Speckle Photography Research Articles

New Schemes of Digital Speckle Photography

This paper presents a review and an analysis of the development of speckle photography from the first experiments performed at the National Physical Laboratory of Great Britain up to the present time. The possibility of diagnosing turbulent and three-dimensional flows by the speckle photography methods has been shown. The single-exposure scheme of speckle photography has been analyzed and the possibility of using multiaspect speckle photography in reconstructive tomography of complex three-dimensional flows has been shown. Methods based on speckle technologies such as Talbot interferometry, PIV and BOS techniques have been analyzed. Particular consideration is given to the application of new measuring technologies in diagnosing turbulent flows and reconstructive tomography of three-dimensional flows.

A New Method of Creating High-Temperature Speckle Patterns and Its Application in the Determination of the High-Temperature Mechanical Properties of Metals

Speckle techniques, such as DIC (Digital Image Correlation) and DSP (Digital Speckle Photography), are frequently used to determine the high-temperature mechanical properties of materials. Speckle techniques require the creation of a random speckle pattern on the surface of the specimen. The most commonly used approach to creating a high-contrast speckle pattern is to spray a layer of black paint particles on a white background. However, in a high-temperature environment, the paint particles tend to peel off or burn off. In this paper, we present an approach that uses a novel laser-engraving technology, where the created speckles will sustain temperatures as high as the melting temperature of the specimen. The size, density, depth and distribution of the speckles can be controlled to suit a particular situation. Since the pattern is part of the specimen, it will never disappear, until the melting temperature of the metal is reached. As an application, we used the technique to determine the elastic modulus of Ti up to 600°C and tungsten up to 1000°C.

A versatile system for in-situ speckle and thermography-based diagnostics of artifacts

Holographic nondestructive techniques already showed their effectiveness in detecting structural defects on or beneath the surface of historical artifacts, not emerging from conventional visual inspections. Unfortunately, such tools are difficult to be operated out of the stability conditions guaranteed by an optics laboratory. The peculiar features of many works of art, however, make often impracticable their transport to dedicated facilities. In this work, a compact, stable, portable holographic system was designed and implemented: the resulting apparatus allows in-situ acquisitions, even in non-laboratory conditions, and provides rapid and clearly interpretable diagnostic information by the completely non-invasive procedure of speckle pattern photography (SPP). Complementary data are furnished by an integrated infrared thermography system. A map of the artifact surface can be obtained, representing the distribution of defects and their extension. The technique will hopefully help or replace conventional methods, more invasive or less systematic (visual inspections, surface tapping), still largely used for the same purposes.

Effect of noise on measurements of diffusivity in transparent liquid mixtures by digital speckle photography

Interfacing two liquid mixtures in a diffusion cell induces noise in the initial state of the diffusing system, which produces a gap between the diffusion boundary and the ideally boundary assumed in the theory. Measured diffusivity values systematically drift with time and they are often corrected by using a linear shift of the zero-time of the process after sufficiently long time when the system reaches the free one-dimensional diffusion regime. In data analysis methods which involve optical correlation between pairs of successive digital images of the cell, it is not easy to establish how long the transient lasts. We show that when the initial perturbation between solution and solvent relaxes slowly toward the diffusive regime no simple zero-time correction can be applied.

Digital Speckle Photography of Subpixel Displacements of Speckle Structures Based on Analysis of Their Spatial Spectra

We have investigated the capabilities of the method of digital speckle interferometry for determining subpixel displacements of a speckle structure formed by a displaceable or deformable object with a scattering surface. An analysis of spatial spectra of speckle structures makes it possible to perform measurements with a subpixel accuracy and to extend the lower boundary of the range of measurements of displacements of speckle structures to the range of subpixel values. The method is realized on the basis of digital recording of the images of undisplaced and displaced speckle structures, their spatial frequency analysis using numerically specified constant phase shifts, and correlation analysis of spatial spectra of speckle structures. Transformation into the frequency range makes it possible to obtain quantities to be measured with a subpixel accuracy from the shift of the interference-pattern minimum in the diffraction halo by introducing an additional phase shift into the complex spatial spectrum of the speckle structure or from the slope of the linear plot of the function of accumulated phase difference in the field of the complex spatial spectrum of the displaced speckle structure. The capabilities of the method have been investigated in natural experiment.

Investigation of the Effect of Small Hardening Spots Created on the Sample Surface by Laser Complex with Solid-State Laser

This paper describes the results of an investigation of the effect of small hardening spots (about 1 mm) created on the surface of a sample by laser complex with solid-state laser. The melted area of the steel sample is not exceed 5%. Steel microhardness change in the region subjected to laser treatment is studied. Also there is a graph of the deformation of samples dependence on the tension. As a result, the yield plateau and plastic properties changes were detected. The flow line was tracked in the series of speckle photographs. As a result we can see how mm surface inhomogeneity can influence on the deformation and strength properties of steel.

Precise In-Process Strain Measurements for the Investigation of Surface Modification Mechanisms

The question, how certain surface layer properties (for example, hardness or roughness) can be specifically influenced in different manufacturing processes, is of great economic interest. A prerequisite for the investigation of the formation of surface layer properties is the metrological assessment of the material stresses during processing. Up to now, no commercial in-process measuring system exists, which is able to determine material stresses in the form of mechanical strains in high-dynamic manufacturing processes with sufficient accuracy. A detailed analysis of the resolution limits shows that speckle photography enables deformation measurements with a resolution in the single-digit nanometer range. Thus, speckle photography basically offers the potential to measure material stresses during processing. Using the example of single-tooth milling, the applicability of speckle photography for in-process stress measurements is demonstrated. Even in such highly dynamic manufacturing processes with cutting speeds up to 10 m/s, the absolute measurement uncertainty of the strain is less than 0.05%. This is more than one order of magnitude lower than the occurring maximal strain. Therefore, speckle photography is suitable for characterizing the dynamic stresses and the material deformations in manufacturing processes.

A multi-technique nondestructive approach for characterizing the state of conservation of ancient bookbindings

The aim of this work is to evaluate the potentiality of a multi-technique nondestructive approach for characterizing the state of conservation of precious bookbindings. In particular, the bookbinding of an ancient book dating back nineteenth century was inspected by infrared thermography, near-infrared reflectography and transmittography, digital speckle photography, holographic interferometry, and proton magnetic resonance relaxometry. Data were processed by different and innovative methodologies, among which, a calibration procedure of the camera for correlation analyses based on specklegrams. The results were compared, showing a promising synergy for the diagnostic purposes, on the basis of the relationship between structure, properties, and uses of the analyzed materials.

Digital Laser Speckle Technologies in Measuring Blood Flow in Biotissues and the Stressed-Strained State of the Maxillodental System

A method has been developed for estimating the stressed-strained state in the ″orthodontic apparatus–dentin″ system with the use of laser-optical diagnostics based on speckle photography. We have determined the indices of the stressed-strained state in the ″orthodontic apparatus–dentin″ system depending on the composition and form of the orthodontic arch cross section. We have determined the optimum indices of the stressed-strained state of orthodontic arches in patients with periodontium diseases in combination with maxillodental anomalies and deformations, to which the following arches correspond: from copper–nickel–titanium (CuNiTi) alloy with circular (0.012″, 0.013″, 0.014″, 0.016″, 0.018″), and rectangular (0.014 × 0.025″, 0.016 × 0.025″) cross sections, from titanium–molybdenum alloy (TMA) with a rectangular cross section (0.016 × 0.025″), and from stainless steel (SS) with a circular (0.016″, 0.018″) cross section. Direct correlation has been established between indices of the stressed-strained state in the ″orthodontic apparatus–dentin″ system and the periodontium capillary pressure (r = 0.78, p < 0.05), as well as inverse strong correlation with the periodontium microcirculation intensity (r = –0.88, p < 0.05).

Accuracy concerns in digital speckle photography combined with Fresnel digital holographic interferometry

A combination of digital holographic interferometry (DHI) and digital speckle photography (DSP) allows in-plane and out-of-plane displacement measurement between two states of an object. The former can be determined by correlating the two speckle patterns whereas the latter is given by the phase difference obtained from DHI. We show that the amplitude of numerically reconstructed object wavefront obtained from Fresnel in-line digital holography (DH), in combination with phase shifting techniques, can be used as speckle patterns in DSP. The accuracy of in-plane measurement is improved after correcting the phase errors induced by reference wave during reconstruction process. Furthermore, unlike conventional imaging system, Fresnel DH offers the possibility to resize the pixel size of speckle patterns situated on the reconstruction plane under the same optical configuration simply by zero-padding the hologram. The flexibility of speckle size adjustment in Fresnel DH ensures the accuracy of estimation result using DSP.

Fundamental uncertainty limit for speckle displacement measurements.

The basic metrological task in speckle photography is to quantify displacements of speckle patterns, allowing for instance the investigation of the mechanical load and modification of objects with rough surfaces. However, the fundamental limit of the measurement uncertainty due to photon shot noise is unknown. For this reason, the Cramér-Rao bound (CRB) is derived for speckle displacement measurements, representing the squared minimal achievable measurement uncertainty. As result, the CRB for speckle patterns is only two times the CRB for an ideal point light source. Hence, speckle photography is an optimal measurement approach for contactless displacement measurements on rough surfaces. In agreement with a derivation from Heisenberg's uncertainty principle, the CRB depends on the number of detected photons and the diffraction limit of the imaging system described by the speckle size. The theoretical results are verified and validated, demonstrating the capability for displacement measurements with nanometer resolution.

Determination of subpixel microdisplacements of speckle structure using the phase shift of spatial spectrum field

An experimental method for measurement of subpixel microdisplacements of speckle structure based on the parameters of the linear phase shift in the field of the complex spatial spectrum of the displaced structure is proposed and experimentally implemented. The phase shift is determined when a phase shift of spectrum is numerically added and the correlation analysis of the resulting linear phase shift in the spatial spectrum of specklegrams is performed. The method provides additional possibilities in the measurements using digital speckle photography when the period of interference fringes formed in the total spatial spectrum of specklegrams is significantly greater than the spectral width.

Mapping interior deformation of a composite sandwich beam using Digital Volumetric Speckle Photography with X-ray computed tomography

Sandwich beams or plates with a soft core and stiff face sheets have been used extensively as a structural engineering material because of its high strength-to-weight ratio. While many studies have been devoted to understanding its failure mechanism, much remain unknown. For example while its main failure mode is a 45° shear crack in the core and delamination at the core/face sheet interface, its exact mechanism remains somewhat a mystery: where and how the crack is initiated and what effects its propagation. One of the main reasons is the lack of an effective experimental tool to monitor, in situ, the failure process and thereby quantify the stress/strain field as it progresses. In this paper we present a new tool called DVSP (Digital Volumetric Speckle Photography) that is capable of mapping quantitatively the interior deformation of sandwich beams/plates as it is being loaded. We have applied the technique to a sandwich beam under 3-point bending. Full field deformation in the form of displacement and shear strain contours of 6 transverse and 3 longitudinal sections of the beam are presented. It is believed that this technique will shed much light on the failure mechanism of sandwich beams/plates in the future.

Influence of hydrogen embrittlement on the localization of plastic strain in Al–Cu–Mg alloy

The influence of hydrogen embrittlement of the aluminum Al–4%Cu–0.2%Mg alloy on the parameters of localized plastic strain is studied. Digital speckle photography is used to visualize the localized strain patterns in the D1 alloy. The velocity of localized deformation bands as a function of the yield strength for nonhydrogenated and hydrogenated samples is found.

Detection Method on Slight Deformation of Rotation Axis Based on Digital Speckle Photography Frequency Domain Analysis

Detection Method on Slight Deformation of Rotation Axis Based on Digital Speckle Photography Frequency Domain Analysis

Digital speckle photography with the ring aperture diaphragm

In this paper theoretical and the experimental studies of application of optical systems with the ring apertures diaphragm in double-exposure speckle photography at the registration of the specklegram are presented. The application of optical systems with the ring apertures allows to increase sensitivity and to expand effective range at measurement of displacements by the double-exposure speckle photography. Theoretical estimates of the expansion of the range of measured displacements are presented. Experimental results of the analysis of interference patterns reconstructed by Young’s method with double-exposure speckle photography confirming theoretical calculations are presented.

Influence of hydrogen on the localization of plastic strain in low-carbon steel

The localization of plastic strain in the extension of 08ps low-carbon steel is compared after hot rolling and after electrolytic saturation with hydrogen by means of a three-electrode electrochemical cell. Analysis of the strain curves shows that they include a plasticity region, a stage of linear strain hardening, and a stage of parabolic strain hardening (Taylor hardening). Double-exposure speckle photography is used to visualize the localized strain zones. On that basis, the quantitative deformation characteristics may be obtained. In other words, the displacement-vector field in a plane sample under extension may be determined, and then the components of the plastic distortion tensor (the local elongation exx, shear exy, and rotation ωz) may be calculated. The basic types of plastic flow localization are identified at different stages of strain hardening, and their parameters are determined.

Full-field 3D deformation measurement: comparison between speckle phase and displacement evaluation.

The objective of this paper is to describe a full-field deformation measurement method based on 3D speckle displacements. The deformation is evaluated from the slope of the speckle displacement function that connects the different reconstruction planes. For our experiment, a symmetrical arrangement with four illuminations parallel to the planes (x,z) and (y,z) was used. Four sets of speckle patterns were sequentially recorded by illuminating an object from the four directions, respectively. A single camera is used to record the holograms before and after deformations. Digital speckle photography is then used to calculate relative speckle displacements in each direction between two numerically propagated planes. The 3D speckle displacements vector is calculated as a combination of the speckle displacements from the holograms recorded in each illumination direction. Using the speckle displacements, problems associated with rigid body movements and phase wrapping are avoided. In our experiment, the procedure is shown to give the theoretical accuracy of 0.17 pixels yielding the accuracy of 2×10^-3 in the measurement of deformation gradients.

Experimental methodology for study of granular material flow using digital speckle photography

Granular material flow occurs in many industrial applications, and the characteristics of such flow is challenging to measure. Therefore, an experimental method that captures the flow behavior at different loading situations is desired.In this work, experimental measurements of granular material flow with digital speckle photography (DSP) are carried out. The granular flow process is recorded with a high-speed camera; the image series are then analyzed using the DSP method. This approach enables field data such as displacement, velocity, and strain fields to be visualized during the granular material flow process. Three different scenarios were studied: free surface flow in a fill shoe, flow without a free surface in a fill shoe, and the rearrangement of material in a cavity. The results showed that it is possible to obtain field data of the motion of particles for all three scenarios with the DSP technique. The presented experimental methodology can be used to capture complex flow behavior of granular material.

Modelling laser speckle photographs of decayed teeth by applying a digital image information technique

We report on the application of a digital image model to assess early carious lesions on teeth. When decay is in its early stages, the lesions were illuminated with a laser and the laser speckle images were obtained. Due to the differences in the optical properties between healthy and carious tissue, both regions produced different scatter patterns. The digital image information technique allowed us to produce colour-coded 3D surface plots of the intensity information in the speckle images, where the height (on the z-axis) and the colour in the rendering correlate with the intensity of a pixel in the image. The quantitative changes in colour component density enhance the contrast between the decayed and sound tissue, and visualization of the carious lesions become significantly evident. Therefore, the proposed technique may be adopted in the early diagnosis of carious lesions.