Fringe Tracing Research Articles

One example of image digital processing of experimental data obtained by the method of photoelasticity

The article is devoted to the digital processing interferograms (isochromatic fringe patterns) obtained by the photoelasticity method. The application to interpretate the isochromatic fringe patterns allows us to automate image processing, gets to avoid the routine and time-consuming work. The features of the developed software package are described in detail by means the problem of stretching plate with two symmetrical edge notches. The application’s algorithm includes following main steps: image pre-processing, localization interference fringe, fringe tracing. The application creates a text file containing all the necessary data to further determine the stress-strain state (isochromatic fringe’s number and locus of fringes).

DIGITAL PROCESSING OF INTERFEROGRAMS OBTAINED BY THE PHOTOELASTICITY METHOD

The article is devoted to the digital processing interferograms (isochromatic fringe patterns) obtained by the photoelasticity method. An application to interpretate the isochromatic fringe patterns is developed. It allows us to automate this procedure, gets to avoid the routine and time-consuming work. The features of the developed software package are described in detail by means of the classic problem of a diametrically compressed disk. The applications algorithm includes following main steps: image pre-processing, localization interference fringe, fringe tracing. The application creates a text file containing all the necessary data to further determine the stress-strain state (isochromatic fringes number and locus of fringes).

Digital image processing in interference-optical methodsof solid mechanics

In the paper the detailed review and comparison of modern methods of digital object processing in present-day interference optical techniques (namely, photomechanics) are given. The methods of fringe thinnig, fringe clustering, fringe tracing are discussed. Applications of methods to digital photomechanics and fracture mechanics parameter determination are considered. The example of cracked semidisc for compression is analyzed.

Retrieval of a wave front with small deformations from a sample of interferograms with a variable number and orientation of interference fringes

A new approach to phase retrieval from an interferogram through the application of the classical fringe tracing method to a sample in which interference fringes change their number and orientation is proposed. Wave fronts retrieved from individual interferograms are averaged, and the quality of the studied surface is judged by the estimate found. Phase averaging over several interferograms allows a decrease in the error variance for the retrieved phase, caused not only by random noise, but also nonuniformity of the aperture covering by interference fringes and light source heterogeneity. Results of model experiments carried out show prospects for interferometry in the nanometrology of high-quality optical components.

Method for increasing the accuracy of wavefront reconstruction from a set of interferograms

A new approach to the wavefront (phase) reconstruction from interferograms is proposed based on the classical method of fringe tracing using a sample of interferograms with different numbers and orientations of interference fringes. Wavefronts reconstructed from individual interferograms are averaged, and the test surface quality is judged by the data obtained. Averaging the phase by several interferograms makes it possible to reduce the variance of the phase reconstruction error caused by random noise and inhomogeneities in aperture illumination and interference fringe distribution, as well as to reduce the effect of instability of external conditions.

Novel processing tools for automated doppler picture veocimetry (DPV) evaluation

A unique technique, the Doppler Picture Velocimetry (DPV), for measuring and visualizing velocities especially in hypersonic gas flows is presented. By means of a Michelson interferometer (MI) the Doppler shifted light scattered by tracers is transformed into an image showing Fizeau fringes. The image fringe distribution provides information on the Doppler frequency shift which is related to the velocity of the particles crossing a light sheet plane. To overcome former disadvantages of DPV, the optical set-up as well as the processing scheme have been improved significantly. Two schemes have been tested in recent times for fringe processing: 1) an alternative process, the Least Square Estimators (LES) and 2) an algorithm based on a technique using Fast Fourier Transformations (FFT) to examine the DPV images in the frequency domain. The new DPV algorithms now allow an automated calculation of the velocity profiles from the Doppler pictures without manual fringe tracing as it had to be done in the past. Both methods are compared by means of Mach 6 flows around a wedge and a sphere produced in the ISL high energy shock tunnel STA. TiO$_2$ particles are illuminated by a laser light sheet which was perpendicularly arranged to the main flow direction. Light observation via the MI from the side (90° to illumination) informs on the vertically oriented velocity through the DPV images.

Error analysis and thermal expansion measurement with electron-beam moiré

In this paper, the authors study erros incurred when using the experimental technique of electron-beam moire. There are two sources of error: error manifested as an apparent magnificant drift and error due to fringe tracing. The error due to fringe tracing is nearly negligible in comparison to the error due to magnification drift. By investigating the thermal expansion of commercially pure copper, the authors demonstrate the usefulness of the error estimate. The average result for the coefficient of thermal expansion is within 1.8 percent of handbook values for this materials, with a possible error due to apparent magnification drift of 9 percent.

Fringe tracing by image processing

The optical techniques of stress analysis, such as, holography, photoelasticity, and moiré, produce results in the form of whole-field fringe patterns representing physical measurements. The accuracy of results will depend on how precisely fringe orders and fringe locations are determined. A detailed account of an accurate and automatic method of tracing the centerlines of dark and bright fringes using image processing is presented. The variance of gray-level values across fringe width is used as a criterion for determining fringe direction. The determination of fringe direction is done in an operator window of size 5 x 5, 7 x 7, or 9 x 9 pixels at the central pixel of the window. Sequential application of this condition enables the user to trace the fringe centerline. Using the capabilities of image-processing equipment, the cursor, and the specially developed computer program, holographic fringes of different patterns were traced. A few of these cases are presented as illustrative examples. It is also shown that the technique can be successfully applied to fringes that close on themselves so as to form closed loops.

Direct observation of the pressure developed in a liquid during cavitation-bubble collapse

A Mach–Zehnder interferometer with a pulsed dye-laser light source is used to observe the collapse of laser-produced cavitation bubbles in water near a rigid transparent polymeric boundary. Vapor bubbles with a maximum radius of about a millimeter are formed using a Q-switched Nd:YAG laser operating at 1.06 μm with a pulse energy of 4.6 mJ and a pulse duration of 8 ns. The pressure in the liquid around the bubbles is determined directly from the resulting interferograms using a fringe tracing method. Prior to bubble rebound, a region of compressed water develops adjacent to the cavity and a weak acoustic wave is directed towards the rigid boundary.

Automated Digital Analysis of Fiber Interferograms

An automatic method for the analysis of fiber interferograms is described. The system consists of an Aus Jena interference microscope fitted with a solid state video camera, an image array processor, and a dedicated DEC microcomputer. A key factor in the speed and accuracy of the analysis is an automatic fringe tracing procedure that eliminates operator bias. The system outputs both thickness profiles and chord-averaged refractive index profiles across the fiber sample. Examples of application to high speed spun nylon and PET and Kevlar® fibers are given.