Digital Fringe Patterns Research Articles

Adaptive Phase Error Suppression Concerning 3D surface Deformation Measurement on Color Digital Fringe Projection Profilometry

3D surface measurement based on phase-shifting profilometry (PSP) has been actively developed in recent years. Three color channels of RGB that are modulated to generate a one-shot PSP method is a concept of color digital fringe pattern profilometry (CDFPP). The CDFPP is a promising technique for the 3D imaging profile of dynamic surface deformation if several phase errors in the one-shot PSP method can be suppressed. This work proposes a processing scheme for phase error suppression schemes (PESS) based on retrieving the modulated sinusoidal fringe and color fringe normalization in PSP using RGB color channel. The processing of PESS consists of tunable bandpass filtering (BPF) followed by fringe normalization. The initial BPF function is defined based on a smoothing spline data set of frequency and power spectrum from the baseline color fringe image. The predefine BPF function could be tunable during the imaging process by considering each frame's condition and RGB channel spectrum mapping. The corrected fringe images are then normalized from the color imbalance, and the phase shift is calculated using the conventional three-step PSP. For evaluation, PESS is performed to reconstruct simulator membrane deformation from four different static profiles and tested to observe the 3D surface of continuous membrane deformation. The PESS could suppress the phase errors of less than 30% less absolute errors than the conventional method and successfully reconstruct the 3D surface for low-frequency continuous membrane deformation with minimizing phase errors.

Application of S-transform and sub-pixel parallax for optical three-dimensional measurement without phase unwrapping

In the optical three-dimensional measurements of complex objects, the process of phase unwrapping affects the measurement accuracy and measurement speed. To avoid phase unwrapping, S-transform and sub-pixel matching are applied. Firstly, a digital fringe pattern is projected to an object and the wrapped phase can be extracted. Secondly, an initial parallax is calculated based on the deformed fringe pattern. According to the principle of stereo vision, candidate matching points can be calculated. Thirdly, phase slope function can be defined and the edge points can be confirmed from the phase slope. Finally, the sub-pixel parallax is calculated and 3D information can be obtained based on system calibration parameters. Experiments and results verify that the proposed method can provide high-speed and accurate 3D information, especially for discontinuous objects and shiny objects.

Computed time average digital holographic fringe pattern under random excitation.

Time average digital holography under random excitation or square wave excitation is established as an on-site non-destructive testing tool for defect detection in large metallic and composite sandwich structures due to its high sensitivity, single exposure interferogram, and fast inspection capability. However, extensive calibration studies are necessary to corroborate the defect type and defect size with the excitation frequency range and excitation magnitude. In this paper, a method to simulate a time average digital holographic fringe pattern under random excitation is proposed with the idea to minimize the number of calibration experiments and also for better evaluation of the size and type of defect. The proposed method circumvents the requirement for a closed form expression for the complex characteristic fringe function for time average interferometry under random excitation. The computed fringe pattern is illustratively compared with an experimental time average digital holographic fringe pattern.

High-resolution Three-dimensional Surface Imaging Microscope Based on Digital Fringe Projection Technique

Abstract The three-dimensional (3-D) micro-scale surface imaging system based on the digital fringe projection technique for the assessments of microfiber and metric screw is presented in this paper. The proposed system comprises a digital light processing (DLP) projector, a set of optical lenses, a microscope, and a charge coupled device (CCD). The digital seven-step fringe patterns from the DLP projector pass through a set of optical lenses before being focused on the target surface. A set of optical lenses is designed for adjustment and size coupling of fringe patterns. A high-resolution CCD camera is employed to picture these distorted fringe patterns. The wrapped phase map is calculated by seven-step phase-shifting calculation from these distorted fringe patterns. The unwrapping calculation with quality guided path is introduced to compute the absolute phase values. The dimensional calibration methods are used to acquire the transformation between real 3-D shape and the absolute phase value. The capability of complex surface measurement for our system is demonstrated by using ISO standard screw M1.6. The experimental results for microfiber with 3 μm diameter indicate that the spatial and vertical resolutions can reach about 3 μm in our system. The proposed system provides a fast digital imaging system to examine the surface features with high-resolution for automatic optical inspection industry.

Nonlinear gamma correction via normed bicoherence minimization in optical fringe projection metrology

We propose a method to compensate for the projector intensity nonlinearity induced by gamma effect in three-dimensional (3-D) fringe projection metrology by extending high-order spectra analysis and bispectral norm minimization to digital sinusoidal fringe pattern analysis. The bispectrum estimate allows extraction of vital signal information features such as spectral component correlation relationships in fringe pattern images. Our approach exploits the fact that gamma introduces high-order harmonic correlations in the affected fringe pattern image. Estimation and compensation of projector nonlinearity is realized by detecting and minimizing the normed bispectral coherence of these correlations. The proposed technique does not require calibration information and technical knowledge or specification of fringe projection unit. This is promising for developing a modular and calibration-invariant model for intensity nonlinear gamma compensation in digital fringe pattern projection profilometry. Experimental and numerical simulation results demonstrate this method to be efficient and effective in improving the phase measuring accuracies with phase-shifting fringe pattern projection profilometry.

Optimal defocus selection based on normed Fourier transform for digital fringe pattern profilometry.

Owing to gamma-effect robustness and high-speed imaging capabilities, projector defocusing of binary-coded fringe patterns is by far the most widely used and effective technique in generating sinusoidal fringe patterns for three-dimensional optical topography measurement with digital fringe projection techniques. However, this technique is not trouble-free. It is borne with uncertainty and challenges mainly because it remains somewhat difficult to quantify and ascertain the level of defocus required for desired fidelity in sinuousness of the projected fringe pattern. Too much or too little defocusing will affect the sinuosity accuracy of fringe patterns and consequently jeopardize the quality of the measurement results. In this paper, by combining intrinsic phase spectral sensitivities and normed Fourier transform, a method to quantify the amount of defocus and subsequently select the optimal degree of sinuosity for generating digital sinusoidal fringe patterns with projector defocusing for fringe pattern optical three-dimensional profilometry is proposed. Numerical simulations plus experiments give evidence of the feasibility and validity of the proposed method in enabling an improved digital binary defocusing technique for optical phase-shift profilometry using the digital fringe projection technique.

Experimental study of temporal-spatial binary pattern projection for 3D shape acquisition.

Three-dimensional (3D) acquisition of an object with modest accuracy and speed is of particular concern in practice. The performance of digital sinusoidal fringe pattern projection using an off-the-shelf digital video projector is generally discounted by the nonlinearity and low switch rate. In this paper, a binary encoding method to encode one computer-generated standard sinusoidal fringe pattern is presented for circumventing such deficiencies. In previous work [Opt. Eng.54, 054108 (2015)OPEGAR0091-328610.1117/1.OE.54.5.054108], we have developed a 3D system based on this encoding tactic and showed its prospective application. Here, we first build a physical model to explain the mechanism of how to generate good sinusoidality. The phase accuracy with respect to the conventional spatial binary encoding method and sinusoidal fringe pattern is also comparatively evaluated through simulation and experiments. We also adopt two phase-height mapping relationships to experimentally compare the measurement accuracy among them. The results indicate that the proposed binary encoding strategy has a comparable performance to that of sinusoidal fringe pattern projection and enjoys advantages over the spatial binary method under the same conditions.

Real-time structured light-based otoscopy for quantitative measurement of eardrum deformation.

An otological profilometry device based on real-time structured light triangulation is presented. A clinical otoscope head is mounted onto a custom-handheld unit containing both a small digital light projector and a high-speed digital camera. Digital fringe patterns are projected onto the eardrum surface and are recorded at a rate of 120 unique frames per second. The relative angle between projection and camera axes causes the projected patterns to appear deformed by the eardrum shape, allowing its full-field three-dimensional (3-D) surface map to be reconstructed. By combining hardware triggering between projector and camera with a dedicated parallel processing pipeline, the proposed system is capable of acquiring a live stream of point clouds of over 300,000 data points per frame at a rate of 40 Hz. Real-time eardrum profilometry adds an additional dimension of depth to the standard two-dimensional otoscopy image and provides a noninvasive tool to enhance the qualitative depth perception of the clinical operator with quantitative 3-D data. Visualization of the eardrum from different perspectives can improve the diagnosis of existing and the detection of impending middle ear pathology. The capability of the device to detect small middle ear pressure changes by monitoring eardrum deformation in real time is demonstrated.

Phase Error Caused by Speed Mismatch Analysis in the Line-Scan Defect Detection by Using Fourier Transform Technique

The phase error caused by the speed mismatch issue is researched in the line-scan images capturing 3D profile measurement. The experimental system is constructed by a line-scan CCD camera, an object moving device, a digital fringe pattern projector, and a personal computer. In the experiment procedure, the detected object is moving relative to the image capturing system by using a motorized translation stage in a stable velocity. The digital fringe pattern is projected onto the detected object, and then the deformed patterns are captured and recorded in the computer. The object surface profile can be calculated by the Fourier transform profilometry. However, the moving speed mismatch error will still exist in most of the engineering application occasion even after an image system calibration. When the moving speed of the detected object is faster than the expected value, the captured image will be compressed in the moving direction of the detected object. In order to overcome this kind of measurement error, an image recovering algorithm is proposed to reconstruct the original compressed image. Thus, the phase values can be extracted much more accurately by the reconstructed images. And then, the phase error distribution caused by the speed mismatch is analyzed by the simulation and experimental methods.

Parameter discretization in two-dimensional continuous wavelet transform for fast fringe pattern analysis

The two-dimensional continuous wavelet transform (2D-CWT) technique provides robust processing for digital fringe pattern analysis. To cope with the problem of long computation time, a concept called the cover map is introduced to speed up the 2D-CWT analysis. The cover map is constructed by discretizing the continuous dilation and rotation parameters. The discretized parameters help substantially reduce the processing time without affecting the analysis accuracy. The theories are presented and the validity and effectiveness of the proposed concept are demonstrated by computer simulation and real experiment.

Comparison of Multi-Frequency Phase Unwrapping Algorithm for Shape Measurement Using Digital Fringe Projection

As technology development, projected digital fringe pattern is widely used in measuring three- dimensional (3D) shape, even in the presence of surface discontinuities. This paper investigates the reliability and accuracy of two multi-frequency phase unwrapping algorithms by experimental means. The first, which unwraps through a sequence of phase maps produced with exponentially change in spatial frequency with time, is found to be less robust and accuracy than the second, which only use three different frequency to beat new frequency for unwrapping the phase. Through experiment, the best frequency, adjacent ratio for the first method and frequency difference for the second method are proposed for the high efficiency and accuracy measurement.

1239 POSTER Radiodermatitis affects more than a half of cancer patients submitted to radiotherapy, hence skin support measures mainly with urea are now widely recommended. The ESPI project

Objective: To examine the in-plane and out-of-plane response of human dentine to thermal loads in real time.Methods: An Electronic Speckle Pattern Interferometry (ESPI) system sensitive to both the in-plane and out-of-plane displacements was configured and used in conjunction with an advanced fringe processing technique. Specimens were prepared from freshly extracted lower central incisor teeth and were separately mounted on a thermal block to apply thermal loads from room temperature (25°C) to 60°C. The real time speckle patterns were acquired using a digital camera. These digital fringe patterns were subjected to further image processing to enhance the quality of fringes. The resultant images were later analyzed to study the out-of-plane and in-plane displacement gradients in the facio-lingual plane of the dentine.Results: The out-of-plane deformations were observed in the plane perpendicular to the long axis of the tooth, while the in-plane deformations occurred in the plane parallel to the long axis of the tooth.Conclusion: The ESPI analysis revealed whole-field and distinct thermal response in human dentine in-plane and out-of-plane. The cervical dentine experienced distinct and conspicuous displacement to the temperature changes.

Automatic Full-Field 3-D Profilometry Using White Light Confocal Microscopy with DMD-Based Fringe Projection

A new full-field 3-D micro surface profilometer using digital micromirror device (DMD)-based fringe projection strategy and confocal principle is presented in the article. In viewing the fact that conventional laser confocal measurement method not only easily encounters undesired irregular scattering problems, but also lack scanning efficiency due to its single-point type measurement, the newly developed automatic surface profilometer deploys a DMD chip to project spatially encoded digital fringe patterns with dynamic light intensity, onto the object to obtain excellent measurement performance. A novel digital fringe pattern design with adaptive sinusoidal intensity modulation was developed for active fringe projection, to obtain optimized depth resolution with a micrometer lateral resolution in confocal measurement. Some of semiconductor components have been measured to attest the feasibility of the developed approach. The depth measurement resolution can reach better than 0.1μm and the maximal measured error was verified to be less than less than 0.5 % of the measured step size.

Mechanical vibration measurement by high-resolution time-averaged digital holography

In digital holography, a challenge inherited from speckle interferometry is the quantitative data processing of time-averaged holograms in view of estimating the full field of vibration amplitudes. Fundamentally, the greatest obstacle comes from the orthogonal components of time-averaged digital holograms, corrupted by multiplicative, high-frequency phase noise covering the deterministic vibration-related phase. The paper describes a novel method of obtaining through digital holography high-resolution time-averaged fringe patterns. The three-level approach includes the elimination of the multiplicative high-frequency phase noise by synchronous detection followed by low-pass filtering. The high-frequency information needed by this procedure is taken from the digital hologram of the object at rest. The complete procedure includes an automatic correction of the environmental phase drift introduced by the synchronous detection. The different stages of data processing are illustrated by experimental results.

Analysis on the nature of thermally induced deformation in human dentine by electronic speckle pattern interferometry (ESPI)

Objective: To examine the in-plane and out-of-plane response of human dentine to thermal loads in real time. Methods: An Electronic Speckle Pattern Interferometry (ESPI) system sensitive to both the in-plane and out-of-plane displacements was configured and used in conjunction with an advanced fringe processing technique. Specimens were prepared from freshly extracted lower central incisor teeth and were separately mounted on a thermal block to apply thermal loads from room temperature (25°C) to 60°C. The real time speckle patterns were acquired using a digital camera. These digital fringe patterns were subjected to further image processing to enhance the quality of fringes. The resultant images were later analyzed to study the out-of-plane and in-plane displacement gradients in the facio-lingual plane of the dentine. Results: The out-of-plane deformations were observed in the plane perpendicular to the long axis of the tooth, while the in-plane deformations occurred in the plane parallel to the long axis of the tooth. Conclusion: The ESPI analysis revealed whole-field and distinct thermal response in human dentine in-plane and out-of-plane. The cervical dentine experienced distinct and conspicuous displacement to the temperature changes.

Stress distribution in the dento-alveolar system using digital photoelasticity.

Past research has confirmed that the governing factors in cellular modelling and remodelling adhere to sound principles of engineering mechanics. Hence studies of stress distributions would provide better understanding of the functional adaptation of dental supporting structures. Photoelasticity is an established experimental tool to study whole-field stress distribution in structures subjected to forces. However, it has certain limitations that make its application in biological specimens tedious. In this investigation an advanced digital photoelastic system is used to visualize and study the nature of the stress distribution in dental supporting structures. These digital fringe patterns are analysed using a phase-shift technique. The present biomechanical study shows that dental supporting structures exhibit a characteristic stress distribution, promoting structural adaptation based on needs. Furthermore, the advantage of using a digital image processing system along with the circular polariscope is discussed.

Optically enhanced tiling (OET) in digital fringe pattern analysis

AbstractIn digital fringe pattern analysis information is usually lost in high fringe density zones due to digitisation and quantisation errors. A generic approach namely optically enhanced tiling (OET) is presented to construct a composite picture from multiple optically zoomed images. The procedure is explained with the help of the problem of data acquisition in digital photoelasticity using phase shifting technique.

Shear measurement in digital speckle shearing interferometry using digital correlation

Accurate computations of surface slope deformation based on digital speckle shearing interferometry fringe patterns require a precise measure of the amount of shear introduced. A previous method of employing image processing, while providing accurate and fast measurements, is limited by the need to place a tape target on the object surface. In this paper, a novel method employing digital correlation is demonstrated to obviate this requirement while maintaining accurate mensurations of shear. It also allows automated measurement procedures to be readily incorporated.