Fringe Projection Method Research Articles

Fringe Projection Method’s Evolving Role in Cultural Asset Cataloging and Conservation: Potentiality in Testing Conservative Restoration Techniques

Fringe Projection Method’s Evolving Role in Cultural Asset Cataloging and Conservation: Potentiality in Testing Conservative Restoration Techniques

Experimental and computational investigation into the damage mechanisms in composite metal hybrid panels subjected to high pressure blast loading

In this work, a free piston shock tube is utilized to conduct blast experiments on unidirectional glass fiber reinforced thermoplastic composite-metal hybrid panels. Damage and deflection characteristics are obtained in real time during the blast test using fringe projection methods, and post blast damage is assessed by nondestructive evaluation methods. Finite element models are developed in LS Dyna for the blast apparatus and hybrid composite panel and the results are compared to the experimental blast tests. The delamination in the computational model matched both size and shape to the delamination of the composite C-Scan. Furthermore, both dynamic deflection and permanent deflection of the hybrid panel correlated well between the experimental blast test and the LS Dyna blast simulation.

Quantitative phase modulation monitoring based on a shearing interferometry fringe projection method

It is known that digital projectors have a nonlinear effect that deforms the ideal sinusoidal fringe pattern to be nonsinusoidal and finally introduces errors. To obtain high-density and pure sine projection fringes, this paper proposes a shearing interferometry fringe projection (SIFP) system (also called interferometry phase measuring deflectometry, IPMD) as well as a unidirectional slope Fourier transform reconstruction algorithm. The experiments of the phase modulation monitoring are performed, and the result shows that the root-mean-square (RMS) values are 0.1044λ (for a Gaussian modulated phase) and 0.0848λ (for an aspheric modulated phase). Also, the experiment of the diopter detection for the ophthalmic glasses is carried out. The result shows that the measured diopter is −3.0073D (the ideal diopter is −3.0D), and the prediction error of the diopter is 0.0073D. All the results have proved that the proposed SIFP (IPMD) system and the unidirectional slope Fourier transform reconstruction algorithm are effective. The density of the interference fringe in the SIFP system can be easily adjusted, which effectively avoids the nonlinear effect caused by digital projectors. With the improvement of the unidirectional slope Fourier transform reconstruction algorithm, the application of the SIFP system in phase quantitative measurement can be further promoted. Simultaneously, the successful application of the proposed method in the diopter detection of the ophthalmic glasses may provide a new scheme for wavefront aberrations detection of human eyes. © 2020 Optica Publishing Group.

Phase-aided online self-correction method for high-accuracy three-dimensional measurement.

The binocular structured light 3D measurement system is widely used in situ industrial inspection and shape measurement, where the system structure is generally unstable due to mechanical loosening or environmental disturbance. Timely corrections to the changing structural parameters thus is an essential task for online high-accuracy measurement, which is difficult for traditional unidirectional fringe projection methods to self-correct the structural change. To this end, we propose an online self-correction method based on the investigation that orthogonal fringe projection can intrinsically relax the constraint on the epipolar geometry relationship and provide bidirectional phases for accurate corresponding point searching. Since orthogonal fringe projection may sacrifice the measurement efficiency, we further design a searching strategy by locally unwrapping one directional phase to reduce the number of projection patterns. Experimental results demonstrate that the proposed method is effective for online self-correction of unstable system structure to achieve high-accuracy 3D measurement under complex measurement environments.

An adaptive fringe projection method for 3D measurement with high-reflective surfaces

Fringe projection profilometry (FPP) has the advantages of non-contact, quick and high accuracy, which is popular in the fields of industrial inspection and measurement. However, the coded fringe patterns may be failure suffered from the high reflectivity of the measured surfaces. Aiming at this problem, an adaptive fringe projection method is proposed in this paper by the best projection intensity strategy and the coordinate mapping establishment between camera and projection regions. Combining the local camera response function, the dichotomizing search is used to confirm the best projection intensity of overexposed regions by pixel-wise. The unwrapping phase map of background is performed to establish the coordinate mapping relationship of measured overexposed regions between the camera and projection images, which is robust and convenient. The mapping errors caused by inaccurate phase information in the overexposed regions are effectively avoided, and the fringe projection patterns are adjusted adaptively, without additional equipment and low-expose patterns. Both theory and experiments have verified the feasibility and superiority of the proposed method.

Accurate measurement of high-reflective surface based on adaptive fringe projection technique

Fringe projection profilometry (FPP) is one of the most effective methods to obtain three-dimensional surface topography information about an object. However, for objects with large variations in surface reflectance, it is difficult to measure the saturated regions with conventional FPP. In this paper, a new adaptive fringe projection (AFP) method is proposed to solve this problem. Compared with existing methods, the proposed method can automatically generate low-intensity projected fringe patterns instead of requiring manual adjustment. First, we built a projection intensity model for the captured pixels and defined the surface reflection factor and environmental factor for saturated pixels. Then, a small number of uniform grayscale pattern sequences were projected to mark the saturated areas, while the surface reflection factor and the environment factor of individual pixels were calculated and a surface coefficient look-up table (SCLUT) was created. The low-intensity projected fringe patterns were automatically calculated and generated based on the captured uniform grayscale sequences of images to establish the mapping relationship between the camera image plane and the projector image plane. Finally, the optimal projection intensities of pixels in the saturated regions were calculated according to the SCLUT, and adaptive fringe patterns were generated. Experiments show that using the adaptive fringe patterns generated by the proposed method to measure objects can accurately adjust the projection intensity and substantially improve the measurement accuracy of high-reflective surface.

Comparison of Optical and Stylus Methods for Surface Texture Characterisation in Industrial Quality Assurance of Post-Processed Laser Metal Additive Ti-6Al-4V.

Additive manufacturing technologies enable lightweight, functionally integrated designs and development of biomimetic structures. They contribute to the reduction in material waste and decrease in overall process duration. A major challenge for the qualification for aerospace applications is the surface quality. Considering Ti-64 laser powder bed fusion (LPBF) parts, particle agglomerations and resulting re-entrant features are characteristic of the upper surface layer. Wet-chemical post-processing of the components ensures reproducible surface quality for improved fatigue behaviour and application of functional coatings. The 3D SurFin® and chemical milling treatments result in smoother surface finishes with characteristic properties. In order to characterise these surfaces, three methods for surface texture measurement (contact and non-contact) were applied, namely confocal microscopy, fringe projection and stylus profilometry. The aim of this work was to show their suitability for measurement of laser powder bed fusion as-built and post-processed surfaces and compare results across the evaluated surface conditions. A user-oriented rating of the methods, summarising advantages and disadvantages of the used instruments specifically and the methods in general, is provided. Confocal microscopy reaches the highest resolution amongst the methods, but measurements take a long time. The raw data exhibit large measurement artefacts for as-built and chemically milled conditions, requiring proper data post-processing. The stylus method can only capture 2D profiles and the measurement was restricted by particle agglomerations and craters. However, the method (process and instrument) is entirely standardised and handheld devices are inexpensive, making it accessible for a large group of users. The fringe projection method was the quickest and easiest regarding measurement and data post-processing. Due to large areal coverage, reproduction of location when performing repeat measurements is possible. The spatial resolution is lower than for confocal microscopy but is still considered sufficiently high to characterise the investigated surface conditions.

Influence of Contour Scan Variation on Surface, Bulk and Mechanical Properties of LPBF-Processed AlSi7Mg0.6.

Metal additive manufacturing technologies have great potential for future use in load-bearing aerospace applications, requiring a deeper understanding of mechanical performance and influencing factors. The objective of this study was to investigate the influence of contour scan variation on surface quality, tensile and fatigue strength for laser powder bed fusion samples made of AlSi7Mg0.6 material and to create high-quality as-built surfaces. The samples were produced with identical bulk and different contour scan parameters to accommodate the investigation of the impact of as-built surface texture on mechanical properties. The bulk quality was evaluated by density measurements according to Archimedes' principle and tensile testing. The surfaces were investigated using the optical fringe projection method, and surface quality was assessed by the areal surface texture parameters Sa (arithmetic mean height) and Sk (core height, derived from material ratio curve). Fatigue life was tested at different load levels, and the endurance limit was estimated based on a logarithmic-linear relation between number of cycles and stress. All samples were found to have a relative density of more than 99%. Surface conditions distinctive in Sa and Sk were successfully created. The resulting mean values of the ultimate tensile strength σult are between 375 and 405 MPa for 7 different surface conditions. It was confirmed that the influence of contour scan variation on bulk quality is insignificant for the assessed samples. Regarding fatigue, one as-built condition was found to perform as well as surface post-processed parts and better than the as-cast material (compared to literature values). The fatigue strength at the endurance limit for 106 cycles is between 45 and 84 MPa for the three considered surface conditions.

Fringe Projection Method for 3D High-Resolution Reconstruction of Oil Painting Surfaces

The fringe projection (FP) method is an outstanding tool for reconstructing painted surfaces. This technique, which has been used for conservation and digitization, does not damage the artwork and can reach sub-millimeter accuracy. To carry out this type of analysis, it is necessary to achieve the most accurate measurements possible. Measuring the precision that a projector-camera-object arrangement can achieve is a complex task. In this paper, we show an experimental method used to measure the accuracy of this technique with instrumentation within the reach of most conservation laboratories. The method consists of capturing, as a reference model, a stepped cylindrical Nylamid® pyramid, as a construction whose shape, size, and manufacturing accuracy are known with high precision. The pyramid has eight well-defined steps, which are fashioned with an accuracy more exact than that of the fringe projection method. The height of each step was measured, obtaining the mean and variance of the height measurements fitted to a Gaussian distribution. In this work, we show the measured heights of the steps, obtained by varying the period of the fringes. The smallest detectable step height was less than 44.1 µm; however, this was obtained with a variance in the order of the step height. The smallest detectable step height with a small variance was 0.1008 mm. In addition to this accuracy measurement, a qualitative evaluation of a painting was carried out, finding the presence of possible superimposed thin layers, fabric, and microcracks, which commonly occur in the drying and aging processes. Further research would provide an experimental measurement of the method’s accuracy and its variance as essential for obtaining a confidence criterion that could then be applied to the model of the painting’s surface.

A new method for high dynamic range 3D measurement combining adaptive fringe projection and original-inverse fringe projection

For fringe projection profilometry (FPP), high reflections on metal, plastic, and ceramic surfaces cause intensity saturation, resulting in three dimensional (3D) measurement errors. The measurement efficiency using the multiple exposure method (MEM) is low and the exposure time is difficult to quantify. The original and inverse fringe projection method (OIFPM) suffers from saturation of the opposite fringes on highly reflective surfaces. The adaptive fringe projection method (AFPM) is still limited by image dynamic range and pixel mapping accuracy. To this end, this paper proposes a new scheme combining OIFPM and AFPM, called the original and inverse adaptive fringe projection method (OIAFPM). Firstly, an adaptive intensity template in the camera coordinate system is generated. Secondly, a pixel mapping between the camera and projector images is established. Thirdly, original and inverse adaptive fringes are generated and projected. Finally, 72 wrapped phase calculation equations are given in detail. Experiments confirm that the 3D measurement employing the proposed OIAFPM is much better than that employing AFPM and OIFPM. Although the 3D measurement employing MEM is similar to that employing OIAFPM, the measurement efficiency employing OIAFPM is much higher than that employing MEM.

Three-Dimensional Measurement Method for High Dynamic Range Surfaces Based on Adaptive Fringe Projection

As an optical measurement method, fringe projection three-dimensional (3D) measurement technology will have poor measurement effect due to overexposure or too dark of the fringes when measuring high dynamic range (HDR) surfaces. The adaptive fringe projection method is one of the effective methods to solve the HDR surfaces measurement, but pixel matching is the difficulty. The existing matching methods generally use phase for matching, which requires at least 24 additional patterns to be projected, seriously affecting the measurement efficiency. In order to solve these problems, an adaptive fringe projection measurement method using speckle pattern for pixel matching is proposed in this paper. First, two grayscale patterns are projected to calculate the adaptive projection intensity. Then a speckle pattern is projected to match the pixels of the captured pattern and the projected pattern. Finally, the adaptive fringe patterns are generated. Better than the methods before, only three additional patterns are needed to be projected to realize the measurement of the HDR surfaces. The experimental results show that the proposed method can reduce the number of projected patterns by more than 59.5% while achieving similar measurement accuracy, which greatly improves the measurement efficiency.

Warpage characterization of a large size fan-out panel subjected to inhomogeneous heating by a digital fringe projection system

Warpage is an essential issue of fan-out wafer/panel level packaging. Increasing size of wafers/panels poses new challenges to the warpage characterization system. In this work, a warpage measurement system based on digital fringe projection method is reported. It can cover 600 mm × 600 mm area with 4 × 4 camera array. Benchmark tests were carried out to evaluate the measurement error, which was found to be 0.053 %. The warpage evolution of a 515 mm × 510 mm panel with inhomogeneous heating was then studied by heating from room temperature to 90 °C. It was found that the warpage evolution of a large size panel can be very complex. Based on finite element modeling results, this can be attributed to temperature gradient, and several other factors, including mismatch of coefficient of thermal expansion, gravity, initial stress state and original layout design of the panel (such as size and count of chips, RDL design and chip position).

Three-dimensional reconstruction from a fringe projection system through a planar transparent medium.

A vision measurement system is placed in a protective cover made of a transparent medium to avoid environmental influences. Due to the deflection of light rays on the front and rear surfaces of the transparent medium, the imaging position of an object on the camera target plane is deviated, which makes the traditional vision detection methods based on the triangulation principle produce large measurement errors. This work introduces a three-dimensional (3D) reconstruction method by fringe projection system through a planar transparent medium. We derive the coordinate transformation relationship between a real-object point and a pseudo-object point caused by light refraction based on Snell's law of flat refraction. Based on the relationship, a modified fringe projection method is proposed for unbiased 3D reconstruction. Two experiments, including 3D shape measurement of a white plate with ring markers and 3D shape measurement of a regular spherical object are conducted. The results demonstrate the effectiveness of the proposed method in such measurement environment.

Rapid 3D measurement of colour objects based on three-channel sinusoidal fringe projection

A rapid 3D measurement method based on three-channel projection is proposed in this paper. Project a composite color fringe map and an all-white map to the object to be measured. Firstly, crosstalk correction is performed on the composite color fringe pattern collected by the camera then the reflectivity of fringe image is corrected by all-white image. Then, the background normalization principle is used to solve the spectrum aliasing problem when the Fourier transform obtains a truncated phase. Finally, the three-frequency heterodyne method is used for phase unwrapping to obtain the continuous phase and 3D shape. Compared with the traditional color fringe projection method, this method not only eliminates the spectrum aliasing problem but also solves the uneven fringe reflectivity problem when measuring color objects using an all-white map. This method only requires two images to achieve measurement of 3D color dynamic objects.

A 3D shape measurement method for high-reflective surface based on accurate adaptive fringe projection

Fringe projection profilometry (FPP) is a popular three-dimensional (3D) shape measurement technique with advantages of high-precision and non-contact. However, measuring high-reflective surface is still a challenging task for conventional FPP because image saturation leads to absolute phase errors and reconstruction errors. In this paper, a new adaptive fringe projection (AFP) method is proposed. Compared with existing AFP methods, we simplify the experimental process and get more accurate coordinate mapping. First, we built the projection intensity model for each projector pixel rather than camera pixel. Then, a uniform gray pattern was projected and two images were captured under high and low exposures respectively to calculate the quantitative low projection intensity. In coordinate mapping, sub-pixel coordinates were mapped from camera image onto projector image. They were filtered with absolute phase to deal with resolution difference between camera and projector and get accurate coordinate correspondence. Finally, adaptive fringe patterns were generated. Besides, we proposed a novel quantitative criterion to evaluate the effectiveness of AFP methods called point cloud integrality (PCI), which is calculated based on the number of 3D points of high-reflective areas. The experiments demonstrate that the proposed method increases both the measurement accuracy and PCI compared with conventional FPP and some existing AFP methods.

Improved dual-frequency phase-coding fringe projection method for 3D shape measurement

The phase-coding method is useful in 3D shape measurement due to its low sensitivity to surface contrast and camera noise. However, as the number of fringe periods increases, the wrapped phase recovered from the sinusoidal phase-shifting patterns is always misaligned with the fringe order calculated by the stair-coding patterns. To solve this, an improved dual-frequency phase-coding fringe projection method is designed to realize absolute phase retrieval. Specifically, the auxiliary fringe order determined by phase-shifting is used to get the absolute phase, which can eliminate fringe-order errors. Moreover, to increase speed and the number of codewords, this paper presents a scheme that only requires seven patterns, including a uniform gray image, two high-frequency sinusoidal fringe patterns, two low-frequency sinusoidal fringe patterns and two phase-coding fringe patterns. Therefore, this method requires fewer patterns to realize high-resolution measurement than existing methods. The success of the proposed method can be proved by experiments.

Long depth-range measurement for fringe projection photogrammetry using calibration method with two reference planes

Using a calibration method with two reference planes, this study proposes a simple method to extend a measurement range in an optical direction for a fringe projection technique. The proposed method only inserts a baseline to the conventional fringe pattern projected onto an object surface and computes an exact phase alignment in the extended area concerning the baseline. The correction coefficient for the fundamental equation calculating 3D position coordinates of the object surface is developed to prevent accuracy degradation caused by extending the measurement range. Regardless of the extension of the measurement range, the accuracy verification test demonstrates that the proposed fringe projection method provides a measurement result with the same accuracy as the conventional method. Furthermore, through the surface shape measurement of the stationary parabola antenna reflector having a diameter of 1.5 m, a pendulum motion of the flat plate with a large amplitude, and a complicated deployment behavior of the flexible membrane structure, effectiveness and applicability of the proposed method for actual structures are demonstrated. This study offers a fringe projection photogrammetry applicable to capture dynamic responses of the object surface with a large amplitude and the large-sized stationary object surface with high spatial resolution.

Simultaneous extraction of profile and surface roughness of 3D SLM components using fringe projection method

PurposeThe purpose of this study was expected to simultaneously monitor the surface roughness of each solidified layer, the surface roughness of the metal powder, the outline of the solidified layer, and the height difference between the solidified layer and the metal powder.Design/methodology/approachIn the proposed approach, color images with red, green and blue fringes are used to measure the shape of the built object using a three-step phase-shift algorithm and phase-unwrapping method. In addition, the surface roughness is extracted from the speckle information in the captured image using a predetermined autocorrelation function.FindingsThe feasibility and accuracy of the proposed system were validated by comparing it with a commercial system for an identical set of samples fabricated by a selective laser melting process. The maximum and minimum errors between the two systems are approximately 24% and 0.8%, respectively.Originality/valueIn the additive manufacturing field, the authors are the first to use fringe detection technology to simultaneously measure the profile of the printed layer and its surface roughness.

Fringe-width encoded patterns for 3D surface profilometry.

This paper presents a new fringe projection method for surface-shape measurement that uses novel fringe-width encoded fringe patterns. Specifically, the projection patterns are adjusted with the width of the fringe as the codeword. The wrapped phase with coding information is obtained by using the conventional wrapped phase calculation method, and the fringe order can be identified from the wrapped phase. After the fringe order is corrected based on the region growing algorithm, the fringe order and the wrapped phase can be directly used to reconstruct the surface. Static and dynamic measurements demonstrated the ability of the method to perform 3D shape measurement with only three projected patterns, single camera and projector in the least case.

Single-shot circular fringe projection for the profiling of objects having surface discontinuities.

Fringe projection profilometry (FPP) is a widely used non-contact optical method for 3D profiling of objects. The commonly used linear fringe pattern in FPP has periodic intensity variations along the lateral direction. As a result, the linear fringe pattern used in FPP cannot uniquely represent the lateral shift induced by the objects having surface discontinuities. Thus, unambiguous surface profiling of objects, especially with surface discontinuities, using a single linear fringe image having a single fringe frequency, is unfeasible. This paper proposes using a radially symmetric circular fringe pattern as the structured light pattern for accurate unambiguous surface profiling of sudden height-discontinuous objects. To the best of our knowledge, this is the only method that can reconstruct discontinuous height profiles with the help of a single fringe image having a single frequency. The performance of the proposed algorithm is evaluated on several synthetic and real objects having smooth variations and discontinuities. Compared to the well-known fringe projection methods, the results depict that for a tolerable range of error, the proposed method can be applied for the reconstruction of objects with 4 times higher dynamic range and even at much lower fringe frequencies.