Fringe Modulation Research Articles

Structured light 3D shape measurement for translucent media base on deep Bayesian inference

Traditional structured light technique faces challenges in measuring translucent media due to low fringe modulation and strong random noise caused by subsurface scattering, thereby significantly reducing phase quality. In addition, the difficulty in obtaining ground truth makes it hard to assess reliability even though obtaining measured results. Here, we proposed a 3D measurement method for translucent media base on deep Bayesian inference to achieve both fringe enhancement and phase uncertainty evaluation. Specifically, a deep network incorporated with quatuor-branch residual block is developed to significantly enhance the fringe modulation and signal-to-noise ratio (SNR) for accurate phase recovery. Subsequently, a Bayesian inference mechanism is established for probabilistic statistics, which allows for the optimization of fringe output and provides uncertainty self-evaluation based on Monte Carlo (MC) sampling. Furthermore, by incorporating both numerical and physical constraints into the supervised learning, the network can effectively mitigate phase-shifted errors in the final results. The proposed method shows high efficiency and flexibility since it requires no additional patterns or hardware setup. Experiments validate the feasibility of the proposed method.

Sinusoidal parameter estimation technique for three-dimensional object reconstruction from a single object capture

Fringe projection profilometry is widely used for three-dimensional measurements. Sinusoidal patterns are commonly used; those patterns are modulated by the object’s topography, raising the necessity to relate this modulation with the object’s shape. Several approaches have been proposed and can be classified as spatial or temporal algorithms, making them suitable for dynamic or static objects. In this paper, an improvement of a different approach to analyze the modulated fringe pattern is proposed; from a single fringe pattern, the object’s shape can be found with the determination of maximums and minimums of the signal, avoiding the use of Fourier transform, wavelet transform, and Riez transform techniques.

Multi-wavelength pinhole point diffraction interferometry for optics metrology with interferometric intensity based phase retrieval method

In this paper, we propose a multi-wavelength pinhole point diffraction interferometry (MPPDI) and a two-step phase extraction method based on the interferometric intensity information. MPPDI is mainly used to extend the dynamic measurement range of traditional pinhole point diffraction interferometry (PPDI) to realize the measurement of large-aperture and high-order aspheric surface. Spherical/aspherical optics will be test under three different wavelengths, and the corresponding interferograms are recoded by 3CMOS detector at the same moment. By combining different wavelengths, MPPDI can obtain a synthetic wavelength with a larger measuring range and greater flexibility. In interferometry, the choice of the wavelength of the light source has a great influence on the measurement range and accuracy. In MPPDI, we can choose different wavelength combinations to select the most suitable synthetic wavelength according to the asphericity of the tested mirror. To enhance efficiency and reduce the impact of air disturbances during phase shifts, we propose a new phase extraction method based on interferometric intensity to interpret the phase map of surface information. The background intensity of diffraction wavefront is firstly pre-recorded by 3CMOS, then phase-shift interferograms of test mirror is acquired with two-step piezoelectric ceramic driver (PZT) movement. From the analysis, it can be found that fringe modulation has tiny influence on the interferograms intensity. Therefore, based on global intensity distribution of fringe pattern and grayscale information, phase map can be retrieval using proposed two-step processing method (only single phase-shift). The two-step phase extraction method can reduce the number of phase shifts from 9 to 3, which improves the efficiency and reduces errors due to multiple phase shifts. MPPDI and two-step phase extraction we proposed have a larger measurement while reducing the number of phase shifts and avoiding error accumulation due to too many phase shifts. Suitable for large-aperture and high-order aspheric surface measurement.

Hilbert transform for modulation of interference fringes of liquid substances

The demodulation of interferometric signals presents a challenge in the study of object, especially when working in environments with disturbances, instability or noise. This document presents a procedure to demodulate a pair of highly noisy interferograms obtained with a fiber optic Mach Zehnder interferometer, the procedure is based on using the reconfiguration of the interference fringes as a result of Hilbert Transform. The method shown is fast, it does not present susceptibilities at high frequencies, it avoids demodulating interference patterns of closed fringes and with little carrier signal, in addition, it allows to verify the mathematical equivalence of the spectral analysis using Fourier Transform. As an application model, samples of liquid substances were used, where it was possible to know changes between substances according to their density

Image Quality Improvement in Sparse-View X-Ray Phase-Contrast Trimodal CBCT With Multifrequency Fringe Modulation and Iterative Methods

X-ray grating interferometer can simultaneously retrieve attenuation, refraction and scattering signals of the imaged sample, which shows its great potential in biomedical imaging. However, fast scan of gratings and high centrifugal forces during rotating in X-ray grating-based cone-beam computed tomography (CBCT) system may lead to phase-stepping errors and grating position fluctuations, which cause reconstruction failure with phase-stepping (PS) method. Whats more, the reconstructed slices usually suffer from severe artifacts under low dose conditions. Herein, we propose a multi-frequency model to simulate the mechanical instability of the system and put forward novel iterative image reconstruction methods named MFM-TLI and MLEM-FISTA-TV to improve image quality in sparse-view trimodal CBCT. Results reveal that the model and MFM-TLI is effective to retrieve high quality absorption, differential phase contrast (DPC) and dark-field (DF) signals under mechanical instability, significantly outperforming the PS method. Image quality reconstructed by MLEM-FISTA-TV is improved compared to other iterative and analytical algorithms with sparse-view data. To the best of our knowledge, this is the first time multi-frequency method is applied in X-ray grating interferometer and iterative MLEM-TV algorithm is evaluated on experimental sparse-view dark-field CBCT data. This work might provide support for future trimodal CBCT reconstruction.

Estimation Method Based on Extended Kalman Filter for Uncertain Phase Shifts in Phase-Measuring Profilometry

Phase-measuring profilometry (PMP) is increasingly applied in high-accuracy three-dimensional shape measurement. However, various factors may result in the uncertainty of phase shift values in the PMP system, and phase errors induced by actual phase shift often bring about the reconstruction failure of a measured object. A quadratic phase estimation method using the extended Kalman filter is proposed to remove the phase error introduced by uncertain phase shift. After eliminating the background and fringe modulation, the state estimation is employed to evaluate the quadratic phase coefficients in a selected mask window, and the phase shifts of adjacent fringe patterns can be estimated to compute the unwrapping phase. This paper presents a novel method for improving the accuracy of the PMP system influenced by phase shift errors, and the proposed region-wise method significantly enhances the reconstruction quality and efficiency. Experimental results show that the proposed algorithm effectively evaluates the actual phase shift and directly compensates the phase error, and has the advantages of high speed, high accuracy, and robustness.

Ultrafast beam pattern modulation by superposition of chirped optical vortex pulses

As an extension of pulse shaping techniques using the space–time coupling of ultrashort pulses or chirped pulses, we demonstrated the ultrafast beam pattern modulation by the superposition of chirped optical vortex pulses with orthogonal spatial modes. The stable and robust modulations with a modulation frequency of sub-THz were carried out by using the precise phase control technique of the constituent pulses in both the spatial and time/frequency domains. The performed modulations were ultrafast ring-shaped optical lattice modulation with 2, 4 and 6 petals, and beam pattern modulations in the radial direction. The simple linear fringe modulation was also demonstrated with chirped spatially Gaussian pulses. While the input pulse energy of the pulses to be modulated was 360 upmu J, the output pulse energy of the modulated pulses was 115 upmu J with the conversion efficiency of sim 32%. Demonstrating the superposition of orthogonal spatial modes in several ways, this ultrafast beam pattern modulation technique with high intensity can be applicable to the spatially coherent excitation of quasi-particles or collective excitation of charge and spin with dynamic degrees of freedom. Furthermore, we analyzed the Poynting vector and OAM of the composed chirped OV pulses. Although the ring-shaped optical lattice composed of OV pulse with topological charges of pm , ell is rotated in a sub-THz frequency, the net orbital angular momentum (OAM) averaged over one optical period is found to be negligible. Hence, it is necessary to require careful attention to the application of the OAM transfer interaction with matter by employing such rotating ring-shaped optical lattices.

Evaluation method for noise-induced phase error in fringe projection profilometry.

For noise-induced phase error and phase unwrapping reliability in fringe projection profilometry (FPP), a new evaluation method is proposed. By introducing a Gaussian noise model, the maximum variance of the noise-induced phase error and phase unwrapping reliability are directly obtained. According to the proposed evaluation method, the variance of the noise-induced phase error is proportional to the noise variance and inversely proportional to the fringe modulation. The phase unwrapping reliability is not only inversely proportional to the noise-induced phase error, but also inversely proportional to the fringe frequency ratio. This work analyzes and compares three efficient dual-frequency phase unwrapping algorithms. The results show that the wrapped phase accuracy and phase unwrapping reliability based on 4fH+4fL are the best, followed by 3fH+3fL and 3fH+2fL. However, 3fH+2fL has the highest measurement efficiency.

Surface Shape Distortion Online Measurement Method for Compact Laser Cavities Based on Phase Measuring Deflectometry

Conventional phase measuring deflectometry (PMD) takes up a large measurement space and is not suitable for compact online measurement, as the liquid crystal display (LCD) has to be placed in parallel with the mirror under test. In this paper, a compact online phase measuring deflectometry (COPMD) with the LCD screen set perpendicular to the mirror under test is presented for surface shape distortion real-time measurement. The configuration of the COPMD in an enclosed laser cavity is proposed, and the principle of the method is theoretically derived by using the vector-form reflection law. Based on the analysis model, the fringe modulation regulation of the LCD is revealed, and the measurement errors caused by misalignments of the components are illustrated. The validity and flexibility of the COPMD method are verified in the experiment by using a single-actuator deformable mirror as the mirror under test and the PMD method as the comparison. The proposed COPMD method remarkably expands the application range of the conventional PMD method, as it could make efficient use of compact space and is applicable for real-time measurement in enclosed laser facilities and assembled laser systems.

Production of good quality holograms by the THz pulsed vortex beams

In this paper, we discuss the quality of holograms based on the calculation of the modulation depth. It’s shown that the terahertz (THz) pulsed vortex beams play a vital role in holography filed, where two lasers with frequency difference have used. The THz vortex beams give new regions of larger frequency detuning and an important value of the modulation depth and fringe contrast (MDFC ratio) for obtaining the best holograms contrary to the Gaussian beams for which the best holograms are realized for small frequency detuning. The particular cases such as, Gaussian beam and single cycle pulses are deduced from our result. Numerical simulations are also presented to study the dependence of the MDFC ratio on the frequency detuning for THz vortex beams and Gaussian beams. This research could be beneficial in holographic interferometry, and it will firmly establish as a tool for scientific and engineering studies.

Induced gyrotropy in thin PET films before and after swift heavy ion irradiation evidenced from analysis of optical interference fringes

We present here experimental studies of the UV/vis/near IR transmission spectra of polyethylene terephthalate (PET) films before and after irradiation with Kr+15 ions of energy 1.75 MeV/a.u. and fluence of 1.9 × 10 10 cm−2. Our data show small variations (∼0.5%) in transmission between individual pristine PET film samples in the IR region of the spectrum, as would be expected for films produced to certain manufacturing tolerances. Swift heavy ion (SHI) irradiation of a sample reduces transmission in this region, but by less than the observed variation in transmission between individual pristine samples, and does not affect local transmission variations in the spectrum, indicating that SHI irradiation has not destroyed the molecular structure of the PET film responsible for these variations. We use established methods to separate our experimental spectra into interference-free transmission spectra and isolated interference fringes and estimate the absorption coefficient and dispersion of the refractive index for pristine and irradiated PET films. We show that the observed amplitude modulation of the interference fringes is due to the optical activity of our PET samples and is a signature of induced gyrotropy. In some spectral regions, we observe a refractive index that is negative relative to the Cauchy approximation. We provide evidence that this is due to the ordering/texture of chiral molecules in PET films, suggesting that very highly textured PET films may exhibit a greater degree of negativity.

High dynamic range fringe pattern acquisition based on deep neural network

Fringe projection profilometry has wide applications in three-dimensional measurement. When the fringes are projected onto the shiny surfaces with a large reflectivity, however, the saturation regions and dark regions are often generated on the surface due to the inconsistent reflection characteristics of the measured object. The saturation and dark regions make the modulation of captured fringe patterns degraded, and the accuracy of the three-dimensional measurement is significantly affected. To solve this problem, an improved network for high dynamic range fringe pattern is designed In this network, we introduce a low modulation region detection module based on U-net to obtain the saturation and dark regions in the fringe patterns. Then, the detailed fringe information in the low modulation regions can be predicted through a fringe enhancement module which takes advantage of the global distribution and detail intensity of the fringe patterns. In the experiment, a mass of simulated and actual low modulation fringe patterns are processed through the proposed network. The experimental results show that the designed network can effectively remove the noise of the captured fringe patterns and repair the low modulation regions captured from the shiny surfaces. Combined with phase calculation, the improved high dynamic range fringe patterns are used for three-dimensional data calculation. A standard metal gauge block with a height of 5 mm is measured and the root mean square error of the height is improved from 0.55 mm to 0.06 mm. This proves the proposed method can effectively improve the quality of fringe patterns projected on complex reflective surfaces.

Neuroblastoma Cells Classification Through Learning Approaches by Direct Analysis of Digital Holograms

The label-free single cell analysis by machine and Deep Learning, in combination with digital holography in transmission microscope configuration, is becoming a powerful framework exploited for phenotyping biological samples. Usually, quantitative phase images of cells are retrieved from the reconstructed complex diffraction patterns and used as inputs of a deep neural network. However, the phase retrieval process can be very time consuming and prone to errors. Here we address the classification of cells by using learning strategies with images coming directly from the raw recorded digital holograms, i.e. without any data processing or refocusing involved. Indeed, in the raw digital hologram the entire complex amplitude information of the sample is intrinsically embedded in the form of modulated fringes. We develop a training strategy, based on deep and feature based machine learning models, in order extract such information by skipping the classical reconstruction process for classifying different neuroblastoma cells. We provided an experimental validation by using the proposed strategy to classify two neuroblastoma cell lines.

A New Three-Part Phase Unwrapping Algorithm for 3-D Measurement of Dynamic Objects

For three-dimensional (3-D) measurement of dynamic objects, a new three-part phase unwrapping algorithm is proposed. First, a novel fringe modulation degree threshold estimation method is proposed. Then, the unwrapped phase can be obtained by three segments. The first-segment is calculated by 2-D spacial phase unwrapping algorithm while the fringe modulation degree is within the threshold. The second-segment is calculated between two frames when their fringe modulation degree difference is within the threshold. The third-segment is calculated by a estimation method of 2π jump order. This method only need one wrapped phase to calculate the unwrapped phase. Simulations and experiments have proved the effectiveness of the proposed method.

Lens-free digital holographic microscopy for cell imaging and tracking by Fresnel diffraction from a phase discontinuity

In this Letter, a very simple, stable, and portable lensless digital holographic (DH) microscopy method is presented relying on the Fresnel diffraction (FD) of light from a phase discontinuity (PD). A phase plate in the transmission or a physical step in the reflection can be employed in the path of the divergent beam of a coherent light source as a component imposing the PD. The recorded diffraction pattern in the vicinity of the PD is a hologram produced by off-axis overlapping of two diffracted waves in both sides of the boundary region with adjustable fringe modulation. To validate the method, measurements are performed on the amplitude and phase specimens as well as on the dynamic processes of water evaporation and 3D tracking of floating cells. A reflective configuration of FD from a physical step can be used as a powerful platform for lensless DH microscopy using high-energy electromagnetic radiation, e.g., x-ray and UV sources for the high-resolution imaging of moving samples.

Smooth Surface Defect Detection by Deep Learning Based on Wrapped Phase Map

Object detection is widely used in the industry for visual inspection of defects. But it fails to fully reflect the defects on the smooth surface due to the difficulty in illuminating, and the high performance of defect detection tends to be a challenge. In this paper, we proposed a novel defect detection method for laptop panels. On the one hand, we utilized the wrapped phase map obtained by phase-shift reflection fringes for defect detection and used the DenseFuse network to fuse the wrapped phase map with the fringe modulation map to optimize the dataset. On the other hand, we proposed an improved deep learning-based network Vovecwnet. The experiments indicated that our image-based multi-information fusion system could achieve excellent performance in defect detection on the smooth surface (e.g., laptop panels surface), with high accuracy of 97.5% and a few reference time 0.0762s on GTX 2080ti GPU. Thus, our proposed method outperformed and tackled the challenges encountered in the defect detection of smooth surfaces.

Generation of coherent multicolor noise-like pulse complex in Yb-doped fiber laser mode-locked by GIMF-SA

We have demonstrated the generation of multicolor noise-like pulse complex in a passively Yb-doped mode-locked fiber laser based on a single mode-graded index multimode-single mode fiber (SMF-GIMF-SMF) device as the saturable absorber (SA). The stimulated Raman scattering (SRS) effect leads to the cascaded generation of the main noise-like pulse (NLP) at 1028.8 nm together with the noise like Raman pulse (RP) at 1076.1 nm. The generated dual wavelength pulses demonstrate the unique properties of mutually synchronization and coherence. The autocorrelation traces show that each of the synchronously mode-locked pulses exhibits a double-scale structure with a narrow peak which consists of a train of quasi-periodic beat pulses with a 35.7 fs pulse width and a pulse separation of about 77.2 fs. The total output power reaches 102.5 mW with 34% of it belonging to the RP. And furthermore, by separating the two pulses with spectral filters, the modulation fringes cannot be observed anymore. These results indicate that the Raman component participates in the mode-locking operation as a 'signal' instead of 'noise'. Such a coherent Raman pulse source provides a novel platform for numerous applications, such as frequency comb spectroscopy and so on.

Depth segmentation using disparity and self-adaption fringe modulation analysis

In this paper, we propose a depth segmentation method combining disparity information with fringe modulation information, which aims at improving the robustness under the noisy scene. Firstly, we investigate the continuity characteristics in the U-V disparity map and distinguishing modulation distribution in the wrapped phase images, respectively. Then, the disparity-based method is optimized to detect and classify the valid regions containing objects and their supporting surfaces. Besides, we provide an evaluation criterion to reduce the noise represented as scattered points. Furthermore, through the analysis of the different fringe modulation distributions between objects and their shadow areas, a method with adjustable thresholds is presented to suppress shadow noise. Our approach is evaluated on both simulation scenes and real-world scenes to verify the availability and robustness. Sufficient experimental results indicate that the proposed method can effectively separate the valid region into objects and detect their incomplete supporting surfaces.

New quality weight used for phase unwrapping in color fringe reflection method

The fringe -reflection technology has been widely applied in the fields of industrial inspection and measurements. In the present study, three-step phase-shifting is developed using the color fringe-reflection approach to inspect a work-piece surface with rotational symmetry characteristics, and an improved experiment is proposed. In addition, by combining Robert gradient with fringe modulation, a new quality weight, referred to as ‘Modulation-Robert gradient variance’ (MRGV), is proposed for quality-guiding the phase-unwrapping (PU) algorithm. In comparison to the traditional quality weights, MRGV presented higher noise immunity and lower PU error in the root mean square (RMS). In addition, in terms of defect detection, MRGV performed better than the other weights. Finally, after MRGV-guided PU conduction, the proposed experiment method is used in combination with an improved Southwell integration algorithm to realize the 3D reconstruction. The reconstruction error is approximately 3.09% lower than that presented by certain other methods. Furthermore, the results indicated that the combination of the proposed method and the new quality weight could effectively detect the defects and offer an accurate measurement of a rotationally symmetrical surface to a certain extent.

Adaptive Phase Correction for Phase Measuring Deflectometry Based on Light Field Modulation

The phase measuring deflectometry is a powerful measuring method for complex optical surfaces, which captures the reflected fringe images associated with a displaying screen and calculates the normal vectors of the surface under test (SUT) accordingly. The captured images are usually set conjugate to the SUT, which in turn makes the screen defocused. As a result, the blurring effect caused by the point spread function (PSF) of the off-axis catadioptric imaging system can bias the solved phases. In order to correct the phase errors, a light field is constructed based on the Fourier compressive sensing method to describe the light transmission between the screen and camera pixels. Fringe modulation is conducted to enhance the robustness against noise, and then, space-variant PSFs can be extracted from the light field. The true phases are obtained by solving a Wiener deconvolution problem, with the merit function adaptively regularized by adjusting the damping parameter. The proposed method can correct adaptively the phase errors induced by the complex aberrations. Compared to the reference measurements, the form accuracy can be improved by four times.