Arthur Ho
The University of New South Wales, School of Optometry and Vision Science, Department Member
Soft contact lenses (SCL) incorporating aberration-correction have the potential of providing acuity improvements beyond those offered by traditional spherical surface SCL. With the availability of such SCL offered by manufacturers, it is... more
Soft contact lenses (SCL) incorporating aberration-correction have the potential of providing acuity improvements beyond those offered by traditional spherical surface SCL. With the availability of such SCL offered by manufacturers, it is worthwhile to consider the extent of benefits such designs offer. One factor that can potentially influence the performance of aberration-corrected SCL is the optical effect introduced by the post-lens tear film (PoTLF). We modelled the optical impact of the PoTLF assuming three models: (1) worst case (2) representative Zernike surfaces, and (3) most-probable case. PoTLF parameters were based on measured or modelled values from previous studies. The optical contributions of the PoLTF was modelled as a pair of standard aspheric surfaces representing the posterior SCL surface and the anterior corneal/epithelial surface with ro=7.80, k=-0.26. For each model, we analysed the MTF and calculated the Strehl ratio as a result of the optical aberrations introduced by PoTLF. Our results suggest that the PoTLF introduces a detectable amount of degradation in images and may slightly limit the visual benefits from aberration-corrected SCL. However, compared to the performance of traditional spherical surface SCL, we predict that worthwhile visual improvements should still be afforded by aberration-corrected SCL.
Research Interests:
ABSTRACT
Research Interests:
We present a method for measuring lens power from extended depth OCT biometry, corneal topography, and refraction using an improvement on the Bennett method. A reduced eye model was used to derive a formula for lens power in terms of... more
We present a method for measuring lens power from extended depth OCT biometry, corneal topography, and refraction using an improvement on the Bennett method. A reduced eye model was used to derive a formula for lens power in terms of ocular distances, corneal power, and objective spherical equivalent refraction. An error analysis shows that the formula predicts relaxed lens power with a theoretical accuracy of ± 0.5 D for refractive error ranging from -10 D to + 10 D. The formula was used to calculate lens power in 16 eyes of 8 human subjects. Mean lens power was 24.3 D ± 1.7 D.
To determine if the lens volume changes during accommodation. The study used data acquired on 36 cynomolgus monkey lenses that were stretched in a stepwise fashion to simulate disaccommodation. At each step, stretching force and dioptric... more
To determine if the lens volume changes during accommodation. The study used data acquired on 36 cynomolgus monkey lenses that were stretched in a stepwise fashion to simulate disaccommodation. At each step, stretching force and dioptric power were measured and a cross-sectional image of the lens was acquired using an optical coherence tomography system. Images were corrected for refractive distortions and lens volume was calculated assuming rotational symmetry. The average change in lens volume was calculated and the relation between volume change and power change, and between volume change and stretching force, were quantified. Linear regressions of volume-power and volume-force plots were calculated. The mean (±SD) volume in the unstretched (accommodated) state was 97 ± 8 mm3. On average, there was a small but statistically significant (P = 0.002) increase in measured lens volume with stretching. The mean change in lens volume was +0.8 ± 1.3 mm3. The mean volume-power and volume-...
Research Interests:
Soft contact lenses (SCL) incorporating aberration-correction have the potential of providing acuity improvements beyond those offered by traditional spherical surface SCL. With the availability of such SCL offered by manufacturers, it is... more
Soft contact lenses (SCL) incorporating aberration-correction have the potential of providing acuity improvements beyond those offered by traditional spherical surface SCL. With the availability of such SCL offered by manufacturers, it is worthwhile to consider the extent of benefits such designs offer. One factor that can potentially influence the performance of aberration-corrected SCL is the optical effect introduced by the post-lens tear film (PoTLF). We modelled the optical impact of the PoTLF assuming three models: (1) worst case (2) representative Zernike surfaces, and (3) most-probable case. PoTLF parameters were based on measured or modelled values from previous studies. The optical contributions of the PoLTF was modelled as a pair of standard aspheric surfaces representing the posterior SCL surface and the anterior corneal/epithelial surface with ro=7.80, k=-0.26. For each model, we analysed the MTF and calculated the Strehl ratio as a result of the optical aberrations introduced by PoTLF. Our results suggest that the PoTLF introduces a detectable amount of degradation in images and may slightly limit the visual benefits from aberration-corrected SCL. However, compared to the performance of traditional spherical surface SCL, we predict that worthwhile visual improvements should still be afforded by aberration-corrected SCL.
Research Interests:
Research Interests:
ABSTRACT
Research Interests:
Research Interests:
Research Interests:
Research Interests:
PURPOSE: To investigate the relationship between the sign of spherical aberration (SA) and the corresponding depth-of-focus (DoF) values around best focus, at three different spatial frequencies (SF). Additionally, to study the influence... more
PURPOSE: To investigate the relationship between the sign of spherical aberration (SA) and the corresponding depth-of-focus (DoF) values around best focus, at three different spatial frequencies (SF). Additionally, to study the influence of the Stiles-Crawford effect (SCE) on ...
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The role of inherent spherical aberration (SA) in the optical performance of presbyopic eyes corrected with simultaneous vision multifocal contact lenses was investigated.
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The purpose of this study was to determine the contribution of the gradient refractive index to the change in lens power in hamadryas baboon and cynomolgus monkey lenses during simulated accommodation in a lens stretcher. Thirty-six... more
The purpose of this study was to determine the contribution of the gradient refractive index to the change in lens power in hamadryas baboon and cynomolgus monkey lenses during simulated accommodation in a lens stretcher. Thirty-six monkey lenses (1.4–14.1 years) and twenty-five baboon lenses (1.8–28.0 years) were stretched in discrete steps. At each stretching step, the lens back vertex power was measured and the lens cross-section was imaged with optical coherence tomography. The radii of curvature for the lens anterior and posterior surfaces were calculated for each step. The power of each lens surface was determined using refractive indices of 1.365 for the outer cortex and 1.336 for the aqueous. The gradient contribution was calculated by subtracting the power of the surfaces from the measured lens power. In all lenses, the contribution of the surfaces and gradient increased linearly with the amplitude of accommodation. The gradient contributes on average 65 T 3% for monkeys and 66 T 3% for baboons to the total power change during accommodation. When expressed in percent of the total power change, the relative contribution of the gradient remains constant with accommodation and age in both species. These findings are consistent with Gullstrand's intracapsular theory of accommodation.
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We develop a new method to compute in situ corneal oxygen uptake during soft-contact-lens (SCL) wear using a micro-polarographic Clark electrode. After steady SCL wear and subsequent removal, a membrane-covered polarographic... more
We develop a new method to compute in situ corneal oxygen uptake during soft-contact-lens (SCL) wear using a micro-polarographic Clark electrode. After steady SCL wear and subsequent removal, a membrane-covered polarographic microelectrode is immediately placed onto the cornea. The resulting polarographic signal is related to the steady-state corneal oxygen uptake rate during soft-contact-lens wear. We devise a new analysis to quantify oxygen uptake into the cornea during lens wear. The proposed procedure is applied to new polarographic data for 10 human subjects with 12 different commercial lenses during open eye. We compare our results with recent theory. Average corneal oxygen uptake rates at open eye during SCL wear for 10 subjects wearing 12 different commercial lenses vary from 2 to 10 μL(STP)/cm(2)/h. High oxygen permeability lenses have uptake rates of -10 μL(STP)/cm(2)/h, in close agreement with our previously obtained no-lens human uptake rates of 9 to 13 μL(STP)/cm(2)/h at open eye.(40) Application of the classical data-interpretation procedure to our experimental data gives corneal-uptake results that are approximately three to five times smaller than those obtained with our new interpretation scheme. We provide a simple and reliable tool to quantify corneal-oxygen-uptake rates during in vivo soft-contact-lens wear. Comparison of our newly measured in vivo oxygen uptakes to model prediction for SCLs of varying oxygen transmissibility is in good agreement with available theory.