High-precision surface profilometry on a micron-groove based on dual-comb electronically controlled optical sampling.

We demonstrate an optical method for 3D profilometry of micro-nano devices with large step structures. The measurement principle is based on a dual-comb direct time-of-flight detection. An electronically controlled optical sampling (ECOPS) approach is used to improve the acquisition rate. In a proof-of-principle distance measurement experiment, the measurement precision reaches 15 nm at 4000-times averages. The method has been used to characterize the profile of a large aspect-ratio rectangular micron-groove with 10 µm width and 62.3 µm depth. By point-by-point scanning, a 3D point cloud image is obtained, and the 3D profile of the micro-structure is quantitatively reconstructed with sub-micrometer precision. The proposed high-precision, high-speed surface 3D profile measurement technology could be applied to profilometry and inspection of complex microelectronics devices in the future.

Automatic and rapid whole-body 3D shape measurement based on multinode 3D sensing and speckle projection.

Automatic and rapid whole-body 3D shape measurement has attracted extensive attention in recent years and been widely used in many fields. Rapid 3D reconstruction, automatic 3D registration, and dedicated system layout are critical factors to enable an excellent 3D measurement system. In this paper, we present an automatic and rapid whole- body 3D measurement system that is based on multinode 3D sensors using speckle projection. A rapid algorithm for searching homologous point pairs is suggested, which takes advantage of the optimized projective rectification and simplified subpixel matching techniques, leading to an improved time efficiency of 3D reconstruction. Meanwhile, a low-cost automatic system with a flexible setup and an improved calibration strategy are proposed, where system parameters of each node sensor can be simultaneously estimated with the assistance of a cubic and a planar target. Furthermore, an automatic range data registration strategy by employing two aided cameras is investigated. Experiment results show that the presented approach can realize automatic whole-body 3D shape measurement with high efficiency and accuracy.

Phase-3D mapping method developed from back-projection stereovision model for fringe projection profilometry.

Two major methods for 3D reconstruction in fringe projection profilometry, phase-height mapping and stereovision, have their respective problems: the former has low-flexibility in practical application due to system restrictions and the latter requires time-consuming homogenous points searching. Given these limitations, we propose a phase-3D mapping method developed from back-projection stereovision model to achieve flexible and high-efficient 3D reconstruction for fringe projection profilometry. We showed that all dimensional coordinates (X, Y, and Z), but not just the height coordinate (Z), of a measured point can be mapped from phase through corresponding rational functions directly and independently. To determine the phase-3D mapping coefficients, we designed a flexible two-step calibration strategy. The first step, ray reprojection calibration, is to determine the stereovision system parameters; the second step, sampling-mapping calibration, is to fit the mapping coefficients using the calibrated stereovision system parameters. Experimental results demonstrated that the proposed method was suitable for flexible and high-efficient 3D reconstruction that eliminates practical restrictions and dispenses with the time-consuming homogenous point searching.

Structured light field 3D imaging.

In this paper, we propose a method by means of light field imaging under structured illumination to deal with high dynamic range 3D imaging. Fringe patterns are projected onto a scene and modulated by the scene depth then a structured light field is detected using light field recording devices. The structured light field contains information about ray direction and phase-encoded depth, via which the scene depth can be estimated from different directions. The multidirectional depth estimation can achieve high dynamic 3D imaging effectively. We analyzed and derived the phase-depth mapping in the structured light field and then proposed a flexible ray-based calibration approach to determine the independent mapping coefficients for each ray. Experimental results demonstrated the validity of the proposed method to perform high-quality 3D imaging for highly and lowly reflective surfaces.

Dynamic 3D imaging based on acousto-optic heterodyne fringe interferometry.

An acoustic-optics heterodyne fringe interferometry coupled with a three-camera system is developed for dynamic 3D imaging. In this system, first-order beams with a slight frequency difference diffracted from two acousto-optic deflectors (AODs) form a beat intensity fringe pattern. Setting the frequency of the trigger signal for the CCD cameras into four times the beat frequency, four-step phase-shifting fringe patterns can be obtained, and the wrapped phase map (WPM) can be calculated. Under the epipolar constraint among three cameras, the homologous points can be determined unambiguously with the assistant of a WPM; thus the 3D shape can be reconstructed while skipping the phase unwrapping step. Experimental results are presented to validate this approach.

Calibration of fringe projection profilometry with bundle adjustment strategy.

The measurement accuracy of fringe projection profilometry (FPP) largely depends on the calibration procedure. A more reliable calibration approach based on the stereo vision model of the FPP scheme in conjunction with the bundle adjustment strategy is presented. It can adjust the coordinates of benchmarks and thereby estimate the scheme parameters more accurately even with an imperfect target. The experiment results shows that the proposed approach can reach highly accurate calibration by solely using a printed target pattern, which verifies the proposed approach.

Optical measurement network for large-scale and shell-like objects.

An optical measurement method for large-scale and shell-like objects is proposed and is verified by experiments. The underlying concept is a model-based optical measurement network consisting of multinode three-dimensional (3D) sensors. To achieve this, a synthetic calibration method is presented to enable the measurement. A phase-aided active stereoscopy is thus applied to each node sensor for acquiring partial range images from different viewpoints. The multiple range images are then registered to obtain a 3D reconstructed model, which is compared with the computer-aided design (CAD) model to quantitatively reveal the differences between the two models. Experiment results are also presented to validate the proposed approach.