Browsing by Author "Ivanov B."

Now showing 1 - 9 of 9
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    Basic holographic characteristics of a panchromatic light sensitive material for reflective autostereoscopic 3D display
    (2009-02-16) Sainov V.; Petrova T.; Ivanov B.; Zdravkov K.; Nazarova D.; Stoykova E.; Minchev G.
    Basic holographic characteristics of a newly developed panchromatic ultrafine grain silver halide light sensitive material for RGB recording of reflective holographic screen for autostereoscopic 3D display are presented. The average grain size is less than 10nm which ensures high resolution, diffraction efficiency, and signal-to-noise ratio (more than 100:1) in a large dynamic range for RGB reflective holographic recording. The decrease of the diffraction efficiency in recording of scattering objects is less than 30 from the maximal values for specular reflection. The analysis of color recording of the reflective holographic screen with one viewing zone is presented on the basis of the so-called sandwich structure built of two layers for multiple holographic recording in blue, green, and red spectral regions.
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    Dynamic speckle analysis with smoothed intensity-based activity maps
    (2017-06-01) Stoykova E.; Berberova N.; Kim Y.; Nazarova D.; Ivanov B.; Gotchev A.; Hong J.; Kang H.
    Pointwise intensity-based algorithms are the most popular algorithms in dynamic laser speckle measurement of physical or biological activity. The output of this measurement is a two-dimensional map which qualitatively separates regions of higher or lower activity. In the paper, we have proposed filtering of activity maps to enhance visualization and to enable quantitative determination of activity time scales. As a first step, we have proved that the severe spatial fluctuations within the map resemble a signal-dependent noise. As a second step, we have illustrated implementation of the proposed idea by applying filters to non-normalized and normalized activity estimates derived from synthetic and experimental data. Statistical behavior of the estimates has been analyzed to choose the filter parameters, and substantial narrowing of the probability density functions of the estimates has been achieved after the filtering. The filtered maps exhibit an improved contrast and allowed for quantitative description of activity.
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    Dynamic speckle imaging with SVD compression
    (2022-01-01) Stoykova E.; Levchenko M.; Ivanov B.; Madjarova V.; Nazarova D.; Nedelchev L.; Machikhin A.; Park J.
    Dynamic speckle imaging (DSI) of areas with different speed of processes ongoing in industrial or biological objects relies on statistical processing of a large number of images of the speckle patterns formed on the objects surface under laser illumination. The DSI visualizes the speed spatial distribution as an activity map. We propose compression of the raw DSI data by applying singular value decomposition (SVD). A specific feature of speckle images for DSI is lack of a structure with areas of close intensity values. The gain from the direct SVD application may be modest in cases when a great number of non-zero singular values is needed to build an activity map comparable in quality to the ground truth map from bitmap images. For higher compression, we propose SVD to be applied to the 2D arrays containing the differences between the successive images. The SVD compression has been verified by using synthetic and experimental data.
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    Intensity-based dynamic speckle method for analysis of variable-rate dynamic events
    (2023-01-01) Stoykova E.; Nedelchev L.; Blagoeva B.; Ivanov B.; Levchenko M.; Berberova-Buhova N.; Nazarova D.
    We study efficiency of intensity-based dynamic speckle method for characterization of dynamic events which occur at variable rate in time within the temporal averaging interval. We checked the ability of the method to describe the speed of evolution by i) numerical simulation at variable speed, ii) processing of speckle patterns obtained from phase distributions fed to a SLM at controllable change of the temporal correlation radius of speckle intensity fluctuations and iii) conducting experiments with a polymer solution drying by using a hot-stage. The numerical and SLM simulation experiments allowed for modification of the used estimates in order to obtain relevant information.
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    Noise analysis in outdoor dynamic speckle measurement
    (2023-04-01) Levchenko M.; Stoykova E.; Ivanov B.; Nedelchev L.; Nazarova D.; Choi K.; Park J.
    The dynamic speckle method (DSM) is an effective tool for the estimation of speed of processes. The speed distribution is encoded in a map built by statistical pointwise processing of time-correlated speckle patterns. For industrial inspection, the outdoor noisy measurement is required. The paper analyzes the efficiency of the DSM in the presence of environmental noise as phase fluctuations due to the lack of vibration isolation and shot noise due to ambient light. The usage of normalized estimates for the case of non-uniform laser illumination is studied. The feasibility of the outdoor measurement has been proven by numerical simulations of noisy image capture and real experiments with test objects. Good agreement has been demonstrated in both the simulation and experiment between the ground truth map and the maps extracted from noisy data.
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    Normalization in dynamic speckle analysis for non-destructive monitoring of speed of processes
    (2021-12-17) Stoykova E.; Nazarova D.; Nedelchev L.; Levchenko M.; Berberova-Buhova N.; Ivanov B.
    The paper is dedicated to analysis of normalized intensity-based pointwise algorithms for processing dynamic speckle images with spatially varying speckle statistics in non-destructive visualization of regions of faster or slower changes across an object. Both existing and newly proposed algorithms are analyzed. Extraction of speed of changes is done by acquiring correlated in time speckle images formed on the object surface under laser illumination. The studied algorithms have been applied to simulated low and high contrast speckle data. Their performance has been compared to processing of binary patterns as another approach for dealing with varying speckle statistics in the acquired images. The efficiency of the algorithms have been checked on the experimental data, including data in a compressed format. We have proven that the algorithms with normalization at successive instants by a sum of two intensities or a single intensity outperform as a whole the algorithms which apply the time-averaged estimates of the mean value and the variance of speckle intensity.
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    Polarization Diffraction Gratings in PAZO Polymer Thin Films Recorded with Digital Polarization Holography: Polarization Properties and Surface Relief Formation
    (2024-05-01) Berberova-Buhova N.; Nedelchev L.; Mateev G.; Nikolova L.; Stoykova E.; Ivanov B.; Strijkova V.; Hong K.; Nazarova D.
    In this work, we study the polarization properties of diffraction gratings recorded in thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt]) using digital polarization holography. Using two quarter-wave plates, the phase retardation of each pixel of the SLM is converted into the azimuth rotation of linearly polarized light. When recording from the azopolymer side of the sample, significant surface relief amplitude is observed with atomic force microscopy. In contrast, recording from the substrate side of the sample allows the reduction of the surface relief modulation and the obtaining of polarization gratings with characteristics close to an ideal grating, recorded with two orthogonal circular polarizations. This can be achieved even with a four-pixel period of grating, as demonstrated by our results.
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    Polarization properties of two-dimensional polarization holographic gratings inscribed in azopolymer thin films
    (2023-01-01) Mateev G.; Nedelchev L.; Nazarova D.; Nikolova L.; Ivanov B.; Strijkova V.; Stoykova E.
    Azopolymers are efficient photoanisotropic materials and for this reason are commonly used for inscription of polarization-selective diffraction gratings by polarization holography. More often however, the characteristics of one-dimensional (1D) gratings are studied. Here, we present real time diffraction efficiency kinetics and polarization properties of two-dimensional (2D) polarization holographic gratings, each of them obtained by overlapping two perpendicular 1D gratings. The gratings are consecutively inscribed using as recording medium the azopolymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt], or PAZO. To record the gratings, He-Cd laser (λ = 442 nm) was used with left and right circular polarizations (LCP and RCP) of the recording beams. Our results show that the volume polarization grating has lower influence on the polarization properties of the 2D grating than the surface relief scalar grating.
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    Two-Dimensional Polarization Holographic Gratings in Azopolymer Thin Films: Polarization Properties in the Presence or Absence of Surface Relief
    (2023-07-01) Mateev G.; Nedelchev L.; Nikolova L.; Ivanov B.; Strijkova V.; Stoykova E.; Choi K.; Park J.; Nazarova D.
    During polarization holographic recording in azopolymer thin films, usually together with the volume anisotropic grating, a surface relief grating (SRG) is also formed. By using two consecutive exposures, it is possible to obtain a two-dimensional (2D) grating. To the best of our knowledge, the polarization properties of such gratings have not been studied yet. To determine the influence of the surface relief on the polarization selectivity of the 2D gratings, we propose two methods to suppress the SRG formation: by varying the recording conditions or varying the sample structure. In these experiments we have used the commercially available azopolymer PAZO, poly[1-4-(3-carboxy-4-hydrophenylazo) benzene sulfonamido]-1,2-ethanediyl, sodium salt] to perform the polarization holographic recording using a 442 nm He-Cd laser. As indicated by our results, when the surface relief is present, it strongly dominates the response of the 2D grating and it behaves almost as a scalar polarization insensitive grating. Conversely, when the SRG formation is suppressed, the polarization properties of the 2D grating in all four diffracted orders are very well pronounced. In this way, we demonstrate that we can easily control SRG formation and, if desired, obtain 2D grating with high surface relief modulation, or alternatively record polarization-selective 2D gratings with virtually no surface relief.