Bio

Bio


My interest in optics research started during my undergraduate studies. I worked on better methods to detect the optical degradation in the images of ground-based astronomical telescopes due to atmospheric turbulence as part of my Ph.D. studies at the Indian Institute of Science. During my first postdoctoral position in University College Dublin, I developed digital methods using spatial light modulators for measuring the wave distortions in optical microscopy and vision science. Pyramid, point diffraction, Hartmann-Shack, and confocal signal-based wavefront sensors were explored. At the Institute of Optics in Madrid, as part of my second postdoctoral position, I worked on accurate optical quality evaluation techniques for patients implanted with multifocal intraocular lenses, and contributed to the development of mathematical methods and experimental validations to facilitate the demonstration of multifocal vision for prospective refractive surgery patients and contact lens wearers using a simultaneous vision simulator based on the tunable lens that works on the principle of temporal multiplexing.

Current Role at Stanford


We develop next-generation ocular imaging devices to allow non-invasive visualization of subcellular structures in the eye with the goal of building clinically useful tools that help in early disease diagnosis and monitoring.

Honors & Awards


  • Outstanding Reviewer Recognition, Optical Society of America (2016)
  • Robert S. Hilbert Memorial Student Travel Grant, Optical Society of America and Optical Research Associates (2011)

Education & Certifications


  • Ph.D., Indian Institute of Science (2012)
  • Master of Science, Sri Sathya Sai Institute of Higher Learning (2006)
  • Bachelor of Science, Sri Sathya Sai Institute of Higher Learning (2004)

Professional

Work Experience


  • Senior Post Doctoral Researcher, Instituto de Optica (IO-CSIC) (12/22/2014 - 2/13/2018)

    Location

    Madrid, Spain

  • Post Doctoral Researcher, University College Dublin (3/14/2012 - 8/31/2014)

    Location

    Dublin, Ireland

Professional Affiliations and Activities


  • Chair, Vision Technical Group, Optical Society of America (2019 - Present)
  • Committee Member, Optical Society of America (Optical Metrology Technical Group) (2014 - 2018)

Publications

All Publications


  • 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

  • Visual simulators replicate vision with multifocal lenses. Scientific reports Vinas, M., Benedi-Garcia, C., Aissati, S., Pascual, D., Akondi, V., Dorronsoro, C., Marcos, S. 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

  • 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

  • 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

  • Experimental validations of a tunable-lens-based visual demonstrator of multifocal corrections. Biomedical optics express Akondi, V., Sawides, L., Marrakchi, Y., Gambra, E., Marcos, S., Dorronsoro, C. 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

  • 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
  • Evaluation of the true wavefront aberrations in eyes implanted with a rotationally asymmetric multifocal intraocular lens Journal of Refractive Surgery Akondi, V., Pérez-Merino, P., Martinez-Enriquez, E., Dorronsoro, C., Alejandre, N., Jiménez-Alfaro, I., Marcos, S. 2017; 33 (4): 257-265
  • 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

  • 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

  • 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
  • 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
  • Multi-faceted digital pyramid wavefront sensor Optics Communications Akondi, V., Castillo, S., Vohnsen, B. 2014; 323: 77-86
  • 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
  • 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

  • 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