz between two 2.3° fields corresponding to test (peripheral pupil) and reference (pupil centre) positions. Participants determined thresholds at 13 positions along the horizontal pupil meridian by indicating if the test field was brighter or dimmer than the reference field. Thresholds were determined by a staircase procedure after four reversals at each pupil location. After pupil dilation, seven emmetropes were tested at 0 D to 6 D accommodation stimulus levels in 2 D intervals. Data were fit by the Gaussian function, both when the fits were unforced or forced to pass through the sensitivity expected for the reference point. Directionality (ρ) and peak location values (xmax) were determined for unforced and forced fits. RESULTS: Regression slopes for ρ as a function of accommodation stimulus were not significant. There was a tendency for xmax to shift temporally with increasing accommodation across the 6 D stimulus range. This was not significant for regression fitting (-0.059 mm/D, R2  = 0.06, p = 0.20), but a paired t-test for 0 and 6 D stimuli showed a weakly significant change of 0.62 mm (p = 0.05). The differences between the two fitting approaches were small and non-significant. CONCLUSIONS: Directionality did not change with accommodation, but the pupil peak location showed a significant temporal shift of approximately 0.62 mm with 6 D accommodation stimulus. It is possible that substantial changes in the directionality and a shift in the direction of peak location might occur at very high levels of accommodation.

Digital holographic microscope for human eye retinal structures recording in vivo.

We introduce the digital holographic microscope for recording in vivo human eye retinal structures. Current eye imaging technologies cannot provide images with resolutions better than 1 µm within depths of a few hundred micrometers. This can be improved with digital holography, in which a hologram of the eye captured with digital camera contains information about structures over the full depth of the eye. This information can be reconstructed either optically or numerically. Our hologram recording scheme utilizes working principles of the off-axis digital holographic microscope, designed for reflective micro-object investigation. The eye cornea and lens form the microscope objective. We can record in vivo digital holograms of the human eye retina with resolution after reconstruction of at least 1.3 micrometer.

Standardizing sum-of-segments axial length using refractive index models.

Optical biometry uses interferometry to measure the axial length (AL) of the eye. Traditionally, one-variable regression formulas have converted the optical path length measured by a biometer to a geometric AL. An alternate calculation of axial length sums the individual segments of the eye (sum-of-segments AL). This calculation has been shown to improve predictions of some intraocular lens power formulas when used in place of traditional axial length. Sum-of-segments ALs are determined from 13 refractive index models. As measured in 1695 eyes, these yield different ocular axial lengths. A path to standardization from these models is presented.

Nasal-temporal asymmetry in peripheral refraction with an aspheric myopia control contact lens.

A combination of human subject data and optical modelling was used to investigate unexpected nasal-temporal asymmetry in peripheral refraction with an aspheric myopia control lens. Peripheral refraction was measured with an auto-refractor and an aberrometer. Peripheral refraction with the lens was highly dependent upon instrument and method (e.g. pupil size and the number of aberration orders). A model that did not account for on-eye conformation did not mirror the clinical results, but a model assuming complete lens conformation to the anterior corneal topography accounted for the positive shift in clinically measured refraction at larger nasal field angles. The findings indicate that peripheral refraction of highly aspheric contact lenses is dependent on lens conformation and the method of measurement. These measurement methods must be reported, and care must be used in interpreting results.

Fixation Stability with Bessel Beams.

SIGNIFICANCE: Ophthalmic imaging instruments that require stable fixation can benefit by using Bessel beams in the form of monitor-based Bessel images. PURPOSE: The purpose of this study was to investigate fixation stability using laser Bessel and Gaussian beams and monitor-based images of these targets. METHODS: The right eyes of 16 participants were presented with seven fixation targets: monitor-based images of a bull's eye/cross hair, a Gaussian beam, a Bessel beam with four rings and a Bessel beam with three rings; laser Gaussian beam, laser Bessel beam with four rings, and laser Bessel beam with three rings. Participants fixated target centers for five runs, in which each run presented the seven targets for 20 seconds each. An Eye Tribe tracker sampled eye positions at 30 Hz. Standard deviations along horizontal (σx) and vertical meridians (σy) and areas of bivariate contour ellipses (BCEAs) of fixation positions were calculated, and statistical significances of target differences for these parameters were determined. RESULTS: Average σx, σy, and BCEAs ranged from 0.26 to 0.35°, 0.38 to 0.55°, and 0.78 to 1.31 degrees, respectively. Target differences in σx (χ6 = 13.0, P = .04), (σy) (χ6 = 36.819, P < .001), and BCEA (χ6 = 34.406, P < .001) were statistically significant. There were significant post hoc differences between some of the target pairs for σy and BCEA, but not for σx. Monitor-based Bessel beam targets provided significantly smaller σy and BCEAs than the bull's eye/cross hair combination and the monitor- and laser-based Gaussian beam targets. CONCLUSIONS: Monitor-based images of Bessel beams provided better fixation targets than did a bull's eye/cross hair combination, monitor-based Gaussian images, and laser Gaussian beams, but no claim can be made that laser Bessel beams provide better fixation targets than do laser Gaussian beams. Monitor-based Bessel images should be useful for ophthalmic imaging instruments requiring stable fixation.

Experimental Study of Refraction Effects of Nominally Plano Ophthalmic Prisms and Magnifying Lenses.

SIGNIFICANCE: Nominally plano ophthalmic prisms give autorefraction results similar to those predicted on the basis of how effective powers change with pantoscopic tilt, and magnifying lenses give autorefraction results similar to those predicted on the basis of vergence changes. Without appreciation of the optics involved, these effects might wrongly be considered artifacts. PURPOSE: The purpose of this study was to investigate the interactions of autorefractors with lenses and prisms. METHODS: There were 15 adult participants across three experiments, with a range of ages and refractions. In experiments 1 and 2, participants wore frames containing base-up and base-down nominally plano prisms. In experiment 3, participants wore a lens that produced either 6.3% magnification or 5.9% minification, depending on which surface faced the eye. Autorefracting instruments with different operating principles were used: Shin-Nippon SRW5000 autorefractor, Grand Seiko 5100K autorefractor, Hoya AR-530 autorefractor, a Complete Ophthalmic Analysis System-High Definition wavefront sensor, and Tomey FC-800 autorefractor. A theory on the likely effects of magnifying lenses was presented. RESULTS: For ophthalmic prisms, refractions showed results similar to those predicted on the basis of how effective prism powers change with pantoscopic tilt. As tilt increased, base-up prism gave more positive mean refractions and more negative horizontal/vertical astigmatism and vice versa for base-down prisms. In the presence of 10° tilt, 8Δ base-up prisms and 8Δ base-down prisms had different effects by a mean of 0.36 diopters. Magnifying lenses affected refractions similar to those predicted on the basis of vergence changes, with 6% magnification and minification producing mean changes of -11 and +8%, respectively, in absolute mean refraction. There was no strong evidence that different instruments had different effects. CONCLUSIONS: The results have implications for studies in which prisms and lenses are placed in the front eyes, such as accommodation studies using thick lenses close to the eyes to stimulate accommodation rather than by changing object distance.

Theoretical Study of Refraction Effects of Plano Ophthalmic Prisms.

SIGNIFICANCE: Nominally plano prisms can have appreciable refractive errors that exceed the usual prescribing step of 0.25 D, particularly when an eye rotates to view off-axis objects. PURPOSE: The purpose of this study was to determine theoretically the refractive power effects of nominally plano-refracting power prisms. METHODS: Plano prisms with zero refraction were designed for the as-worn condition. A basic method was developed to determine refractive effects in the presence of pantoscopic tilt. A refined method was developed that considers the eye rotating behind the lens, and this and the basic method were compared with accurate raytracing. RESULTS: Plano prisms of 4 and 8 Δ were designed with astigmatic back surfaces to compensate for oblique incidence, and tangential and sagittal image vergence errors were investigated for base-up (BU) and base-down (BD) directions, 0 and -3.33 D object vergences, and pantoscopic tilts up to 10°. Basic and refined results did not differ from accurate results by more than 0.04 and 0.08 D, respectively. Errors for 8 Δ prisms were approximately twice those for 4 Δ prisms. Errors were approximately proportional to tilt. With 10° tilt, the errors ranged between -0.65 D/-0.23 D (8 Δ BD, -3.33 D object vergence) and +0.36 D/+0.15 D (8 Δ BU, 0 D object vergence). Sagittal errors were generally about one third of corresponding tangential errors. In the presence of tilt, BU prisms had positive errors, and BD prisms had similar, but negative, errors for distance objects. At -3.33 D object vergence with tilt, negative errors for BD were greater than positive errors for BU. When the eye rotates to look at objects at different positions, errors can increase beyond those occurring on-axis. CONCLUSIONS: When designed for nontilted conditions, but then subjected to tilt or to viewing off-axis objects, plano prisms can have errors exceeding the usual prescribing step of 0.25 D.

Glare-free retinal imaging using a portable light field fundus camera.

We present the retinal plenoptoscope, a novel light field retinal imaging device designed to overcome many of the problems that limit the use of portable non-mydriatic fundus cameras, including image quality and lack of stereopsis. The design and prototype construction of this device is detailed and the ideal relationship between the eye pupil, system aperture stop and micro-image separation is investigated. A comparison of the theoretical entrance pupil size, multi-view baseline and depth resolution indicates that a higher degree of stereopsis is possible than with stereo fundus cameras. We also show that the effects of corneal backscatter on image quality can be removed through a novel method of glare identification and selective image rendering. This method is then extended to produce glare-free depth maps from densely estimated depth fields, creating representations of retinal topography from a single exposure. These methods are demonstrated on physical models and live human eyes using a prototype device based on a Lytro Illum consumer light field camera. The Retinal Plenoptoscope offers a viable, robust modality for non-mydriatic color and 3-D retinal imaging.

Change in human lens dimensions, lens refractive index distribution and ciliary body ring diameter with accommodation.

We investigated changes in ciliary body ring diameter, lens dimensions and lens refractive index distributions with accommodation in young adults. A 3T clinical magnetic resonance imaging scanner imaged right eyes of 38 18-29 year old participants using a multiple spin echo sequence to determine accommodation-induced changes along lens axial and equatorial directions. Accommodation stimuli were approximately 1 D and 5 D. With accommodation, ciliary body ring diameter, and equatorial lens diameter decreased (-0.43 ± 0.31 mm and -0.30 ± 0.23 mm, respectively), and axial lens thickness increased ( + 0.34 ± 0.16 mm). Lens shape changes cause redistribution of the lens internal structure, leading to change in refractive index distribution profiles. With accommodation, in the axial direction refractive index profiles became flatter in the center and steeper near the periphery of the lens, while in the equatorial direction they became steeper in the center and flatter in the periphery. The results suggest that the anatomical accuracy of lens optical models can be improved by accounting for changes in the refractive index profile during accommodation.

Peripheral Monochromatic Aberrations in Young Adult Caucasian and East Asian Eyes.

SIGNIFICANCE: Myopia prevalence rates differ between racial groups. If the growth of the eye is sensitive to differences in optical input, the difference in spherical aberration between East Asian and Caucasian eyes found in this study may be important in understanding myopia development. PURPOSE: The aim of this study was to determine differences in peripheral wavefront aberrations between two racial groups. METHODS: Wavefront aberrations were measured using a COAS-HD aberrometer across the 42 × 32° central visual field on 37 right eyes of young adults (18 Caucasians, 19 East Asians; mean age 21.5 ± 2.4 years). The mean spherical equivalent refraction was -1.94 ± 1.63 diopters (D) with a range of -5.87 to +0.16 D. Effect of race and visual field position on refractions, individual Zernike aberration coefficients up to the fourth order, higher-order root-mean-square aberration, and total root-mean-square aberration were assessed by repeated-measures analysis of covariance. RESULTS: Caucasians and East Asians had similar relative peripheral myopia across the visual field. All higher-order aberration coefficients were affected by visual field position. Race had no significant effect on any higher-order Zernike coefficient, but the difference in mean vertical coma coefficient (Equation is included in full-text article.)across the visual field (i.e., average of 38 field locations) approached significance, being less positive in Caucasians than in East Asians (P = .08). When correction was made for the Caucasian group being slightly less myopic than the East Asian group, spherical aberration coefficient (Equation is included in full-text article.)was less positive in Caucasians than in East Asians by 0.04 µm (P = .001). The rates of change of coma coefficients across the field were not affected by race. CONCLUSIONS: Caucasians and East Asians had similar relative peripheral myopia, but with less positive spherical aberration coefficient in Caucasians than in East Asians. It remains to be determined whether aberrations have a role in the difference of myopia prevalence rates in different countries.

Improvements to Phakometry Using Bessel Beams.

SIGNIFICANCE: The main problem with phakometry is the low visibility of the third Purkinje image. We built a phakometer using Bessel beams, which have properties of being resistant to diffraction and the potential for self-reconstruction. This instrument had lenticular images three times brighter than those of a conventional phakometer. PURPOSE: To investigate Purkinje image brightness, accuracy, and repeatability of a "Bessel" phakometer compared with those of a conventional phakometer. METHODS: Phakometers were developed with a telecentric imaging system focused at the pupil plane of the eye to capture anterior cornea, anterior lens (PIII), and posterior lens (PIV) Purkinje images. A Bessel beam was generated by a diode laser beam passing through a high-powered doublet with a central obstruction. Software was used to determine image sizes and estimate lens anterior and posterior surface radii of curvature (Ra, Rp), equivalent refractive index (RI), and equivalent power (F). The Bessel phakometer's accuracy was assessed using a model eye. Repeatability (interobserver and intraobserver) and Purkinje images brightnesses of Bessel and conventional phakometers were assessed with six participants. RESULTS: The lens parameters of the model eye determined by the Bessel phakometer were similar to those provided by the model eye's manufacturer with differences (manufacturer - Bessel) in Ra and Rp, RI, and F of +1.18 mm, 0.18 mm, +0.0053, and -0.55 D, respectively. The intraobserver repeatabilities for the Bessel and conventional phakometers were similar. The interobserver repeatabilities of Ra, Rp, and RI for the Bessel phakometer were almost half those (i.e., two times better) for the conventional phakometer. Brightnesses of PIII and PIV were approximately three times higher with the Bessel phakometer than with the conventional phakometer. CONCLUSIONS: The Bessel beam phakometer provided accurate estimates of lens parameters of a model eye and produced brighter Purkinje images and better interobserver repeatability than that of a conventional phakometer.