Optical Wavefront Aberrations Research Articles

Application of Adaptive Optics in Ophthalmology

The eye, the photoreceptive organ used to perceive the external environment, is of great importance to humans. It has been proven that some diseases in humans are accompanied by fundus changes; therefore, the health status of people may be interpreted from retinal images. However, the human eye is not a perfect refractive system for the existence of ocular aberrations. These aberrations not only affect the ability of human visual discrimination and recognition, but restrict the observation of the fine structures of human eye and reduce the possibility of exploring the mechanisms of eye disease. Adaptive optics (AO) is a technique that corrects optical wavefront aberrations. Once integrated into ophthalmoscopes, AO enables retinal imaging at the cellular level. This paper illustrates the principle of AO in correcting wavefront aberrations in human eyes, and then reviews the applications and advances of AO in ophthalmology, including the adaptive optics fundus camera (AO-FC), the adaptive optics scanning laser ophthalmoscope (AO-SLO), the adaptive optics optical coherence tomography (AO-OCT), and their combined multimodal imaging technologies. The future development trend of AO in ophthalmology is also prospected.

Simulated biomechanical effect of aspheric transition zone ablation profiles after conventional hyperopia refractive surgery.

We studied the effects of the aspheric transition zone on the optical wavefront aberrations, corneal surface displacement, and stress induced by the biomechanical properties of the cornea after conventional laser in situ keratomileusis (LASIK) refractive surgery. The findings in this study can help improve visual quality after refractive surgery. Hyperopia correction in 1-5D was simulated using five types of aspheric transition zones with finite element modeling. The algorithm for the simulations was designed according to the optical path difference. Wavefront aberrations were calculated from the displacements on the anterior and posterior corneal surfaces. The vertex displacements and stress on the corneal surface were also evaluated. The results showed that the aspheric transition zone has an effect on the postoperative visual quality. The main wavefront aberrations on the anterior corneal surface are defocus, y-primary astigmatism, x-coma, and spherical aberrations. The wavefront aberrations on the corneal posterior surface were relatively small and vertex displacements on the posterior corneal surface were not significantly affected by the aspheric transition zone. Stress analysis revealed that the stress on the cutting edge of the anterior corneal surface decreased with the number of aspheric transition zone increased, and profile #1 resulted in the maximum stress. The stress on the posterior surface of the cornea was more concentrated in the central region and was less than that on the anterior corneal surface overall. The results showed that the aspheric transition zone has an effect on postoperative aberrations, but wavefront aberrations cannot be eliminated. In addition, the aspheric transition zone influences the postoperative biomechanical properties of the cornea, which significantly affect the postoperative visual quality.

Crystalline lens imaging during accommodation with a slit-scanning tomography system: preliminary results

A slit-scanning tomography system previously developed in the Centre of Physics of University of Minho allows to capture images of the eye and posteriorly calculus of the biometric (thickness and curvature) and optical (wavefront aberrations) properties of the corneal and crystalline lens surfaces. It is intended to optimise the system for an accurate computing of these properties of the crystalline lens in the relaxed state and during ocular accommodation. In this study, preliminary experimental results of the crystalline lens imaging of a subject during accommodation are presented.

Flight-Test Measurement of the Aero-Optical Environment of a Helicopter in Hover

Methods and results are described for flight-test measurements of the aero-optical effect of the near-field flow surrounding a medium-sized helicopter in hover. The data were acquired using a novel, passive measurement approach in which optical wave-front aberrations were computed from the displacements of light-emitting diodes attached to a target array mounted under the helicopter fuselage. The resulting aero-optical data are shown through the rotor downwash and through the engine exhaust of the helicopter, and they are compared to estimates computed using parametric models for the helicopter wake structure and rotor–vortex diffusion. The experimental data provide additional information on the magnitude and frequency content of the aero-optical aberrations, as well as vibration data, that will be valuable for future beam-control and modeling studies.

Towards model-based adaptive optics optical coherence tomography.

The transfer function for optical wavefront aberrations in single-mode fiber based optical coherence tomography is determined. The loss in measured OCT signal due to optical wavefront aberrations is quantified using Fresnel propagation and the calculation of overlap integrals. A distinction is made between a model for a mirror and a scattering medium model. The model predictions are validated with measurements on a mirror and a scattering medium obtained with an adaptive optics optical coherence tomography setup. Furthermore, a one-step defocus correction, based on a single A-scan measurement, is derived from the model and verified. Finally, the pseudo-convex structure of the optical coherence tomography transfer function is validated with the convergence of a hill climbing algorithm. The implications of this model for wavefront sensorless aberration correction are discussed.

热载荷下激光扩束系统的光机优化设计

In order to improve the temperature adaptability of vehicle-borne beam expander system, a structural optimization method is proposed to directly deal with optical wavefront aberrations. After sensitivity analysis, root mean square (RMS) value of the optical wavefront aberration can be calculated from the strain of the finite element nodes through homogeneous coordinate transformation. Taking the minimum wavefront aberration as the optimization objective, an optimized topology is obtained. According to the topology of material, the frame is divided into blocks, and optimized secondly in the form of size optimization. Finally, a manufacturable structure with good temperature flexibility is obtained. When the thermal load is -50C, the wavefront aberration RMS value is reduced from 0.728 to 0.196 , = 632.8 nm. And in the range of 50C, the optimized wavefront aberration RMS value is about 1/4 times as large as that before optimization at the same thermal load conditions.

Evaluation of quality of vision in early stage after sub-bowman keratomileusis

To assess the quality of vision in the early stage after sub-bowman keratomileusis (SBK) for the correction of moderate and high myopia. Retrospective case series. Sixty eight myopic eyes of 35 consecutive patients in Chongqing Medal Eye Institute from May to November 2011 were enrolled in this study. All patients were evaluated at 2 weeks, 1 and 3 months postoperatively. The main measurements included visual acuity, corneal and total optical wavefront aberration, photopic and mesopic contrast sensitivity. The median values of preoperative uncorrected central visual acuity (UCVA) and best corrected visual acuity (BCVA) were 0.05 and 1.00, respectively. The median value of postoperative UCVA was 1.20. There was no significant difference between the preoperative and postoperative anterior corneal total aberration (F = 0.712, P > 0.05). Compared with preoperative value (0.47 ± 0.17)µm, the corneal high orders aberrations in 2 weeks after SBK (1.37 ± 0.72) µm increased by 3 times. The total optical aberrations in 2 weeks after SBK (0.99 ± 0.58) µm decreased to one-eighth of preoperative measurements (8.26 ± 2.93) µm. Compared with preoperative value (0.30 ± 0.12)µm, the total high order aberrations in 2 weeks after SBK (0.46 ± 0.19) µm increased by 1.5 times. Although photopic and mesopic contrast sensitivity fluctuated mildly after SBK, the measurements did not decline significantly. Compared with preoperative data, mesopic contrast sensitivity at 3 months increased 0.294 (F = 20.88,P < 0.01) and 0.296 (F = 13.64, P < 0.01) at 12 and 18 c/d, respectively. Patients could obtain good quality of vision at the early stage after SBK.

Variations in image optical quality of the eye and the sampling limit of resolution of the cone mosaic with axial length in young adults

To evaluate the variation in higher-order ocular wavefront aberrations and the Nyquist limit of resolution of the cone mosaic (N(c)) in a population of young healthy subjects and the relation to axial length (AL). Fondazione G.B. Bietti IRCCS, Rome, Italy. Case series. An adaptive optics retinal camera prototype (rtx1) was used to image the cone mosaic. Cone density and N(c) were calculated at fixed eccentricity between 260 μm and 600 μm from the foveal center. Ocular higher-order wavefront aberrations were measured using the OPD Scan II device. The coefficient of variation (CoV) was used to analyze the variation in optical and retinal parameters. The correlation of optical and retinal parameters with AL was performed using Pearson analysis. Twelve subjects (age 24 to 38 years; AL 22.61 to 26.63 mm) were evaluated. A high interindividual variation in the higher-order wavefront aberrations was found, ranging from 26% for corneal higher-order aberrations (HOAs) to 41% for intraocular HOAs. The CoV of cone density and N(c) were 16% and 5%, respectively. The decline in cone density and N(c) with AL was statistically significant at all retinal eccentricities (R(2) > 0.44, P<.001). Although there appeared to be random variation in the eye's optical wavefront aberration from subject to subject, the cone-packing density and N(c) were highly correlated with AL. Although the eye's overall image optical quality in the emmetropic group and the myopic group was comparable, the spatial sampling of the cone mosaic decreased with increasing AL. No author has a financial or proprietary interest in any material or method mentioned.

Studies of transverse chromatic aberration based on individual eye model

Two kinds of individual eye models, with and without involving the angle between visual axis and optical axis, are constructed according to the measurements of the corneal surfaces, the optical axis lengths and wavefront aberrations. The constructed eye models are then used to investigate the properties of the transverse chromatic aberration (TCA) and its influence on visual performance. The statistical distributions of the horizontal TCA, vertical TCA and the absolute magnitude of the TCA over the spectrum from 470 to 650 nm are provided, respectively. The variations of TCA with wavelength as referring to the central wavelength of 555 nm for five subjects are also provided. Then we evaluate the impact of TCA on image performance in comparison with longitudinal chromatic aberration (LCA). It is shown that LCA is more detrimental than foveal TCA in image quality. It is found that, while LCA was corrected well, the foveal TCA is simultaneously corrected. Finally, we investigate the correction of peripheral TCA with an achromatizing element. The correction of peripheral TCA of human eye strongly depends on the location of the achromatizing element, and with the element in front of the eye, the peripheral TCA cannot be corrected.

Generation of acoustic waves in the eye via cavitation during photoablation of corneal tissue

During LASIK and other refractive correction surgeries, the shape of the cornea is modified by removal of tissue with high energy laser pulses. The laser beam deposits enough energy over a small spot size (approximately 1 mm diameter) to create a cavitation event from localized heating. Multiple pulses are delivered at different locations on the cornea to remove tissue selectively and reshape its surface in effort to minimize optical wavefront aberrations. Cavitation with each pulse can generate an acoustic wave that propagates within the eye and potentially damage sensory cells in the retina. A mathematical model was created to investigate this phenomenon and understand the effects of laser spot size and pulse repetition frequency on characteristics of the acoustic wave. Motion associated with the cavitation event was treated as a point source on the surface of a spherically shaped eye globe. A spatial map of internal acoustic pressure amplitudes was then created; results indicate that operational parameters can be adjusted to help prevent deleritous effects of cavitation-induced stress waves on the retina during refractive correction surgery.

Correcting the ocular aberrations of a healthy adult population using microelectromechanical (MEMS) deformable mirrors

We model the capabilities of several commercially available microelectromechanical deformable mirrors to compensate for the typical optical wavefront aberrations that occur in the human eye. The simulations include the nonlinear effects of actuator clipping, ocular pupil diameter, beam diameter on the mirror and the number of mirror modes.

Effect of tear film break-up on higher-order aberrations measured with wavefront sensor

PURPOSE: To investigate whether optical wavefront aberrations vary with tear film break-up.DESIGN: Observational case series.METHODS: Higher-order aberrations were examined for 20 eyes of 20 normal subjects with a Hartmann-Shack wavefront sensor before and after tear film break-up.RESULTS: Higher-order aberrations for photopic vision (central 4 mm diameter) after tear film break-up increased 1.44 fold compared to higher-order aberrations before tear film break-up (P = .001, paired t-test). Higher-order aberrations after tear film break-up for scotopic vision (central 6 mm diameter) were also 1.23 times higher than those before break-up (P = .005, paired t-test).CONCLUSION: Wavefront sensing enabled us to evaluate the induced irregular astigmatism caused by tear film break-up quantitatively. Wavefront aberrations should be measured carefully to avoid the effects of tear film break-up, especially in wavefront-guided refractive surgery.

Optical real-time defect-enhancement diagnostic system

We have demonstrated an all-optical diagnostic system that enhances the observation of defects in periodic structures. This real-time technique employs a spatial light modulator as a smart-pixel array for information processing in the Fourier transform plane of a lens. The system also includes a phase-conjugate mirror for autoalignment and for correction of optical wave-front aberrations that are imparted on the object light by the smart-pixel processor and its associated optical train.

Estimating the contribution of different parts of the atmosphere to optical wavefront aberration

Current adaptive optical telescope designs use a single deformable mirror (DM), usually conjugated to the aperture plane, to compensate for the cumulative effects of optical turbulence. The corrected field of view (FOV) of an adaptive optics system could theoretically be increased through the use of multiple DMs conjugated to a like number of corresponding planes which sample the turbulence region in altitude. Often, the atmospheric turbulence responsible for the degradation of long-exposure telescope images is concentrated in several relatively strong layers. The logical location for the planes of correction in a multiconjugate adaptive optics (MCAO) system would be the same as these “seeing layers.” Each DM would correct for the component of the total wavefront contributed by its associated turbulent layer. However, there is no known method of isolating a particular layer so that its component may be measured. Somehow, the individual components must be estimated using available measurements of the cumulative wavefront at the aperture of the telescope. This paper presents a theoretical analysis of a signal processing technique for determining these phase contributions. The method takes advantage of the spatial diversity of wavefront sensor (WFS) measurements from two or more reference sources. These separate wavefront sensor measurements are processed via minimum mean square error filtering to yield an estimate of the phase perturbation caused by a particular turbulent layer of the atmosphere. Our results indicate that multiple wavefront corrector adaptive optics systems will require much brighter reference sources than single wavefront corrector systems.

Wavefront sensor noise reduction and dynamic range expansion by means of optical image intensification

A Hartmann wavefront sensor measures optical wavefront aberrations by using a lens array to break the input sample into subapertures. The lens in each subaperture produces an image whose centroid position depends on the average phase front tilt in the subaperture. The simplest measurement implementation, a quad cell detector, is limited in tilt dynamic range to 1 wave because the (diffraction-limited) image has a radius equivalent to 1 wave of tilt. For greater tilts, the image is off-null and no positional information is available. Dynamic range can be increased by defocusing the spot but only at the expense of increased centroid variance. However, if the spot image is optically intensified before being detected, the intensified image can be defocused without increasing the centroid variance. A theoretical derivation of sensor performance with and without intensification is given. Results of simulations and laboratory experiment are presented showing good agreement with theory. The use of this technique may greatly extend the capabilities of wavefront sensor systems at modest increase in cost or complexity.

Ray-trace analysis and data reduction methods for the Ritchey-Common test

The relationships between figure errors on an optical flat and wave-front aberrations when the flat is tested in the Ritchey-Common configuration are examined by the real ray-tracing lens analysis program polycon. A new data reduction method for the test is demonstrated. The test is simulated, and rays are traced using polycon. Unit magnitude Zernike term surface errors are added, one by one, to the flat under simulated test, and the effect on the wave front is derived by real ray tracing. This results in a table of influence functions for the particular test geometry. The simulation is repeated for one or more different field angles or azimuthal rotations of the flat to obtain other sets of influence functions. By using these influence functions it is possible to deduce what surface errors on the flat are responsible for any arbitrary test wave-front errors measured up to the order of the influence functions derived. The effectiveness of this method is compared with other existing methods by computer simulations.