Branimir Ivanov

Correlation-based pointwise processing of dynamic speckle patterns.

Correlation-based pointwise processing of dynamic speckle patterns is proposed for spatial characterization of activity in a sample. The result is a set of 2D activity maps of the estimates of temporal correlation, or structure functions, at increasing time lags. Pointwise computation provides spatial resolution, limited by the pixel period of the optical sensor used for acquisition of the speckle patterns. Pointwise normalization of the estimates solves the problem with the nonuniform illumination and varying reflectivity across the sample. The high contrast detailed activity maps obtained from processing of synthetic and experimental speckle patterns confirms efficiency of the proposed approach.

Monitoring of bread cooling by statistical analysis of laser speckle patterns

The phenomenon of laser speckle can be used for detection and visualization of physical or biological activity in various objects (e.g. fruits, seeds, coatings) through statistical description of speckle dynamics. The paper presents the results of non-destructive monitoring of bread cooling by co-occurrence matrix and temporal structure function analysis of speckle patterns which have been recorded continuously within a few days. In total, 72960 and 39680 images were recorded and processed for two similar bread samples respectively. The experiments proved the expected steep decrease of activity related to the processes in the bread samples during the first several hours and revealed its oscillating character within the next few days. Characterization of activity over the bread sample surface was also obtained.

Three-dimensional imaging of cultural heritage artifacts with holographic printers

Holography is defined as a two-steps process of capture and reconstruction of the light wavefront scattered from three-dimensional (3D) objects. Capture of the wavefront is possible due to encoding of both amplitude and phase in the hologram as a result of interference of the light beam coming from the object and mutually coherent reference beam. Three-dimensional imaging provided by holography motivates development of digital holographic imaging methods based on computer generation of holograms as a holographic display or a holographic printer. The holographic printing technique relies on combining digital 3D object representation and encoding of the holographic data with recording of analog white light viewable reflection holograms. The paper considers 3D contents generation for a holographic stereogram printer and a wavefront printer as a means of analogue recording of specific artifacts which are complicated objects with regards to conventional analog holography restrictions.

SLM-based optical simulator for dynamic speckle analysis

The phenomenon of dynamic speckle allows for non-invasive whole-field detection of physical or biological activity in objects through statistical description of laser speckle dynamics. Effective way to improve the statistical analysis is generation of controlled speckle patterns. SLM implementation of an optical simulator of dynamic speckle patterns is proposed by feeding a correlated sequence of 2D random phase distributions to the phase-only SLM. Atthevarying in space correlation radius of the phase fluctuations in the successive frames, the SLM produces regions of different activity on a screen under laser illumination. Feasibility of the proposed approach is proved both by simulation and experiment.

Pointwise implementation of dynamic laser speckle technique

The paper proposes pointwise implementation of correlation-based algorithms for processing time sequences of 2D dynamic speckle patterns of a diffuse object illuminated with a laser light in order to obtain a spatial map of physical or biological activity across its surface. Both temporal correlation and structure functions were studied with and without the pointwise normalization by the variance of intensity fluctuations in time. The performance of the algorithms was checked by simulation of dynamic speckle patterns at a given spatial distribution of the temporal correlation radius of activity and by dynamic speckle measurement of a test object. The obtained set of high contrast and high spatial resolution activity maps proves the efficiency of the normalized correlation-based algorithms to indicate regions of varying activity even at non-uniform illumination and reflectivity across the object.

SLM-Based Fringe Projection Profilometry under Coherent Illumination

The phase-measuring profilometry with projection of pure sinusoidal fringes is known for its high accuracy of 3D object capture that is achieved by comparatively simple means [1]. It offers a number of key advantages when fringe projection is done by a spatial light modulator (SLM) due to software manipulation of fringes [2]. A lot of results have been reported for SLM fringe projection under incoherent illumination by using liquid crystal display or digital light processing technology. The main deficiencies of this type of projection are the discrete-like spatial intensity distribution and the gamma-distortion worsening the spectral content of the fringes [3]. The focus of this report is on the usage of SLMs for coherent projection of the sinusoidal fringes with phase encoding of the sinusoidal function.

Full-field stress analysis by holographic phase-stepping implementation of the photoelastic-coating method

In this paper, we describe a system for polariscopic and holographic phase-shifting implementation of the photoelastic-coating method for a full-field stress analysis. The easiest way to build the combined system is to employ a laser light source. However, coherent illumination introduces a signal-dependent speckle noise which worsens the accurate phase estimation and unwrapping. To answer the question of how it affects the phase retrieval of isochromatics, isoclinics and isopachics, we modeled in the present paper the phase-shifting photoelastic measurement in the presence of speckle noise through the calculation of the complex amplitudes in a Mach–Zender interferometer combined with a circular polariscope and made denoising of simulated and experimental fringe patterns. The latter were recorded at pure tensile load for PhotoStress®-coated samples with a mechanical stress concentrator.

Pointwise intensity-based dynamic speckle analysis with binary patterns

Non-destructive detection of physical or biological activity through statistical processing of speckle patterns on the surface of diffusely reflecting objects is an area of active research. A lot of pointwise intensity-based algorithms have been proposed over the recent years. Efficiency of these algorithms is deteriorated by the signal-dependent speckle data, non-uniform illumination or varying reflectivity across the object, especially when the number of the acquired speckle patterns is limited. Pointwise processing of a sequence of 2D images is also time-consuming. In this paper, we propose to transform the acquired speckle images into binary patterns by using for a sign threshold the mean intensity value estimated at each spatial point from the temporal sequence of intensities at this point. Activity is characterized by the 2D distribution of a temporal polar correlation function estimated at a given time lag from the binary patterns. Processing of synthetic and experimental data confirmed that the algorithm provided correct activity determination with the same accuracy as the temporal normalized correlation function. It is efficient without the necessity to apply normalization at non-uniform distribution of intensity in the illuminating laser beam and offers acceleration of computation.

Dynamic laser speckle metrology with binarization of speckle patterns

Dynamic laser speckle analysis is non-destructive detection of physical or biological activity through statistical processing of speckle patterns on the surface of diffusely reflecting objects. This method is sensitive to microscopic changes of the surface over time and needs simple optical means. Advances in computers and 2D optical sensors forced development of pointwise algorithms. They rely on acquisition of a temporal sequence of correlated speckle images and generate activity data as a 2D spatial contour map of the estimate of a given statistical parameter. The most widely used pointwise estimates are the intensity-based estimates which compose each map entry from a time sequence of intensity values taken at one and the same pixel in the acquired speckle images. Accuracy of the pointwise approach is strongly affected by the signal-dependent nature of the speckle data when the spread of intensity fluctuations depends on the intensity itself. The latter leads to erroneous activity determination at non-uniform distribution of intensity in the laser beam for the non-normalized estimates. Normalization of the estimates, introduces errors. We propose to apply binarization to the acquired speckle images by comparing the intensity values in the temporal sequence for a given spatial point to the mean intensity value estimated for this point and to evaluate a polar correlation function. Efficiency of this new processing algorithm is checked both by simulation and experiment.

Using LED Illumination in Fringe Projection Profilometry with a Sinusoidal Phase Grating

Fringe projection profilometry is a well-established method in optical metrology for capture of 3D objects [1]. The information is retrieved from the phase of the deformed fringe pattern formed by imaging the object onto a CCD camera after projection of a structured light pattern onto its surface. A variety of phase retrieval algorithms requires projection of sinusoidal fringes. Thin diffraction sinusoidal gratings under coherent illumination are a suitable choice for generation of good quality fringes focused in a large measurement volume [2]. The main drawback of coherent projection is the speckle noise. The lateral spacing of fringes produced by coherently illuminated sinusoidal grating does not depend on the wavelength. Thus, a light source with a wider spectrum can be used for reduction of the speckle noise in the fringe patterns recorded by the CCD camera provided the spatial coherence of the source is kept intact [3]. The aim of the present report is to prove speckle reduction in fringes projected with a sinusoidal phase grating (SPG) by using low coherent point light source with emphasis on LED illumination.