Speckle Phenomenon Research Articles

Evaluation of surface roughness of a machined metal surface based on laser speckle pattern

The surface roughness of a machined metal surface is crucial to its appearance and performance. There are hardly any in-situ methods for surface roughness measurement of moving surfaces. A study of the digital speckle patterns generated by rough surfaces illuminated by a laser is performed experimentally. Laser speckle phenomenon can be used to monitor the surface roughness in a non-contact way. By investigating the effect of the surface roughness on the statistical and fractal parameters of the laser speckle pattern, an assessment method for evaluating surface roughness is discussed. The results show that some of the proposed statistical parameters have definite relationships with the surface roughness and can be explored to evaluate the surface roughness. Furthermore, the fractal parameters of the speckle pattern are sensitive to the type of machining process and therefore they can be used to classify the machined surface. The method can be a practical tool to achieve in-situ surface roughness measurement of moving surfaces.

63‐1: Speckle Reduction for Laser Pico‐projector with Dynamic Deformable Mirrors

This research focuses on speckle noise suppression by changing the angle of divergence of the diffuser. Three types of diffuser designs are compared in the experiments. For all three types, the speckle contrast value is lower than 0.027, and the speckle phenomenon is invisible to the human eye.

Speckle reduction using deformable mirrors with diffusers in a laser pico-projector.

We propose a design for speckle reduction in a laser pico-projector adopting diffusers and deformable mirrors. This research focuses on speckle noise suppression by changing the angle of divergence of the diffuser. Moreover, the speckle contrast value can be further reduced by the addition of a deformable mirror. The speckle reduction ability obtained using diffusers with different divergence angles is compared. Three types of diffuser designs are compared in the experiments. For Type 1 which uses a circular symmetric diffuser the speckle contrast value can be decreased to 0.0264. For Type 2, the speckle contrast value can be reduced to 0.0267 because of the inclusion of an elliptical distribution diffuser. With Type 3 which includes a combination of the circular distribution diffuser and elliptical distribution diffuser, the speckle contrast value can be reduced to 0.0236. For all three types, the speckle contrast value is lower than 0.05. Under this speckle value, the speckle phenomenon is invisible to the human eye.

Ultrasound Image Despeckling Based on Statistical Similarity

Ultrasound images are affected by the speckle phenomenon, a multiplicative noise that degrades image quality. Several methods for denoising have been proposed in recent years, based on different approaches. The so-called non-local mean is considered the state-of-the-art method; the idea is to find similar patches across the image and exploit them to regularize the image. The method proposed here is in the non-local family, although instead of partitioning the target image in patches, it works pixelwise. The similarity between pixels is evaluated by analyzing their statistical behavior, in particular, by measuring the Kolmogorov–Smirnov distance between their distributions. To make this possible, a stack of acquired images is required. The proposed method has been tested on both simulated and real data sets and compared with other widely adopted techniques. Performance is interesting, with quality parameters and visual inspection confirming such findings.

Dynamic laser speckle analyzed considering inhomogeneities in the biological sample.

Dynamic laser speckle phenomenon allows a contactless and nondestructive way to monitor biological changes that are quantified by second-order statistics applied in the images in time using a secondary matrix known as time history of the speckle pattern (THSP). To avoid being time consuming, the traditional way to build the THSP restricts the data to a line or column. Our hypothesis is that the spatial restriction of the information could compromise the results, particularly when undesirable and unexpected optical inhomogeneities occur, such as in cell culture media. It tested a spatial random approach to collect the points to form a THSP. Cells in a culture medium and in drying paint, representing homogeneous samples in different levels, were tested, and a comparison with the traditional method was carried out. An alternative random selection based on a Gaussian distribution around a desired position was also presented. The results showed that the traditional protocol presented higher variation than the outcomes using the random method. The higher the inhomogeneity of the activity map, the higher the efficiency of the proposed method using random points. The Gaussian distribution proved to be useful when there was a well-defined area to monitor.

Segmentation of anatomical structures of the heart based on echocardiography

Nowadays, many practical applications in the field of medical image processing require valid and reliable segmentation of images in the capacity of input data. Some of the commonly used imaging techniques are ultrasound, CT, and MRI. However, the main difference between the other medical imaging equipment and EchoCG is that it is safer, low cost, non-invasive and non-traumatic. Three-dimensional EchoCG is a non-invasive imaging modality that is complementary and supplementary to two-dimensional imaging and can be used to examine the cardiovascular function and anatomy in different medical settings. The challenging problems, presented by EchoCG image processing, such as speckle phenomena, noise, temporary non-stationarity of processes, unsharp boundaries, attenuation, etc. forced us to consider and compare existing methods and then to develop an innovative approach that can tackle the problems connected with clinical applications. Actual studies are related to the analysis and development of a cardiac parameters automatic detection system by EchoCG that will provide new data on the dynamics of changes in cardiac parameters and improve the accuracy and reliability of the diagnosis. Research study in image segmentation has highlighted the capabilities of image-based methods for medical applications. The focus of the research is both theoretical and practical aspects of the application of the methods. Some of the segmentation approaches can be interesting for the imaging and medical community. Performance evaluation is carried out by comparing the borders, obtained from the considered methods to those manually prescribed by a medical specialist. Promising results demonstrate the possibilities and the limitations of each technique for image segmentation problems. The developed approach allows: to eliminate errors in calculating the geometric parameters of the heart; perform the necessary conditions, such as speed, accuracy, reliability; build a master model that will be an indispensable assistant for operations on a beating heart.

Determination of the formability limits for Grade 1 titanium sheet by means of ALSAD method

Stamping is one of the most widely used sheet metal forming processes. Esthetics and durability of the deformed products are limited by defects in the form of strain localization, grooves or cracks. Forming limits, e. g. in form of forming limit curve (FLC), help to avoid above-mentioned defects in the sheet metal during forming. Besides the common steel sheets for deep drawing processes, e.g. DC04, components can also be made of metal with specified strength and chemical resistance properties. One of such materials is titanium sheet. In this paper, FLC for GR1 titanium sheet will be presented. It was obtained by means of a new author’s method named as ALSAD (Analysis of Laser Speckle Activity Differences). So far, applied methods of FLC determination have been mainly based on the measurement of the strain distribution in the area of the created groove, deformation gradient and deformation speed. For this reason, detection of strain localization has been dependent on the strain measurement method. The ALSAD, based on laser speckle phenomenon allows detection of defects initiation on the sample surface independently of the strain distribution measurement and it allows, for example, to relate a moment of defect initiation directly to the sample height or forces occurring in the process. In this paper strain values for strain localization, grooves and cracks for GR1 titanium sheet will be shown. Obtained results show good formability of this material, which was verified by means of 3D digital microscope. Such results allow tracing precisely the formation of the defects starting from strain localization up to the crack appearance, which would be helpful for advanced numerical simulations.

Reduction of coherent artefacts in super-resolution fluorescence localisation microscopy.

SummarySuper‐resolution localisation microscopy techniques depend on uniform illumination across the field of view, otherwise the resolution is degraded, resulting in imaging artefacts such as fringes. Lasers are currently the light source of choice for switching fluorophores in PALM/STORM methods due to their high power and narrow bandwidth. However, the high coherence of these sources often creates interference phenomena in the microscopes, with associated fringes/speckle artefacts in the images. We quantitatively demonstrate the use of a polymer membrane speckle scrambler to reduce the effect of the coherence phenomena. The effects of speckle in the illumination plane, at the camera and after software localisation of the fluorophores, were characterised. Speckle phenomena degrade the resolution of the microscope at large length scales in reconstructed images, effects that were suppressed by the speckle scrambler, but the small length scale resolution is unchanged at ∼30 nm.

Multitemporal SAR Image Decomposition into Strong Scatterers, Background, and Speckle

Speckle phenomenon in synthetic aperture radar (SAR) images makes their visual and automatic interpretation a difficult task. To reduce strong fluctuations due to speckle, total variation (TV) regularization has been proposed by several authors to smooth out noise without blurring edges. A specificity of SAR images is the presence of strong scatterers having a radiometry several orders of magnitude larger than their surrounding region. These scatterers, especially present in urban areas, limit the effectiveness of TV regularization as they break the assumption of an image made of regions of constant radiometry. To overcome this limitation, we propose in this paper an image decomposition approach. There exist numerous methods to decompose an image into several components, notably to separate textural and geometrical information. These decomposition models are generally recast as energy minimization problems involving a different penalty term for each of the components. In this framework, we propose an energy suitable for the decomposition of SAR images into speckle, a smooth background, and strong scatterers, and discuss its minimization using max-flow/min-cut algorithms. We make the connection between the minimization problem considered, involving the L0 pseudonorm, and the generalized likelihood ratio test used in detection theory. The proposed decomposition jointly performs the detection of strong scatterers and the estimation of the background radiometry. Given the increasing availability of time series of SAR images, we consider the decomposition of a whole time series. New change detection methods can be based on the temporal analysis of the components obtained from our decomposition.

Biospeckle numerical assessment followed by speckle quality tests

The biospeckle or dynamic speckle phenomenon reflects the information regarding activity of both biological and non-biological material under study. However, due to having a complex interaction between light and material, the analysis of such phenomenon has been a challenge. This study therefore, presents quality tests of biospeckle data obtained from a Plumeria leaf surface, where it was possible to identify some problems in the images related to the tested features. In order to obtain speckle images of best quality, the saturation, the contrast and the homogeneity tests were carried out before the main analysis to be processed. In addition, we present numerical analysis of the biospeckle data to obtain biological information related to the activity of a leaf surface. Significant changes in biospeckle activities were observed. The activity distributions indicated the detection of higher activities around the midvein of the leaf. Activities measured using the IM and AVD values at midvein and lamina regions were found significantly different with a higher correlation observed with the AVD approach.

Recent Clinical Applications of Laser Speckle Flowgraphy in Eyes with Retinal Disease.

Retinal diseases related to ischemia, such as diabetic retinopathy, are the main cause of blindness worldwide. However, the pathogenesis of these diseases remains unclear, as does the time course of associated changes in ocular blood flow. Laser speckle flowgraphy (LSFG), which uses the laser speckle phenomenon to detect and quantify ocular circulation, is a promising candidate for a noninvasive method to measure ocular blood flow in living eyes. A recently developed LSFG measurement parameter, mean blur rate (MBR), can serve as a quantitative and reproducible index of retinal blood cell velocity. Mean blur rate can be used in the study of retinal diseases to evaluate microcirculation in the retinal vessels, choroid, and optic nerve head. In addition to overall MBR (MA), LSFG measurements of optic nerve head microcirculation can be divided into vessel-area MBR (MV) and tissue-area MBR (MT). Absolute values for MT have been shown to be linearly correlated with capillary blood flow, regardless of fundus pigmentation. Recently, there has been an increasing number of reports on the clinical applications of LSFG in retinal disease.

Projection displays: New technologies, challenges, and applications

AbstractThe achievable color gamut and light output of projection displays based on light‐emitting diodes (LEDs), phosphor conversion, and lasers are discussed. The color appearance phenomena, colorfulness, and the Helmholtz–Kohlrausch effect are discussed in the context of LED and laser illumination of projection displays, as well as some pitfalls concerning the interpretation of the Helmholtz–Kohlrausch effect. Also the laser speckle phenomenon and the characterization of it are addressed. The importance of both the American National Standards Institute (ANSI) contrast and the sequential contrast of projectors, as well as the discrepancy between measured and perceived contrasts, are explained. Visibility criteria for contouring artifacts are explained and compared with new emerging electro‐optical transfer functions that are more adapted to this issue. Stereoscopic solutions and early prototypes of autostereoscopic multi‐view, super multi‐view projection displays, and electro‐holographic displays are discussed as well as the limiting factors of these systems in the context of display resolution, and LED and laser light sources.

Speckle Suppression by Controlling the Coherence in Laser Based Projection Systems

In this paper, the relationship between the coherence and the speckle contrast in a laser-based projection display system is studied with the consideration of human visual percept. By using a varifocal liquid-crystal lens and a monochromatic charge-coupled device (CCD), a system which is used for simulating a human eye is set up to record the speckle pattern. An efficient method for controlling the spatial coherence by using a dielectric elastomeric actuator (DEA) is demonstrated. We find that there is a limitation for eliminating the speckle contrast in the laser projection system when only the method of reducing the spatial coherence is used. When the distance between the observer and the screen is sufficiently large ( ${>} \hbox{3 m}$ ), the speckle contrast can be eliminated well ( ${ ) by reducing the spatial coherence and the observer do not observe the speckle phenomenon. When the viewing distance is smaller than 3 m, however, the observer will feel the speckle all of the time. To further reduce the speckle contract, a method of wavelength modulation is used to reduce the temporal coherence while the spatial coherence is well reduced. The speckle contrast is reduced to 4% at the viewing distance of 1 m by reducing both the temporal and spatial coherence of the laser.

转动随机微透镜阵列对激光显示中散斑的抑制

In order to eliminate disturbing speckle phenomenon,this paper discussed the feasibility of speckle reduction,analyzed the superiority in beam control and energy efficiency using the " random micro-lens array" as the random diffuser,and proposed to set the rotating "random micro-lens array" in the optical system to reduce the speckle.The speckle on the screen was measured after setting this component,this method was verified to reduce speckle contrast to 1.15% so that the eyes couldn't distinguish the laser speckle.This method can greatly improve the viewing quality of laser TV.Compared with traditional method of rotating uniform light stick,this method presented the excellent ability in speckle reduction.

Anisotropic diffusion filter with memory based on speckle statistics for ultrasound images.

Ultrasound (US) imaging exhibits considerable difficulties for medical visual inspection and for development of automatic analysis methods due to speckle, which negatively affects the perception of tissue boundaries and the performance of automatic segmentation methods. With the aim of alleviating the effect of speckle, many filtering techniques are usually considered as a preprocessing step prior to automatic analysis methods or visual inspection. Most of the state-of-the-art filters try to reduce the speckle effect without considering its relevance for the characterization of tissue nature. However, the speckle phenomenon is the inherent response of echo signals in tissues and can provide important features for clinical purposes. This loss of information is even magnified due to the iterative process of some speckle filters, e.g., diffusion filters, which tend to produce over-filtering because of the progressive loss of relevant information for diagnostic purposes during the diffusion process. In this paper, we propose an anisotropic diffusion filter with a probabilistic-driven memory mechanism to overcome the over-filtering problem by following a tissue selective philosophy. In particular, we formulate the memory mechanism as a delay differential equation for the diffusion tensor whose behavior depends on the statistics of the tissues, by accelerating the diffusion process in meaningless regions and including the memory effect in regions where relevant details should be preserved. Results both in synthetic and real US images support the inclusion of the probabilistic memory mechanism for maintaining clinical relevant structures, which are removed by the state-of-the-art filters.

Speckle-insensitive fringe detection method based on Hilbert transform for self-mixing interferometry.

A robust phase measurement method to detect self-mixing signals from rough surface targets is addressed. Using the Hilbert transform to create the analytic signal of the monitored optical output power, an accurate instantaneous phase can be extracted without being affected by amplitude variations induced by the speckle phenomenon. In case of fringe fading, a running average filter removes the local bias offset improving the calculations. This also allows one to observe the phase at dynamic feedback levels within the same calculation, avoiding the need of adding external optical, mechanical or electronic components. This method provides confidence for the fringe detection required in subsequent steps to reach subwavelength precision of a reconstructed displacement or for velocimetry applications.

Laser speckle reduction by phase range limited computer generated hologram in laser projection display system.

The speckle phenomenon is an annoyance in laser projection display systems. We propose a novel speckle suppression method that utilizes the interference concept on a pixel point, which reduces the speckle contrast (SC) of the project image by limiting the phase distribution range in the optical field. The SC formula is derived in the uniform interval phase range for partially developed speckle conditions, showing that the SC can be lowered by lessening the phase range limitation. In the ideal simulation model, the SC can be reduced from 98.77% to 0% as the phase range limitation varies from 2π to 0. The phase range limitation model is a novel method using a computer generated hologram to provide beam shaping and phase limitation. In a more realistic simulation model, the SC is reduced from 99.18% to 16.68%.

Basic Technology and Clinical Applications of the Updated Model of Laser Speckle Flowgraphy to Ocular Diseases

Laser speckle flowgraphy (LSFG) allows for quantitative estimation of blood flow in the optic nerve head (ONH), choroid and retina, utilizing the laser speckle phenomenon. The basic technology and clinical applications of LSFG-NAVI, the updated model of LSFG, are summarized in this review. For developing a commercial version of LSFG, the special area sensor was replaced by the ordinary charge-coupled device camera. In LSFG-NAVI, the mean blur rate (MBR) has been introduced as a new parameter. Compared to the original LSFG model, LSFG-NAVI demonstrates a better spatial resolution of the blood flow map of human ocular fundus. The observation area is 24 times larger than the original system. The analysis software can separately calculate MBRs in the blood vessels and tissues (capillaries) of an entire ONH and the measurements have good reproducibility. The absolute values of MBR in the ONH have been shown to linearly correlate with the capillary blood flow. The Analysis of MBR pulse waveform provides parameters including skew, blowout score, blowout time, rising and falling rates, flow acceleration index, acceleration time index, and resistivity index for comparing different eyes. Recently, there have been an increasing number of reports on the clinical applications of LSFG-NAVI to ocular diseases, including glaucoma, retinal and choroidal diseases.

Exploiting Patch Similarity for SAR Image Processing: The nonlocal paradigm

Most current synthetic aperture radar (SAR) systems offer high-resolution images featuring polarimetric, interferometric, multifrequency, multiangle, or multidate information. SAR images, however, suffer from strong fluctuations due to the speckle phenomenon inherent to coherent imagery. Hence, all derived parameters display strong signal-dependent variance, preventing the full exploitation of such a wealth of information. Even with the abundance of despeckling techniques proposed over the last three decades, there is still a pressing need for new methods that can handle this variety of SAR products and efficiently eliminate speckle without sacrificing the spatial resolution. Recently, patch-based filtering has emerged as a highly successful concept in image processing. By exploiting the redundancy between similar patches, it succeeds in suppressing most of the noise with good preservation of texture and thin structures. Extensions of patch-based methods to speckle reduction and joint exploitation of multichannel SAR images (interferometric, polarimetric, or PolInSAR data) have led to the best denoising performance in radar imaging to date. We give a comprehensive survey of patch-based nonlocal filtering of SAR images, focusing on the two main ingredients of the methods: measuring patch similarity and estimating the parameters of interest from a collection of similar patches.

Superresolution imaging in ultrasound B-scan imaging

A number of imaging systems exhibit speckle, which is caused by the interaction of a coherent pulse reflecting off of random reflectors. The limitations of these systems are quite serious since the speckle phenomenon creates a pattern of nulls and peaks from subresolvable scatterers or targets that are difficult to interpret. Another limitation of these pulse-echo imaging systems is that their resolution is dependent on the full spatial extent of the propagating pulse, usually several wavelengths in the axial or propagating dimension and typically longer in the transverse direction. This limits the spatial resolution to many multiples of the wavelength. This paper focuses on the particular case of ultrasound B-scan imaging and develop an inverse filter solution that eliminates both the speckle phenomenon, and the poor resolution dependency on the pulse length and width, to produce SURUS (super-resolution ultrasound) images. The key to the inverse filter is the creation of pulse shapes that have stable inverses. This is derived by use of the standard Z-transform and related properties. Although the focus of this paper is on examples from ultrasound imaging systems, the results are applicable to other pulse-echo imaging systems that also can exhibit speckle statistics.