Community Academic Profiles - Faculty & Researchers

All Publications

Multi-layer Shack-Hartmann wavefront sensing in the point source regime Biomedical Optics Express Akondi, V., Dubra, A. 2021; 12 (1): 409-432
View details for DOI 10.1364/BOE.411189
A two-layer Shack-Hartmann wavefront sensor model of the human and mouse retinas Akondi, V., Dubra, A. 2021
View details for DOI 10.1117/12.2583667
Shack-Hartmann wavefront sensor optical dynamic range Optics Express Akondi, V., Dubra, A. 2021; 29 (6): 8417-8429
Abstract

The widely used lenslet-bound definition of the Shack-Hartmann wavefront sensor (SHWS) dynamic range is based on the permanent association between groups of pixels and individual lenslets. Here, we formalize an alternative definition that we term optical dynamic range, based on avoiding the overlap of lenslet images. The comparison of both definitions for Zernike polynomials up to the third order plus spherical aberration shows that the optical dynamic range is larger by a factor proportional to the number of lenslets across the SHWS pupil. Finally, a pre-centroiding algorithm to facilitate lenslet image location in the presence of defocus and astigmatism is proposed. This approach, based on the SHWS image periodicity, is demonstrated using optometric lenses that translate lenslet images outside the projected lenslet boundaries.

View details for DOI 10.1364/OE.419311
Wavefront distortions in an oscillating resonant galvanometric optical scanner Computational Optical Sensing and Imaging Akondi, V., Kowalski, B., Sredar, N., Dubra, A. 2020: JW2A. 48
Optical and Visual Quality With Physical and Visually Simulated Presbyopic Multifocal Contact Lenses Translational Vision Science & Technology Vinas, M., Aissati, S., Gonzalez-Ramos, A., Romero, M., Sawides, L., Akondi, V., Gambra, E., Dorronsoro, C., Karkkainen, T., Nankivil, D., Marcos, S. 2020; 9 (20): 1-16
View details for DOI 10.1167/tvst.9.10.20
Dynamic distortion in resonant galvanometric optical scanners Optica Akondi, V., Kowalski, B., Burns, S. A., Dubra, A. 2020; 7 (11): 1506-1513
View details for DOI 10.1364/OPTICA.405187
Average gradient of Zernike polynomials over polygons Optics Express Akondi, V., Dubra, A. 2020; 28 (13): 18876-18886
View details for DOI 10.1364/OE.393223
In vivo quantification of Bruch's membrane in humans with visible light OCT Zhang, T., Kho, A., Akondi, V., Dubra, A., Srinivasan, V. 2020
View details for DOI 10.1117/12.2546437
Perceptual and physical limits to temporal multiplexing simulation of multifocal corrections Dorronsoro, C., Rodriguez-Lopez, V., Barcala, X., Gambra, E., Akondi, V., Sawides, L., Marrakchi, Y., Lage, E., Geisler, W. S., Marcos, S. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 2019
View details for Web of Science ID 000488800707189
Shack-Hartmann wavefront sensor bias at the pupil boundary: problem and solution Akondi, V., Dubra, A. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 2019
View details for Web of Science ID 000488800704010
Chromatic Shack-Hartmann wavefront sensor with adaptive optics correction of monochromatic aberrations Steven, S., Akondi, V., Dubra, A. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 2019
View details for Web of Science ID 000488800702373
Optical and visual quality with physical and visually simulated presbyopic multifocal contact lenses Vinas, M., Aissati, S., Gonzalez-Ramos, A., Romero, M., Sawides, L., Akondi, V., Gambra, E., Dorronsoro, C., Martinez-Enriquez, E., Karkkainen, T., Nankivil, D., Marcos, S. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 2019
View details for Web of Science ID 000488800707192
Accounting for focal shift in the Shack-Hartmann wavefront sensor Optics Letters Akondi, V., Dubra, A. 2019; 44 (17): 4151-4154
Abstract

The Shack-Hartmann wavefront sensor samples a beam of light using an array of lenslets, each of which creates an image onto a pixelated sensor. These images translate from their nominal position by a distance proportional to the average wavefront slope over the corresponding lenslet. This principle fails in partially and/or non-uniformly illuminated lenslets when the lenslet array is focused to maximize peak intensity, leading to image centroid bias. Here, we show that this bias is due to the low Fresnel number of the lenslets, which shifts the diffraction focus away from the geometrical focus. We then demonstrate how the geometrical focus can be empirically found by minimizing the bias in partially illuminated lenslets.

View details for DOI 10.1364/OL.44.004151
Tunable lenses: dynamic characterization and fine-tuned control for high-speed applications Optics Express Dorronsoro, C., Barcala, X., Gambra, E., Akondi, V., Sawides, L., Marrakchi, Y., Rodriguez-Lopez, V., Benedi-Garcia, C., Vinas, M., Lage, E., Marcos, S. 2019; 27 (3): 2085-2100
Abstract

Tunable lenses are becoming ubiquitous, in applications including microscopy, optical coherence tomography, computer vision, quality control, and presbyopic corrections. Many applications require an accurate control of the optical power of the lens in response to a time-dependent input waveform. We present a fast focimeter (3.8 KHz) to characterize the dynamic response of tunable lenses, which was demonstrated on different lens models. We found that the temporal response is repetitive and linear, which allowed the development of a robust compensation strategy based on the optimization of the input wave, using a linear time-invariant model. To our knowledge, this work presents the first procedure for a direct characterization of the transient response of tunable lenses and for compensation of their temporal distortions, and broadens the potential of tunable lenses also in high-speed applications.

View details for DOI 10.1364/OE.27.002085
Visual simulators replicate vision with multifocal lenses. Scientific reports Vinas, M. n., Benedi-Garcia, C. n., Aissati, S. n., Pascual, D. n., Akondi, V. n., Dorronsoro, C. n., Marcos, S. n. 2019; 9 (1): 1539
Abstract

Adaptive optics (AO) visual simulators based on deformable mirrors, spatial light modulators or optotunable lenses are increasingly used to simulate vision through different multifocal lens designs. However, the correspondence of this simulation with that obtained through real intraocular lenses (IOLs) tested on the same eyes has not been, to our knowledge, demonstrated. We compare through-focus (TF) optical and visual quality produced by real multifocal IOLs (M-IOLs) -bifocal refractive and trifocal diffractive- projected on the subiect's eye with those same designs simulated with a spatial light modulator (SLM) or an optotunable lens working in temporal multiplexing mode (SimVis technology). Measurements were performed on 7 cyclopleged subjects using a custom-made multichannel 3-active-optical-elements polychromatic AO Visual Simulator in monochromatic light. The same system was used to demonstrate performance of the real IOLs, SLM and SimVis technology simulations on bench using double-pass imaging on an artificial eye. Results show a general good correspondence between the TF performance with the real and simulated M-IOLs, both optically (on bench) and visually (measured visual acuity in patients). We demonstrate that visual simulations in an AO environment capture to a large extent the individual optical and visual performance obtained with real M-IOLs, both in absolute values and in the shape of through-focus curves.

View details for PubMedID 30733540

View details for PubMedCentralID PMC6367467
Centroid error due to non-uniform lenslet illumination in the Shack-Hartmann wavefront sensor Optics Letters Akondi, V., Steven, S., Dubra, A. 2019; 44 (17): 4167-4170
Abstract

Images formed by individual Shack-Hartmann wavefront sensor lenslets are displaced proportionally to the average wavefront slope over their aperture. This principle fails when the lenslet illumination is non-uniform. Here we demonstrate that the resulting error is proportional to the linear component of the illumination intensity, the quadratic wavefront component, and the lenslet size. For illustrative purposes, we compare the error due to centered Gaussian illumination decaying by 30% at the pupil edge against the error due to assuming the wavefront at the lenslet center being equal to the wavefront average across each lenslet. When testing up to ninth-order Zernike polynomial wavefronts and simulating nine lenslets across the pupil, the maximum centroid errors due to non-uniform illumination and sampling are 1.4% and 21%, respectively, and 0.5% and 6.7% when considering 25 lenslets across the pupil in the absence of other sources of error.

View details for DOI 10.1364/OL.44.004167
Comparison of multifocal visual simulations in patients before and after implantation of diffractive trifocal lenses Vinas, M., Romero, M., Aissati, S., Luis Mendez-Gonzalez, J., Benedi, C., Gambra, E., Akondi, V., Garzon, N., Poyales, F., Dorronsoro, C., Marcos, S. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 2018
View details for Web of Science ID 000442912500217
Experimental validations of a tunable-lens-based visual demonstrator of multifocal corrections. Biomedical optics express Akondi, V. n., Sawides, L. n., Marrakchi, Y. n., Gambra, E. n., Marcos, S. n., Dorronsoro, C. n. 2018; 9 (12): 6302?17
Abstract

The Simultaneous Vision simulator (SimVis) is a visual demonstrator of multifocal lens designs for prospective intraocular lens replacement surgery patients and contact lens wearers. This programmable device employs a fast tunable lens and works on the principle of temporal multiplexing. The SimVis input signal is tailored to mimic the optical quality of the multifocal lens using the theoretical SimVis temporal profile, which is evaluated from the through-focus Visual Strehl ratio metric of the multifocal lens. In this paper, for the first time, focimeter-verified on-bench validations of multifocal simulations using SimVis are presented. Two steps are identified as being critical to accurate SimVis simulations. Firstly, a new iterative approach is presented that improves the accuracy of the theoretical SimVis temporal profile for three different multifocal intraocular lens designs - diffractive trifocal, refractive segmented bifocal, and refractive extended depth of focus, while retaining a low sampling. Secondly, a fast focimeter is used to measure the step response of the tunable lens, and the input signal is corrected to include the effects of the transient behavior of the tunable lens. It was found that the root-mean-square of the difference between the estimated through-focus Visual Strehl ratio of the multifocal lens and SimVis is not greater than 0.02 for all the tested multifocal designs.

View details for PubMedID 31065430

View details for PubMedCentralID PMC6490999
On-bench validations of tunable lens based multifocal visual simulations Imaging and Applied Optics Akondi, V., Sawides, L., Marrakchi, Y., Gambra, E., Barcala, X., Marcos, S., Dorronsoro, C. 2018
View details for DOI 10.1364/AIO.2018.AW2A.1
Simulating multifocal intraocular lenses with a spatial light modulator and a tunable lens: a computational evaluation Akondi, V., Gambra, E., Vinas, M., Aissati, S., Dorronsoro, C., Pascual, D., Marcos, S. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 2017
View details for Web of Science ID 000432170303115
Visual simulations of real multifocal lenses in a multi-channel Adaptive Optics system Marcos, S., Vinas, M., Benedi, C., Aissati, S., Akondi, V., Barcala, X., Gambra, E. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 2017
View details for Web of Science ID 000432170303091
In vivo measurement of longitudinal chromatic aberration with multifocal diffractive intraocular lenses. Vinas, M., Gonzalez-Ramos, A., Dorronsoro, C., Akondi, V., Garzon, N., Poyales, F., Marcos, S. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 2017
View details for Web of Science ID 000432170300318
Temporal multiplexing to simulate multifocal intraocular lenses: theoretical considerations Biomedical Optics Express Akondi, V., Dorronsoro, C., Gambra, E., Marcos, S. 2017; 8 (7): 3410-3425
View details for DOI 10.1364/BOE.8.003410
In Vivo Measurement of Longitudinal Chromatic Aberration in Patients Implanted With Trifocal Diffractive Intraocular Lenses Journal of Refractive Surgery Vinas, M., Gonzalez-Ramos, A., Dorronsoro, C., Akondi, V., Garzon, N., Poyales, F., Marcos, S. 2017; 33 (11): 736-742
View details for DOI 10.3928/1081597X-20170814-01
Evaluation of the true wavefront aberrations in eyes implanted with a rotationally asymmetric multifocal intraocular lens Journal of Refractive Surgery Akondi, V., P�rez-Merino, P., Martinez-Enriquez, E., Dorronsoro, C., Alejandre, N., Jim�nez-Alfaro, I., Marcos, S. 2017; 33 (4): 257-265
View details for DOI 10.3928/1081597X-20161206-03
Virtual pyramid wavefront sensor for phase unwrapping Applied optics Akondi, V., Vohnsen, B., Marcos, S. 2016; 55 (29): 8363-8367
View details for DOI 10.1364/AO.55.008363
Temporal multiplexing and simulation of multifocal intraocular lenses Frontiers in Optics Akondi, V., Dorronsoro, C., Gambra, E., Vinas, M., Pascual, D., Aissati, S., Marcos, S. 2016
View details for DOI 10.1364/FIO.2016.FW2A.3
Phase unwrapping with a virtual Hartmann-Shack wavefront sensor Optics Express Akondi, V., Falldorf, C., Marcos, S., Vohnsen, B. 2015; 23 (20): 25425-25439
View details for DOI 10.1364/OE.23.025425
Phase estimation in digital phase-shifting point diffraction interferometry using a virtual Hartmann-Shack wavefront sensor Adaptive Optics: Analysis, Methods & Systems Akondi, V., Marcos, S., Vohnsen, B. 2015
View details for DOI 10.1364/AOMS.2015.AOT2D.3
Optimization of sensing parameters for a confocal signal-based wavefront corrector in microscopy Journal of Modern Optics Jewel, A. R., Akondi, V., Vohnsen, B. 2015; 62 (10): 786-792
View details for DOI 10.1080/09500340.2015.1007100
Closed-loop adaptive optics using a spatial light modulator for sensing and compensating of optical aberrations in ophthalmic applications Journal of Biomedical Optics Akondi, V., Jewel, A. R., Vohnsen, B. 2014; 19 (9): 096014-096014
View details for DOI 10.1117/1.JBO.19.9.096014
3-D Analysis of Pinhole Size Optimization for a Confocal Signal-based Wavefront Sensor Frontiers in Optics Jewel, M. R., Akondi, V., Vohnsen, B. 2014
View details for DOI 10.1364/FIO.2014.JW3A.40
Multi-faceted digital pyramid wavefront sensor Optics Communications Akondi, V., Castillo, S., Vohnsen, B. 2014; 323: 77-86
View details for DOI 10.1016/j.optcom.2014.03.004
Digital phase-shifting point diffraction interferometer Optics Letters Akondi, V., Jewel, A. R., Vohnsen, B. 2014; 39 (6): 1641-1644
View details for DOI 10.1364/OL.39.001641
Myopic aberrations: Simulation based comparison of curvature and Hartmann Shack wavefront sensors Optics Communications Basavaraju, R. M., Akondi, V., Weddell, S. J., Budihal, R. P. 2014; 312: 23-30
View details for DOI 10.1016/j.optcom.2013.09.004
A direct comparison between a MEMS deformable mirror and a liquid crystal spatial light modulator in signal-based wavefront sensing Journal of the European Optical Society-Rapid publications Jewel, A. R., Akondi, V., Vohnsen, B. 2013; 8: 13073-1 - 13073-10
View details for DOI 10.2971/jeos.2013.13073
Digital pyramid wavefront sensor Adaptive Optics: Methods, Analysis and Applications Akondi, V., Castillo, S., Jewel, M. R., Vohnsen, B. 2013
View details for DOI 10.1364/AOPT.2013.OM2A.3
On pinhole size optimization in wavefront sensorless adaptive optics Adaptive Optics: Methods, Analysis and Applications Jewel, M. R., Akondi, V., Vohnsen, B. 2013
View details for DOI 10.1364/AOPT.2013.OTu1A.3
A review of atmospheric wind speed measurement techniques with Shack Hartmann wavefront imaging sensor in adaptive optics Journal of the Indian Institute of Science Mysore Basavaraju, R., Akondi, V., Budihal, R. 2013; 93 (1): 67-84
Digital pyramid wavefront sensor with tunable modulation Optics Express Akondi, V., Castillo, S., Vohnsen, B. 2013; 21 (15): 18261-18272
View details for DOI 10.1364/OE.21.018261
Myopic aberrations: impact of centroiding noise in Hartmann Shack wavefront sensing Ophthalmic and Physiological Optics Akondi, V., Vohnsen, B. 2013; 33 (4): 434-443
View details for DOI 10.1111/opo.12076
X-ray attenuation coefficient of mixtures: Inputs for dual-energy CT Medical Physics Haghighi, R. R., Chatterjee, S., Akondi, V., Kumar, P., Thulkar, S. 2011; 38 (10): 5270?5279
View details for DOI 10.1118/1.3626572
Multi-dither Shack Hartmann sensor for large telescopes: A numerical performance evaluation Adaptive Optics: Methods, Analysis and Applications Akondi, V., Basavaraju, R. M., Budihal, R. 2011
View details for DOI 10.1364/AOPT.2011.ATuA4
Towards Low Cost turbulence generator for AO testing: Utility, control and stability Adaptive Optics: Methods, Analysis and Applications Basavaraju, R. M., Akondi, V., Krishnan, A., Ram, S., Shankar Sai, S., Budihal, R. 2011
View details for DOI 10.1364/AOPT.2011.JMB5
Grid size optimization for atmospheric turbulence phase screen simulations Adaptive Optics: Methods, Analysis and Applications Basavaraju, R. M., Akondi, V., Budihal, R. 2011
View details for DOI 10.1364/AOPT.2011.JMB3
Automated ROI selection and calibration of a microlens array using a MEMS CDM Adaptive Optics: Methods, Analysis and Applications Basavaraju, R. M., Akondi, V., Budihal, R. 2011
View details for DOI 10.1364/AOPT.2011.ATuA5
Evaluation of the performance of centroiding algorithms with varying spot size : case of WFS calibration for the TMT NFIRAOS Adaptive Optics: Methods, Analysis and Applications Akondi, V., Ellerbroek, B. L., Basavaraju, R. M., Anderson, D. R., Budihal, R. 2011
View details for DOI 10.1364/AOPT.2011.ATuA1
Intensity Weighted Noise Reduction in MEMS Based Deformable Mirror Images Vyas, A., Roopshree, M. B., Prasad, B., Predeep, P., Thakur, M., Varma, M. K. AMER INST PHYSICS. 2011
View details for DOI 10.1063/1.3643545

View details for Web of Science ID 000302007600102
Experimental evaluation of centroiding algorithms at different light intensity and noise levels Roopashree, M. B., Vyas, A., Prasad, B., Predeep, P., Thakur, M., Varma, M. K. AMER INST PHYSICS. 2011
View details for DOI 10.1063/1.3643533

View details for Web of Science ID 000302007600090
Influence Function Measurement of Continuous Membrane Deformable Mirror Actuators Using Shack Hartmann Sensor Roopashree, M. B., Vyas, A., Prasad, B., Predeep, P., Thakur, M., Varma, M. K. AMER INST PHYSICS. 2011
View details for DOI 10.1063/1.3643577

View details for Web of Science ID 000302007600134
Noise reduction in the centroiding of laser guide star spot pattern using thresholded Zernike reconstructor Vyas, A., Roopashree, M. B., Prasad, B., Ellerbroek, B. L., Hart, M., Hubin, N., Wizinowich, P. L. SPIE-INT SOC OPTICAL ENGINEERING. 2010
View details for DOI 10.1117/12.856640

View details for Web of Science ID 000285506400148
Dither based sensor for improved consistency of adaptive optics system Vyas, A., Roopashree, M. B., Prasad, B., AtadEttedgui, E., Lemke, D. SPIE-INT SOC OPTICAL ENGINEERING. 2010
View details for DOI 10.1117/12.856636

View details for Web of Science ID 000285833700073
Multilayered temporally evolving phase screens based on statistical interpolation Roopashree, M. B., Vyas, A., Prasad, B., Ellerbroek, B. L., Hart, M., Hubin, N., Wizinowich, P. L. SPIE-INT SOC OPTICAL ENGINEERING. 2010
View details for DOI 10.1117/12.856814

View details for Web of Science ID 000285506400133
Real-time wind speed measurement using wavefront sensor data Roopashree, M. B., Vyas, A., Prasad, B., Korotkova, O. SPIE-INT SOC OPTICAL ENGINEERING. 2010
View details for DOI 10.1117/12.841333

View details for Web of Science ID 000284396600009
Improved Iteratively Weighted Centroiding for accurate spot detection in Laser Guide Star based Shack Hartmann Sensor Vyas, A., Roopashree, M. B., Prasad, B., Korotkova, O. SPIE-INT SOC OPTICAL ENGINEERING. 2010
View details for DOI 10.1117/12.841331

View details for Web of Science ID 000284396600005
Optimizing the modal index of Zernike polynomials for regulated phase screen simulation Vyas, A., Roopashree, M. B., Prasad, B., Ellerbroek, B. L., Hart, M., Hubin, N., Wizinowich, P. L. SPIE-INT SOC OPTICAL ENGINEERING. 2010
View details for DOI 10.1117/12.856816

View details for Web of Science ID 000285506400134
Performance of Centroiding Algorithms at Low Light Level Conditions in Adaptive Optics Vyas, A., Roopashree, M. B., Prasad, B. R., Vyas, A., IEEE IEEE. 2009: 366-+
View details for DOI 10.1109/ARTCom.2009.30

View details for Web of Science ID 000288361500088
Determination of wavelength of laser using modified Newton?s rings setup Physics Education Jha, S., Akondi, V., Sastri, O., Jain, R., Umesh, K. 2005; 22 (3): 195-202

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