A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Digital holography (DH) has emerged as one of the most effective coherent imaging technologies. The technological developments of digital sensors and optical elements have made DH the primary approach in several research fields, from quantitative phase imaging to optical metrology and 3D display technologies, to name a few. Like many other digital imaging techniques, DH must cope with the issue of speckle artifacts, due to the coherent nature of the required light sources. Despite the complexity of the recently proposed de-speckling methods, many have not yet attained the required level of effectiveness. That is, a universal denoising strategy for completely suppressing holographic noise has not yet been established. Thus the removal of speckle noise from holographic images represents a bottleneck for the entire optics and photonics scientific community. This review article provides a broad discussion about the noise issue in DH, with the aim of covering the best-performing noise reduction approaches that have been proposed so far. Quantitative comparisons among these approaches will be presented.

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Strategies for reducing speckle noise in digital holography

Laser speckle based digital optical methods in structural mechanics: A review

Combining ESPI with laser scanning for 3D characterization of racing tyres sections

This paper presents a comparative study of multi-look approaches for de-noising phase maps from digital holographic interferometry. A database of 160 simulated phase fringe patterns with eight different phase fringe patterns with fringe diversity was computed. For each fringe pattern, 20 realistic noise realizations are generated in order to simulate a multi-look process with 20 inputs. A set of 22 de-noising algorithms was selected and processed for each simulation. Three approaches for multi-look processing are evaluated. Quantitative appraisal is obtained using two metrics. The results show good agreement for algorithm rankings obtained with both metrics. One singular and highly practical result of the study is that a multi-look approach with average looks before noise processing performs better than averaging computed with all de-noised looks. The results also demonstrate that the two-dimensional windowed Fourier transform filtering exhibits the best performance in all cases and that the block-matching 3D (BM3D) algorithm is second in the ranking.

Comparative study of multi-look processing for phase map de-noising in digital Fresnel holographic interferometry.

We report on a method for recovering data from a simple portable Digital Speckle Pattern Interferometer, we used for monitoring structural behavior of a painting on wood, hanging on a wall, outside of laboratory conditions, without anti-vibration devices. In such a situation, fringes produced by the object displacements were affected by unpredictable distortions caused by environment vibrations. However, an sufficient number of suitable, i.e., undistorted or barely distorted, fringe patterns usable for processing was found. We performed multiple acquisitions at a frame rate as high as possible. The main task was picking out usable interferograms from large amount of frames. We developed a PC-based method based on jointly analysis of spectral content and fringe image sharpness as selection rules. The selected frames were utilized for off-line processing by using an approach based on Hilbert Transform and Phase Unwrapping via MAx-flow (PUMA) algorithm. We obtained a collection of displacement-maps, that allowed for evaluating the whole structure deformations, caused by environmental thermo-hygrometric fluctuations.

Automatic Frames Extraction and Visualization From Noisy Fringe Sequences for Data Recovering in a Portable Digital Speckle Pattern Interferometer for NDI

Luca Pezzati

Papers

Automatic Frames Extraction and Visualization From Noisy Fringe Sequences for Data Recovering in a Portable Digital Speckle Pattern Interferometer for NDI