Levent Onural

Bilkent University, Ankara, Ankara, Turkey

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Publications (116)132.69 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Three approaches for visualization of transparent micro-objects from holographic data using phase-only SLMs are described. The objects are silicon micro-lenses captured in the near infrared by means of digital holographic microscopy and a simulated weakly refracting 3D object with size in the micrometer range. In the first method, profilometric/tomographic data are retrieved from captured holograms and converted into a 3D point cloud which allows for computer generation of multi-view phase holograms using Rayleigh-Sommerfeld formulation. In the second method, the microlens is computationally placed in front of a textured object to simulate the image of the textured data as seen through the lens. In the third method, direct optical reconstruction of the micrometer object through a digital lens by modifying the phase with the Gerchberg-Saxton algorithm is achieved.
    Optics Express 11/2013; 21(23):28246-57. · 3.55 Impact Factor
  • Erdem Sahin, Levent Onural
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    ABSTRACT: We present a local Gaussian beam decomposition method for calculating the scalar diffraction field due to a two-dimensional field specified on a curved surface. We write the three-dimensional field as a sum of Gaussian beams that propagate toward different directions and whose waist positions are taken at discrete points on the curved surface. The discrete positions of the beam waists are obtained by sampling the curved surface such that transversal components of the positions form a regular grid. The modulated Gaussian window functions corresponding to Gaussian beams are placed on the transversal planes that pass through the discrete beam-waist position. The coefficients of the Gaussian beams are found by solving the linear system of equations where the columns of the system matrix represent the field patterns that the Gaussian beams produce on the given curved surface. As a result of using local beams in the expansion, we end up with sparse system matrices. The sparsity of the system matrices provides important advantages in terms of computational complexity and memory allocation while solving the system of linear equations.
    Journal of the Optical Society of America A 03/2013; 30(3):527-36. · 1.67 Impact Factor
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    Ali Özgür Yöntem, Levent Onural
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    ABSTRACT: We propose a method and present applications of this method that converts a diffraction pattern into an elemental image set in order to display them on an integral imaging based display setup. We generate elemental images based on diffraction calculations as an alternative to commonly used ray tracing methods. Ray tracing methods do not accommodate the interference and diffraction phenomena. Our proposed method enables us to obtain elemental images from a holographic recording of a 3D object/scene. The diffraction pattern can be either numerically generated data or digitally acquired optical data. The method shows the connection between a hologram (diffraction pattern) and an elemental image set of the same 3D object. We showed three examples, one of which is the digitally captured optical diffraction tomography data of an epithelium cell. We obtained optical reconstructions with our integral imaging display setup where we used a digital lenslet array. We also obtained numerical reconstructions, again by using the diffraction calculations, for comparison. The digital and optical reconstruction results are in good agreement.
    Optics Express 10/2012; 20(22):24175-95. · 3.55 Impact Factor
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    IEEE Journal of Selected Topics in Signal Processing 09/2012; 6(5):409-410. · 3.30 Impact Factor
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    ABSTRACT: A circular holographic display that consists of phase-only spatial light modulators is used to reconstruct images in visible light from digital holograms recorded under infrared (10.6 μm) illumination. The reconstruction yields a holographic digital video display of a three-dimensional ghostlike image of an object floating in space where observers can move and rotate around it.
    Optics Letters 08/2012; 37(15):3120-2. · 3.39 Impact Factor
  • Erdem Şahin, Levent Onural
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    ABSTRACT: We introduce a local signal decomposition method for the analysis of three-dimensional (3D) diffraction fields involving curved surfaces. We decompose a given field on a two-dimensional curved surface into a sum of properly shifted and modulated Gaussian-shaped elementary signals. Then we write the 3D diffraction field as a sum of Gaussian beams, each of which corresponds to a modulated Gaussian window function on the curved surface. The Gaussian beams are propagated according to a derived approximate expression that is based on the Rayleigh-Sommerfeld diffraction model. We assume that the given curved surface is smooth enough that the Gaussian window functions on it can be treated as written on planar patches. For the surfaces that satisfy this assumption, the simulation results show that the proposed method produces quite accurate 3D field solutions.
    Journal of the Optical Society of America A 07/2012; 29(7):1459-69. · 1.67 Impact Factor
  • G.B. Esmer, H.M. Ozaktas, L. Onural
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    ABSTRACT: Computer generated holography is one of common methods to obtain three-dimensional visualization. It can be explained by behavior of propagating waves and interference. To calculate the scalar diffraction pattern on a hologram, there are myriad of algorithms in the literature. Some of them employ several approximations, so the calculated fields may not be the exact scalar diffraction field. However, there are algorithms to compute the exact scalar diffraction field with some limitations on the distribution of the given samples over the space. These algorithms are based on “field model” approach. The performance of an algorithm, based on field model, is investigated according to the distribution of given samples over the space. From the simulations, it was observed that the cumulative information provided by the given samples has to be enough to solve the inverse scalar diffraction field. The cumulative information can be increased by having more samples, but there are some scenarios that differential information obtained from the given samples can be infinitesimal, thus the exact diffraction field may not be computed.
    Signal Processing and Communications Applications Conference (SIU), 2012 20th; 01/2012
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    ABSTRACT: Calculation of the scalar diffraction field over the entire space from a given field over a surface is an important problem in computer generated holography. A straightforward approach to compute the diffraction field from field samples given on a surface is to superpose the emanated fields from each such sample. In this approach, possible mutual interactions between the fields at these samples are omitted and the calculated field may be significantly in error. In the proposed diffraction calculation algorithm, mutual interactions are taken into consideration, and thus the exact diffraction field can be calculated. The algorithm is based on posing the problem as the inverse of a problem whose formulation is straightforward. The problem is then solved by a signal decomposition approach. The computational cost of the proposed method is high, but it yields the exact scalar diffraction field over the entire space from the data on a surface.
    Optics Communications 12/2011; 284(24):5537-5548. · 1.44 Impact Factor
  • Ali Özgür Yöntem, Levent Onural
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    ABSTRACT: We present a digital integral imaging system. A Fresnel lenslet array pattern is written on a phase-only LCoS spatial light modulator device (SLM) to replace the regular analog lenslet array in a conventional integral imaging system. We theoretically analyze the capture part of the proposed system based on Fresnel wave propagation formulation. Because of pixelation and quantization of the lenslet array pattern, higher diffraction orders and multiple focal points emerge. Because of the multiple focal planes introduced by the discrete lenslets, multiple image planes are observed. The use of discrete lenslet arrays also causes some other artifacts on the recorded elemental images. The results reduce to those available in the literature when the effects introduced by the discrete nature of the lenslets are omitted. We performed simulations of the capture part. It is possible to obtain the elemental images with an acceptable visual quality. We also constructed an optical integral imaging system with both capture and display parts using the proposed discrete Fresnel lenslet array written on a SLM. Optical results when self-luminous objects, such as an LED array, are used indicate that the proposed system yields satisfactory results.
    Journal of the Optical Society of America A 11/2011; 28(11):2359-75. · 1.67 Impact Factor
  • Erdem Ulusoy, Levent Onural, Haldun M Ozaktas
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    ABSTRACT: Imperfections and nonrobust behavior of practical multilevel spatial light modulators (SLMs) degrade the performance of many proposed full-complex amplitude modulation schemes. We consider the use of more robust binary SLMs for this purpose. We propose a generic method, by which, out of K binary (or 1 bit) SLMs of size M×N, we effectively create a new 2(K)-level (or K bit) SLM of size M×N. The method is a generalization of the well-known concepts of bit plane representation and decomposition for ordinary gray scale digital images and relies on forming a properly weighted superposition of binary SLMs. When K is sufficiently large, the effective SLM can be regarded as a full-complex one. Our method is as efficient as possible from an information theoretical perspective. A 4f system is discussed as a possible optical implementation. This 4f system also provides a means for eliminating the undesirable higher diffraction orders. The components of the 4f system can easily be customized for different production technologies.
    Journal of the Optical Society of America A 11/2011; 28(11):2310-21. · 1.67 Impact Factor
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    F. Yaras, Hoonjong Kang, L. Onural
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    ABSTRACT: The designed circular holographic display system produces ghost-like 3D optical reconstructions of a computer generated 3D model. System uses six phase-only reflective-type spatial light modulators (SLMs) that are configured circularly. Alignment of the SLMs are successful and gap problem is solved by using half-mirrors. The total number of pixels of the resultant display is 11520 × 1080. Reconstructions show that increase in the viewing angle is significant compared to the single SLM case. With the help of the proposed system, observer can see the reconstructions binocularly. As a result, comfortable 3D perception is achieved. In order to avoid eye-hazard, LED illumination is also used as an alternative light source. Experimental results are satisfactory. Proposed system can be used as a holographic display system.
    3DTV Conference: The True Vision - Capture, Transmission and Display of 3D Video (3DTV-CON), 2011; 06/2011
  • Erdem Ulusoy, Levent Onural, Haldun M Ozaktas
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    ABSTRACT: Computation of a binary spatial light modulator (SLM) pattern that generates a desired light field is a challenging quantization problem for which several algorithms have been proposed, mainly for far-field or Fourier plane reconstructions. We study this problem assuming that the desired light field is synthesized within a volumetric region in the non-far-field range after free space propagation from the SLM plane. We use Fresnel and Rayleigh-Sommerfeld scalar diffraction theories for propagation of light. We show that, when the desired field is confined to a sufficiently narrow region of space, the ideal gray-level complex-valued SLM pattern generating it becomes sufficiently low pass (oversampled) so it can be successfully halftoned into a binary SLM pattern by solving two decoupled real-valued constrained halftoning problems. Our simulation results indicate that, when the synthesis region is considered, the binary SLM is indistinguishable from a lower resolution full complex gray-level SLM. In our approach, free space propagation related computations are done only once at the beginning, and the rest of the computation time is spent on carrying out standard image halftoning.
    Journal of the Optical Society of America A 06/2011; 28(6):1211-23. · 1.67 Impact Factor
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    Fahri Yaraş, Hoonjong Kang, Levent Onural
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    ABSTRACT: A circular holographic video display system reconstructs holographic video. Phase-only spatial light modulators are tiled in a circular configuration in order to increase the field of view. A beam-splitter is used to align the active area of the SLMs side by side without any gap. With the help of this configuration observers can see 3D ghost-like image floating in space and can move and rotate around the object. The 3D reconstructions can be observed binocularly. Experimental results are satisfactory.
    Optics Express 05/2011; 19(10):9147-56. · 3.55 Impact Factor
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    ABSTRACT: The European-funded project "Real 3D" brings together nine participants from academia and industry and continues a long-term effort to facilitate the greater presence of digital holography in the three-dimensional capture and display markets.
    Digital Holography and Three-Dimensional Imaging; 05/2011
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    L. Onural, F. Yaraş, Hoonjong Kang
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    ABSTRACT: Holography aims to record and regenerate volume filling light fields to reproduce ghost-like 3-D images that are optically indistinguishable from their physical 3-D originals. Digital holographic video displays are pixelated devices on which digital holograms can be written at video rates. Spatial light modulators (SLMs) are used for such purposes in practice; even though it is desirable to have SLMs that can modulate both the phase and amplitude of the incident light at each pixel, usually amplitude-only or phase-only SLMs are available. Many laboratories have reported working prototypes using different designs. Size and resolution of the SLMs are quite demanding for satisfactory 3-D reconstructions. Space-bandwidth product (SBP) seems like a good metric for quality analysis. Even though moderate SBP is satisfactory for a stationary observer with no lateral or rotational motion, the required SBP quickly increases when such motion is allowed. Multi-SLM designs, especially over curved surfaces, relieve high bandwidth requirements, and therefore, are strong candidates for futuristic holographic video displays. Holograms are quite robust to noise and quantization. It is demonstrated that either laser or light-emitting diode (LED) illumination is feasible. Current research momentum is increasing with many exciting and encouraging results.
    Proceedings of the IEEE 05/2011; · 6.91 Impact Factor
  • Levent Onural
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    ABSTRACT: The diffraction relation between a plane and another plane that is both tilted and translated with respect to the first one is revisited. The derivation of the result becomes easier when the impulse function over a surface is used as a tool. Such an approach converts the original 2D problem to an intermediate 3D problem and thus allows utilization of easy-to-interpret Fourier transform properties due to rotation and translation. An exact solution for the scalar monochromatic propagating waves case when the propagation direction is restricted to be in the forward direction is presented.
    Journal of the Optical Society of America A 03/2011; 28(3):290-5. · 1.67 Impact Factor
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    ABSTRACT: This special issue provides an overview of recent advances and discusses future directions in diverse 3-D media technologies ranging from current stereoscopy to ultrarealistic holographic video, and transmission over the current and future, wired and wireless Internet.
    Proceedings of the IEEE 01/2011; 99(4):536-539. · 6.91 Impact Factor
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    ABSTRACT: Infrared digital holograms of different statuettes are acquired. For each object, a sequence of holograms is recorded rotating the statuette with an angular step of few degrees. The holograms of the moving objects are used to compose dynamic 3D scenes that, then, are optically reconstructed by means of spatial light modulators (SLMs) using an illumination wavelength of 532 nm. This kind of reconstruction allows to obtain a D imaging of the statuettes that could be exploited for virtual museums.
    Proc SPIE 01/2011; 8084.
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    F. Yaraş, Hoonjong Kang, L. Onural
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    ABSTRACT: True-3D imaging and display systems are based on physical duplication of light distribution. Holography is a true-3D technique. There are significant developments in electro-holographic displays in recent years. Liquid crystal, liquid crystal on silicon, optically addressed, mirror-based, holographic polymer-dispersed, and acousto-optic devices are used as holographic displays. There are complete electro-holographic display systems and some of them are already commercialized.
    Journal of Display Technology 11/2010; · 1.66 Impact Factor
  • L. Onural
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    ABSTRACT: Frequently, the most important sporting events are used as platforms to showcase and sometimes launch the latest communication technologies. This article presents an introduction to the technological issues facing a broad deployment of 3DTV systems and discusses some of the signal processing techniques that are used or need to be developed in this area.
    IEEE Signal Processing Magazine 10/2010; · 3.37 Impact Factor

Publication Stats

1k Citations
132.69 Total Impact Points

Institutions

  • 1989–2012
    • Bilkent University
      • Department of Electrical & Electronic Engineering
      Ankara, Ankara, Turkey
  • 2009
    • University of West Bohemia
      • Department of Computer Science and Engineering
      Plzeň, Plzensky kraj, Czech Republic
  • 2007
    • Tampere University of Technology
      Tammerfors, Province of Western Finland, Finland