Demodulation of Spatial Carrier Images: Performance Analysis of Several Algorithms Using a Single Image

Abstract

Optical full-field techniques have a great importance in modern experimental mechanics.Even if they are reasonably spread among the university laboratories, their diffusion in industrial companies remains very narrow for several reasons, especially a lack of metrological performance assessment. A full-field measurement can be characterized by its resolution, bias, measuring range, and by a specific quantity, the spatial resolution. The present paper proposes an original procedure to estimate in one single step the resolution, bias and spatial resolution for a given operator (decoding algorithms such as image correlation, low-pass filters, derivation tools ...). This procedure is based on the construction of a particular multi-frequential field, and a Bode diagram representation of the results. This analysis is applied to various phase demodulating algorithms suited to estimate in-plane displacements. Keywords Fringe • Error assessment • Spatial resolution • Displacement resolution • Benchmarkextraction from a single fringe patterns. Consequently, they should be used especially with the grid method, but also to many high speed implementations (for example: fringe projection, deflectometry or speckle or moiré interferometry).The proposed methodology is based on a generic synthetic image undergoing sinusoidal displacements. The procedure used to generate original images containing very rich information in terms of frequency and phase distributions is presented and justified in the first section of this paper.Displacements are computed by seven fringe analysis codes for various parameters. The measured displacements are compared to the prescribed ones which serve as a reference through a specific method which is described. The two main interests are i-to rely on a reference phase distribution which is perfectly known a priori and ii-to control any additive noise that can potentially be added to this reference, thus leading to a precise assessment of its influence on the measurements provided by various algorithms. Obtained results are presented, analyzed and discussed.

Comparing several spectral methods used to extract displacement fields from checkerboard images

“…This progressively becomes a standard procedure to estimate the bias, as in Ref. [31,26,32] for DIC, and in [33,28,24] for LSA. As mentioned in [9,10], a closed-form expression is available for the prediction of the spatial resolution as a function of λ for LSA.…”

Strain measurements on compliant knitted mesh used in space antennas, by means of 2D Fourier analysis

“…On such kind of mechanical components, severe problems arise due to their repetitiveness and to the presence of a large amount of voids. Most of the phase evaluation methods imply that the output of the optical measurements are fringe patterns whose transformation, during the loading history, is analysed through various mathematical means to extrapolate the corresponding strain field . In the case of grid methods, a pattern is directly applied on the specimen, and it is the target of the measurement .…”

“…In the contest of non‐contacting strain measurement methods, the authors have developed a method that places itself conceptually close to the grid analysis through spatial FT techniques but tries to overcome some of the limitations above discussed. The proposed procedure is capable of deriving directly strains through two‐dimensional Fourier analysis, without the need of any specimen preparation and without the need to know a priori any of its geometrical properties.…”

Characterisation of Knee Brace Migration and Associated Skin Deformation During Flexion by Full-Field Measurements

“…However, the resolution of the full-field measurement system was high enough to measure small magnitudes of slippage, and strain resolution was estimated to be 4×10 −4 . Spatial resolution is defined according to Badulescu et al [3]. Its determination is calculated from the autocorrelation function of pure noise obtained with a repeatability test though the extraction procedure [4].…”