Computer Aided Speckle Interferometry Research Articles

A combination interferometric and morphological image processing approach to rapid quality assessment of additively manufactured cellular truss core components

Advanced manufacturing (AM) processes such as laser powder bed fusion (LPBF) are increasingly capable of fabricating components with useful and unprecedented mechanical properties by incorporating complex internal bracing structures. From the standpoint of quality control and assessment, however, internally complex assemblies present significant build-verification challenges. Here we propose a hybrid approach to the inspection involving the application of computer-aided speckle interferometry (CASI) and morphological image processing as a rapid, inexpensive, and facile method for AM quality control. The described methodology has low capital equipment costs, is full-field and non-contact, can be used in an industrial setting, and has very low requirements in terms of operator training and expertise. Consisting primarily of the combination of image processing software with a simple optical system of variable sensitivity, the method is shown to be effective for inspection of a titanium honeycomb component subjected to differential pressure. Results are compared to those achieved with computed tomography (CT), immersion ultrasound testing (UT), and optical holographic interferometry. Lastly, we propose several possible processing strategies for automated quality assessment based on this powerful hybrid approach.

A Range and Performance Optimized Version of the Computer-Aided Speckle Interferometry Algorithm for Real-Time Displacement-Strain Field Monitoring

This work presents an optimized implementation of the Computer-Aided Speckle Interferometry algorithm which enables full-field determination of displacements and strains on commodity Graphics Processing Units at high resolution and frame rates. By combining careful control of the average speckle size in a laser speckle pattern with a simple sampling rate conversion scheme, a compact representation of the optical speckle is achieved. This allows for optimal use of Graphics Processing Unit architecture with robust range extension. The optimal mapping of the Computer-Aided Speckle Interferometry algorithm to Graphics Processing Unit architecture is shown in detail, and a straightforward method for disambiguating large displacements is illustrated. Lastly, this paper demonstrates a two-step subimage-tapering modification to the original algorithm that enables robust range enhancement while maintaining resolution. Results from numerical simulations on synthetic speckle patterns are shown, and runtime performance metrics are provided, with performance ranging up to 60 frames per second in some cases. The method is suitable for interactive experimental mechanics research, process and testing or any application where real-time high-resolution displacement-strain monitoring is needed. A .NET Framework class library enabling the incorporation of the algorithm into 3rd -party applications is available for download.

Digital laser speckle technique for generating deflection, slope, and curvature contours of bent plates

A digital method is proposed whereby spatial speckles created when an optically rough surface is illuminated by a diffuse coherent laser beam are used to generate slope contour fringes. This is done by photographing the speckles contained in a parallel plane in front of the plate before and after deformation. The resulting photographs are then fast Fourier transformed twice and numerically compared with the digital speckle correlation software called computer-aided speckle interferometry to produce slope contours of the plate. The slope contour data are then stored in a computational array. The curvature contours are generated by numerical differentiation of the slope data; similarly, the deflection contours are generated by numerical integration of the slope data. The curvature data can then be further used in conjunction with plate theory to obtain the bending and twisting moments the plates experience as well as the stress distribution in the surface of the plate. The method is simple and straight forward. A comparison with several other techniques is presented.

Determination of Regional Area Stroke Work with High Spatial Resolution in the Heart

Coronary artery disease generally leads to regional myocardial dysfunction. This necessitates a high spatial resolution technique that is able to determine whole-field regional myocardial function. We have therefore developed the technique of computer-aidedspeckle interferometry (CASI) for measuring myocardial function in the beating heart. Six isolated rabbit hearts were perfused in a modified Langendorff apparatus, which allowed for the hearts to be perfused in the working mode. A pressure transducer was inserted through the apex into the left ventricular (LV) cavity to measure LV pressure. Silicone carbide particles were placed on the anterior surface of LV and used as displacement markers. The anterior surface of the left ventricle was imaged with a CCD camera at 50 frames per second. The CASI technique was used to determine the displacementof regions of myocardium on the basis of the deformation distribution (via displacement vectors) within the region of interest. From this information, changes in regional area were determined as a function of LV pressure, leading to the determination of regional area stroke work (RASW; the integral of LV pressure with respect to regional area). The average region of interest in the six hearts was 19.7 ± 5.9 mm2. The average RASW over the regions of interest was 3.1 ± 0.7 mmHg (with the regional area normalized to end-diastolic area). The region of interest was further divided into subregions, each representing an average of 1 mm2 of myocardium. Within a subregion, more than 20 displacement vectors were determined, which were used to calculate the change in regionalarea and RASW for a given subregion. CASI is an effective whole-field technique to determine regional function with better than 1-mm spatial resolution in the isolated beating heart.

Iteration algorithm for computer-aided speckle interferometry

An iteration algorithm for the analysis of speckle interference patterns is presented. First, four digitized phase-shifted patterns are locally averaged. The phase information is then extracted by the usual phase shift algorithm. The wrapped phase is in turn used to reconstruct four new phase-shifted patterns. These three steps form a cycle. Repetition of the three steps has a great effect on suppressing speckle noise. Theoretical study shows that the iterated phase converges to a perfect result under ideal conditions. In general, the iteration causes little error but improves the phase information a great deal. The signal-to-noise ratio rises when additional iterations are performed.