General phase-shifting algorithm for hybrid errors suppression using variable-frequency fringes.

In measurements based on phase-shifting fringe pattern analysis, residual ripple-like artifacts often appear due to the co-influence of several error sources, e.g., phase-shifting errors, temporal intensity fluctuations and high-order fringe harmonics, when existing algorithms are adopted to retrieve phase using limited number of fringe patterns. To overcome this issue, a general phase-shifting algorithm for hybrid errors suppression by variable-frequency fringes is proposed in this paper for what we believe to be the first time. A corresponding fringe model is deduced to represent real patterns more accurately under the co-influence of these error factors. Variable-frequency fringes are introduced to provide a least and sufficient system of equations, while a least-squares iterative technique with a grouped step-by-step strategy is adopted for stable calculating a larger number of desired parameters in the constructed model. For the phase jump problem caused by non-full rank matrices at certain sampling points, a regularization combined with constraints between coefficients of high-order fringe harmonics is further proposed for identification and processing. Simulations and experimental results have shown that compared with the prior techniques, the accuracies of the proposed algorithm have been significantly enhanced at least 2.1 (simulations) and 1.5 (experiments) times respectively using bi-frequency equal three-step as an example in the study.

Polarization-insensitive achromatic metalens based on computational wavefront coding.

Broadband achromatic metalens imaging is of great interest in various applications, such as integrated imaging and augmented/virtual reality display. Current methods of achromatic metalenses mainly rely on the compensation of a linear phase dispersion implemented with complex nanostructures. Here, we propose and experimentally demonstrate a polarization-insensitive achromatic metalens (PIA-ML) based on computational wavefront coding. In this method, simple circular or square nanopillars are individually coded such that the focal depths at wavelengths at both ends of the achromatic bandwidth overlap at the designed focal plane, which removes the limitation of requiring a linear phase dispersion. An optimized PIA-ML that works in the full optical communication band from 1300 to 1700nm was obtained using a particle swarm optimization algorithm. Experimental results show that both focusing and imaging of the fabricated metalens are consistent with theoretical predictions within the broadband wavelength range, which provides a new methodology for ultra-broadband achromatic imaging with simple-shaped nanostructures.

Fast temporal phase unwrapping method for the fringe reflection technique based on the orthogonal grid fringes.

In traditional temporal phase unwrapping (TPU) algorithms, wrapped phases with different spatial frequencies are obtained from several groups of phase shift fringes to calculate the unwrapped phase. Therefore, the necessary quantity of captured fringes is very large, especially for the fringe reflection technique (FRT), since a pair of phases should be unwrapped to get the slopes of two perpendicular directions. In this paper, we propose a fast TPU algorithm based on the orthogonal grid fringes by which only one image is needed to extract the two integer phases for each frequency instead of two groups of phase shift fringes, and then they can be added into the wrapped phases separately to complete the unwrapping. There are ridge errors in the direct unwrapped phases, but they are significantly suppressed by our pseudo-phase-shift strategy without any extra captured fringes. The proposed method is robust and effective where the fringe amount used for unwrapping is only 1/4 of the previous similar algorithm and 1/6-1/8 of the traditional TPU methods. The detailed comparison of measurement time is also given, which demonstrate that the FRT measurement can be accelerated in most cases by our method. The algorithm is validated by the experiments, which still works well for the severely defocusing fringes or complex specimen.

Flexible structured-light-based three-dimensional profile reconstruction method considering lens projection-imaging distortion.

Structured-light profilometry is a powerful tool to reconstruct the three-dimensional (3D) profile of an object. Accurate profile acquisition is often hindered by not only the nonlinear response (i.e., gamma effect) of electronic devices but also the projection-imaging distortion of lens used in the system. In this paper, a flexible 3D profile reconstruction method based on a nonlinear iterative optimization is proposed to correct the errors caused by the lens distortion. It can be easily extended to measurements for which a more complex projection-imaging distortion model is required. Experimental work shows that the root-mean-square (RMS) error is reduced by eight times and highly accurate results with errors of less than 1‰ can be achieved by the proposed method.

Carrier squeezing interferometry: suppressing phase errors from the inaccurate phase shift.

The carrier squeezing interferometry algorithm is proposed to retrieve the phase from interferograms with phase shift errors. A linear carrier is introduced in the interferograms, and the image data is rearranged by the squeezing interferometry technology. In the spectrum of the rearranged image, the error lobe and the phase lobe are separated so the error-free phase can be retrieved by filtering. The simulated interferograms with phase shift errors are computed, and the precisions are better than 8.4×10(-4)λ. Its validation is verified by experiments, where a mean precision of 0.0040λ is obtained.

Surface profile measurement in white-light scanning interferometry using a three-chip color CCD.

White-light scanning interferometry (WLSI) is a useful technique to measure surface profile when a test object contains discontinuous structures or microstructures. A black and white CCD camera is usually utilized to capture interferograms, and a series of corresponding algorithms is used to achieve the profile measurement. However, the color information in the interferograms is lost. A novel profile measurement method that uses phase information in different color channels (red-green-blue) of an interferogram obtained using a three-chip color CCD in WLSI is proposed. The phase values are extracted by a windowed Fourier transform algorithm. Simulation and experimental results are presented to demonstrate the validity of the proposed method.

[Development of transient pyrometer based on multi-spectral radiation technology].

In modern dynamics system, the radiant temperature of the flame, which caused by the transient plasma stimulated by high-energy-level electromagnetism field, takes an important role in the description of the flying object's status as well as cauterization of the trajectory. Due to its extremely high temperature and transient process, the radiant temperature of the flame can hardly be measured through contracted ways, either static ways such as traditional pyrophotometer or CCD arrays. In the present paper, the authors bring forward a novel pyrophotometer based on classical theory of Planck's law (blackbody radiation law) and multi-channel spectrums radiation method. With this new type pyrophotometer, any spectrum can be selected out from the wavelength of 300 to 860 nm within 2 ns. Also, the application of high-definition diffraction grating and fibers can'ensure the accuracy of selected spectrum. The results through a serial of experiments by using this theory as well as high-speed photodetector indicate that this method is valid and accurate for the measurement of the object's surface's radiant temperature.