Dynamic Speckle Research Articles

Non-destructive speckle imaging of subsurface detail in paper-based cultural materials

We have used Laser Speckle Contrast Imaging (LSCI) to investigate the statistical properties of dynamic speckle reflected from an obscuring scattering surface in order to reveal drawings that are hidden beneath. Here we explore the limitations of this method used with various algorithms when applied to a selection of paper samples. These samples consist of a sketch executed in an assortment of media laid on a base surface that are then hidden beneath a subsequent top layer. The ability to resolve gray scale images was examined as well as the contrast surface temperature relationship. A book with glued pages was investigated in order to demonstrate the technique's applicability to the non-destructive examination of cultural materials. While being shown as a useful tool in revealing obscured drawings the scattering properties of the surface layer present a limitation in its general applicability.

Investigation of the stressed-strained state of cermet dentures using digital laser speckle-photographic analysis

It has been shown that the method of digital dynamic speckle photography is an efficient method of quantitative and qualitative diagnostics in orthopedic stomatology for optimization of the constructions of cermet dentures by evaluating their stressed-strained state. Determination of the stressed-strained state of cermet dentures by the method of digital laser speckle photography at the stages of their manufacture makes it possible to most reliably investigate the quality of manufactured constructions, which diminishes the number of complications and improves the efficiency of treatment in orthopedic stomatology,

Numerical model for simulation of dynamic speckle reference patterns

We propose a numerical model to describe dynamic speckle phenomena. It includes free space propagation, focused and defocused image situations and the effect of the illuminating wavefront phase. We test the model by verifying its statistics and using the results of Okamoto and Asakura for the pure translational, pure boiling and mixed cases of dynamic speckle obtained by moving a rigid diffuser; and also the results of Briers for a plane with a mix of static and moving scatterers. The difficulties associated with the discretization process of the Fresnel propagation formula are analyzed. The model was also compared with controlled experiments and the results suggest that it could be used as a reference to test the performance of activity measurement algorithms under controlled conditions. Three dynamic speckle activity descriptors were also evaluated and compared in boiling speckle simulations: Laser Speckle Contrast Analysis (LASCA), Generalized Differences and Energy of Spectral Bands.

Speckle dynamics for dual-beam optical illumination of a rotating structure

An out-of-plane rotating object is illuminated with two spatially separated coherent beams, giving rise to fully developed speckles, which will translate and gradually decorrelate in the observation plane, located in the far field. The speckle pattern is a compound structure, consisting of random speckles modulated by a smaller and repetitive structure. Generally, these two components of the compound speckle structure will move as rigid structures with individual velocities determined by the characteristics of the two illuminating beams. Closed-form analytical expressions are found for the space- and time-lagged covariance of irradiance and the corresponding power spectrum for the two spatially separated illuminating beams. The present analysis is valid for propagation through an arbitrary ABCD system, though the focus for the experimental evaluation is far-field observations using an optical Fourier transform system. It is shown that the compound speckle structures move as two individual structures with the same decorrelation length. The velocity of the random speckles is a combination of angular and peripheral velocity, where the peripheral velocity is inversely proportional to the radius of the wavefront curvature of the incident beams. The velocity of the repetitive structure is a combination of angular and peripheral velocity, where the peripheral velocity is proportional to the ratio of the angle to the distance between the beams in the object plane. Experimental data demonstrate good agreement between theory and measurements for selected combinations of beam separation, angle between beams, and radius of wavefront curvature at the object.

Bio-speckle activity applied to the assessment of tomato fruit ripening

Dynamic speckle activity is a useful non-destructive tool for the study of certain processes that occur in biological samples. In this study, bio-speckle usefulness in the determination of the ripening degree of different lines of tomatoes was explored. The study was carried out recording the temporal history of the speckle pattern obtained by illuminating the surface of the fruit with a laser beam. The bio-speckle activity was determined by means of the autocorrelation functions of the intensity fluctuations. The intensity of the backscattered light, measured by the average grey level of the speckle diagrams, was also recorded. Both parameters were related to the fruit ripening of each tomato line so that the differences could be observed.

Biospeckle laser spectral analysis under Inertia Moment, Entropy and Cross-Spectrum methods

Biospeckle or dynamic speckle can be used as a method for analysing activity, biologic or not, from materials illuminated with laser beam. The Spatial Temporal Speckle (STS) contains data of time information of dynamic speckle and it is used as input for many techniques allowing the analysis of the activity which is being monitored. One question that rises from the manipulation of the STS is related with the information inside it, in particular, whether it is possible to access different frequency behaviors in the time series presented in the STS pattern. This study presents the Inertia Moment, the Wavelets based Entropy and the Cross-Spectrum analysis as approaches that can be used for evaluating the STS spectral content. In a simulation, STS lines have been created based on many frequencies of the fundamental harmonic. This was done for verifying as each method acts when analysing different frequencies, varying harmonics offset and amplitude. These techniques were applied to real database, to validate their action mechanism in real samples. The results present that all techniques were able to verify the spectral content of different harmonics. Inertia Moment was more efficient on analysing high frequencies, because it is a second order moment, being able to obtain more information from high variations on activity. Entropy and Cross-Spectrum, in turn, were better on differing lower frequencies. This was attributed to the convolution proccess, which is present in both methods, filtering high frequencies. Although, any of them returned informations on both high and low frequencies at the same time, they can be used simultaneously, since Entropy and Cross-Spectrum were complementary to Inertia Moment.

Accuracy of measuring systems using dynamic speckles

Diffraction-limited accuracy is estimated for measuring systems exploiting dynamic speckles. Statistical properties of the signal in the systems with spatial filtering are used to evaluate the signal frequency with precision sufficient to achieve the diffraction-limited accuracy. The results of the analysis allow for designing an optimal measuring system in which components are matched with each other to provide the highest accuracy. Experiments carried out with a range sensor using spatially filtered dynamic speckles are in good agreement with theory.

Statistics of dynamic speckles in application to distance measurements

We present an analysis of statistical properties of dynamic speckles to estimate the limiting accuracy of measurements achievable in a distance sensor using spatially filtered dynamic speckles. The main reason for inaccurate measurements using dynamic speckles is their stochastic nature. It is shown that the average lifetime of dynamic speckles is the key factor defining the measurement accuracy. Main conclusions of the theoretical analysis were confirmed in an experiment carried out with a fast moving rough surface. Special attention is paid to a recently proposed range sensor using dynamic speckles generated by a fast-deflecting laser beam. It is shown that this sensor possesses the best combination of accuracy and response time.

Twente Optical Perfusion Camera: system overview and performance for video rate laser Doppler perfusion imaging

We present the Twente Optical Perfusion Camera (TOPCam), a novel laser Doppler Perfusion Imager based on CMOS technology. The tissue under investigation is illuminated and the resulting dynamic speckle pattern is recorded with a high speed CMOS camera. Based on an overall analysis of the signal-to-noise ratio of CMOS cameras, we have selected the camera which best fits our requirements. We applied a pixel-by-pixel noise correction to minimize the influence of noise in the perfusion images. We can achieve a frame rate of 0.2 fps for a perfusion image of 128x128 pixels (imaged tissue area of 7x7 cm2) if the data is analyzed online. If the analysis of the data is performed offline, we can achieve a frame rate of 26 fps for a duration of 3.9 seconds. By reducing the imaging size to 128x16 pixels, this frame rate can be achieved for up to half a minute. We show the fast imaging capabilities of the system in order of increasing perfusion frame rate. First the increase of skin perfusion after application of capsicum cream, and the perfusion during an occlusion-reperfusion procedure at the fastest frame rate allowed with online analysis is shown. With the highest frame rate allowed with offline analysis, the skin perfusion revealing the heart beat and the perfusion during an occlusion-reperfusion procedure is presented. Hence we have achieved video rate laser Doppler perfusion imaging.

Analysis of bacterial chemotactic response using dynamic laser speckle

Chemotaxis has a meaningful role in several fields, such as microbial physiology, medicine and biotechnology. We present a new application of dynamic laser speckle (or biospeckle) to detect different degrees of bacterial motility during chemotactic response experiments. Encouraging results showed different bacterial dynamic responses due to differences in the hardness of the support in the swarming plates. We compare this method to a conventional technique that uses white light. Both methods showed to be analogous and, in some cases, complementary. The results suggest that biospeckle processed images can be used as an alternative method to evaluate bacterial chemotactic response and can supply additional information about the bacterial motility in different areas of the swarm plate assay that might be useful for biological analysis.

Phase retrieval for improved three-dimensional velocimetry of dynamic x-ray blood speckle

High-resolution synchrotron-based x-ray phase-contrast images of in vitro blood flow have been collected. We demonstrate that the application of a single-image phase retrieval algorithm to the images leads to improved detail in particle image velocimetry correlation peaks. Out-of-plane variation in flow has been extracted from the peaks leading to the three-dimensional reconstruction of velocity across an axially symmetric cylinder.

Mitigation of speckle noise due to laser Doppler vibrometer motion across a vibrating target.

Laser Doppler vibrometers (LDVs) are widely used for remote vibration measurements. In many applications, such as measurements with an LDV on a moving platform or measurements with a continuously scanning beam, an LDV beam moves across a target. The motion of a laser beam across the target generates noise at the LDV output due to dynamic speckles—speckle noise. Speckle noise is caused by phase fluctuations of laser speckles and by Doppler signal dropouts and increases with the speed of the beam. It has been observed that the noise floor increase due to speckles follows a logarithmic curve as the speed increases. Of course as the speed increases, the dwell time over the target area decreases, which limits the signal of interest. This smaller sampling of the vibration signal, combined with the increase in noise due to speckles, places a severe limit on the scanning speed. A way is being investigated to take advantage of this nonlinear increase in noise with respect to speed by using time multiplexing of mul...

Evaluation of laser dynamic speckle signals applying granular computing

The laser dynamic speckle phenomenon is a grained and fluctuant interference produced when a laser light is reflected from an illuminated surface undergoing some kind of activity. This phenomenon allows developing practical applications of unlimited use in biology and technology for being a non-destructive process, enabling the detection of not easily observable activities, such as seeds viability, paints drying, bacteria activities, corrosion processes, food decomposition, fruits bruising, etc. Sequences of intensity images are obtained in order to evaluate the phenomena dynamics, and the signals generated by the intensity changes in each pixel through the sequence are processed with the finality of identifying underlying activity in each point. This paper offers a new methodology based on granular computing to characterize the signals dynamics within the time domain, reducing the time processing and proposing news evaluation parameters to characterize speckle patterns. The methodology is applicable to stationary and non-stationary cases, enabling to monitor the phenomenon in almost real time. Two dynamic processes are analyzed to assess the goodness of the proposed methodology: fast paint drying (non-stationary) and corn seed viability (stationary), being obtained results in agreement with the physical behaviour of the observed processes.

Differentiation of the drying time of paints by dynamic speckle interferometry

This work is part of studies of drying of paints using dynamic speckle interferometry (DSI), a technique based on the use of laser light. DSI is a non-invasive method that does not disturb the drying process. The sample is illuminated by a laser light from a source and the images corresponding to a phenomenon of interference called speckle are registered. If the surface shows activity, the interference pattern changes in time. Following this evolution, we can analyze the drying process by means of an image-processing algorithms. Therefore, it is possible to determine the temporal evolution of surfaces with different drying times. In this work the application of DSI to follow the drying process of paints with different drying times and drying velocities is presented. Different types of paints applied in different thicknesses of wet film and aerosol spray were analyzed. Quantitative measurements were obtained by analysis of the inertia moment of the co-occurrence matrix. Also, qualitative measurements were obtained by sample activity. A new approach for monitoring simultaneously zones with different drying times is discussed and applied. As in previous work, results were compared with those from conventional techniques such as gravimetric analysis providing a good correlation between both techniques.

Statistical properties of dynamic small-N speckles within highly scattering media

Statistics of the phase and intensity of speckles formed with a small number of scattering events (small-N speckles) within multiple-scattering media have been studied. It has been demonstrated that first-order statistics of the intensity fluctuations of small-N speckles nearly obey a Nakagami n distribution in the case considered. The correlation function of the complex amplitude of scattered light is close to a negative exponent. Theoretical results have been experimentally verified using the Shack-Hartmann wavefront analysis technique.

Dynamic speckle imaging with low-cost devices

Light from a rough sample surface illuminated with a laser consists of a speckle pattern. If the surface evolves with time, the pattern becomes dynamic, following the activity of the sample. This phenomenon is used both in research and in industry to monitor processes and systems that change with time. The measuring equipment generally includes high-performance CCD cameras and other expensive devices. Based on the same principle, here we describe a simplified approach where components and devices commonly available in undergraduate physics laboratories are instead used. The technique is described in detail, also reviewing the basic properties of speckle fields. Examples of the application are given, and suggestions for further investigation are provided.

Speckle dynamics for intensity-modulated illumination

We analyze the dynamics of laser speckle patterns, designed for sensing with a receiver, based on spatial filtering. The speckle translation arises after free-space propagation of light scattered from nonspecular surfaces of a solid object in motion. The speckle pattern is manipulated by modulating the intensity of the coherent light, illuminating the target. The space-time normalized cross covariance of speckle patterns incident on the spatial sensor is calculated for the field distribution of three Gaussian beams having arbitrary directions and separations when incident on the target. The modulation of the intensity distribution at the target introduce a higher spatial frequency component in the speckle pattern. The theoretical analysis provides the statistical parameters for both the speckles and the higher spatial frequency component. The analysis reveals that the speckles and the higher spatial frequency component do not necessarily translate as a rigid structure. The theoretical findings are supported by measurements.

Used digital speckle correlation method to realize orientation function of optical mouse

Used digital speckle correlation method (DSCM) to realize orientation function of optical mouse is researched. The experiment is designed to obtain the dynamic speckle patterns which are shot by CCD, then the DSCM is used to process the sequential images and also the experiment is simulated. The experimental results show the DSCM can confirm orientation function of the optical mouse, and have a good agreement with the simulation results, and the resolution we obtained is higher than the resolution of mouse on the market.

Dynamic speckle algorithms comparison using receiver operating characteristic

Dynamic speckle is a useful technique to show biological tissue activity and evolving processes in industry. There are several algorithms for quantitative and qualitative characterization of dynamic speckle. We use the statistical method receiver operating characteristic (ROC) to compare some speckle algorithms. We estimate the capacity of these descriptors to discriminate different activities and the smallest number of images necessary for a correct description of the phenomena. We also use the ROC curves to estimate the best conditions required by some activities. The results are verified using an activity image.

New possibilities of investigating blood flow in soft tissues of the mouth

It has been shown that the method of digital dynamic photography is an effective technique for quantitative and qualitative diagnosis of the microcirculatory state of mouth tissues. As a result of the signal processing, one can obtain information on microcirculation indices from the spectral distribution of low-frequency variations of the speckle structure. The investigated method of digital dynamic speckle photography can be used to elucidate the haemodynamic features in tissues of the oral cavity even at early stages of the disease and then at the examination and treatment stages. New possibilities of digital laser speckle technologies for tomographic investigations of the near-surface blood flow at various depths with the use of two-lens optical configurations are illustrated.