Hybrid 3D Shape Measurement Using the MEMS Scanning Micromirror

Projection superimposition for the generation of high-resolution digital grating

Fringe projection profilometry based on MEMS micro-vibration mirrors is very promising due to its rapid projection, large depth of field, compact size, and low cost. Although high-frequency fringes can achieve accurate reconstruction, the projector must offer sufficient pixel resolution. In this paper, we proposed a high-resolution projection technique called the delay superposition method. During a single exposure time of the camera, the projector projects a group of low-resolution fringe patterns, which are delayed according to the movement characteristics of the vibration mirror. Then, the camera exposure superimposes these low-resolution images to form a high-resolution image. These two steps effectively subdivide the angle intervals, thereby achieving a pixel interpolation. Finally, experimental results show that the proposed method can significantly improve the projector's pixel resolution and reconstruction accuracy. The proposed method allows the MEMS projector's pixel resolution (along one direction) to far exceed that of common DLP projectors. It holds great application potential for high-frequency fringe projection.

Simple modulation of Lissajous MEMS laser beam scanning with reconfigurable structured light patterns for 3D imaging

Structured light 3D imaging systems commonly employ panel-based projectors or 1-axis MEMS mirrors with beam expander lens to project multi-frame barcodes or dot clouds, addressing challenges posed by objects with multi-scale feature sizes. However, these methods often result in large system volumes due to the required projection multi-lens modules, high hardware costs, or limited light pattern generation capabilities that hindering measurement precision enhancement. This paper introduces an innovative approach to reconfigurable spatial light pattern projection using a single bi-axial MEMS mirror with Lissajous scanning. In contrast to the pixel-by-pixel pre-defined image patterns encoding of conventional 2D laser beam scanning, the proposed method simply aligns the MEMS bi-axial resonance frequencies with laser pulse modulation, enabling the projection of diverse structured light patterns such as stripes, lines, dot matrices, and random dot clouds, which can adapt to different 3D imaging algorithms demands. It eliminates the need for multi-frame encoding and streamlines data caching, simplifies digital logic hardware. A prototype 3D imaging system was developed to demonstrate the mathematical model for laser modulation and the technical feasibility based on the proposed principle. Beyond its lens-free essence, the system supports focal-free optics and a compact projection form factor, which accommodates to a broad range of projection distances and field-of-views based on object's location. 3D depth map of polynomial surface and blocks objects are extracted through single-frame pattern projection with a relative high accuracy. The presented modulation theory for diverse structured light pattern generation opens avenues for versatile and compact 3D imaging applications of LiDAR and robotic 3D vision.

Ultra-small, low-cost, and simple-to-control PSP projector based on SLCD technology

Demand for ultra-small, inexpensive, and high-accurate 3D shape measurement devices is growing rapidly, especially in the industrial and consumer electronics sectors. Phase shifting profilometry (PSP) is a powerful candidate due to its advantages of high accuracy, great resolution, and insensitivity to ambient light. As a key component in PSP, the projector used to generate the phase-shifting sinusoidal fringes must be ultra-small (several millimeters), low-cost, and simple to control. However, existing projection methods make it difficult to meet these requirements simultaneously. In this paper, we present a modern technique that can be used to fabricate the desired projector. A specifically designed device based on segmented liquid crystal display (SLCD) technology is used to display the projected patterns, and a cylindrical lens is used as the projection lens. The SLCD device can display four sets of specific filled binary patterns, each yielding a sinusoidal fringe, and all four sinusoidal fringes satisfy the four-step phase shift relation. 3D shape measurement experiments verify the performance of the projector. Considering that the size of SLCD devices can be reduced to a few millimeters, the proposed technique can be easily used to manufacture ultra-small, low-cost, and simple-to-control PSP projectors.

Uniaxial MEMS-based 3D reconstruction using pixel refinement

A uniaxial micro-electro-mechanical systems (MEMS) micro-vibration mirror can be used to construct a new type of fringe projection profilometry (FPP) system. In FPP system calibration, some pixels may be calibrated worse than other pixels due to various error sources, which will affect the final reconstruction accuracy. In addition, there are some difficulties in calibrating the MEMS-based system because a projector using the uniaxial vibration mirror does not have focusing optics and can only project unidirectional fringes. In this paper, we developed an FPP system using a uniaxial MEMS micro-vibration mirror. To solve the calibration problems, we propose a calibration model suitable for the MEMS-based system and a pixel refinement method. These pixels with relatively large calibration errors are called outlier-pixels, which will significantly increase the error of the following 3D mapping. Therefore, the pixel refinement method classifies all pixels based on a frequency distribution histogram of calibration errors during calibration and prevents outlier-pixels from participating in the following 3D mapping. The experimental results show that the proposed method can improve the accuracy of 3D reconstruction, and the feasibility of the self-developed system is verified.

Lissajous scanned variable structured illumination for dynamic stereo depth map

Structured illumination plays an important role in advanced photographic and microscopic imaging applications. Here we report variable structured illumination (VSI) using Lissajous scanning techniques. The variable structured illumination module comprises Lissajous scanning micromirror and fiber-based diode pumped solid state (DPSS) laser with intensity modulation, combined with a stereo camera for dynamic stereo depth map. The micromirror projects static and discrete patterns by modulating the intensity of a laser beam at the least common multiple (LCM) of two scanning frequencies. The pattern density is increased by either decreasing the greatest common divisor (GCD) of scanning frequencies or decreasing the duty rate of the laser modulation. The scanning amplitude also controls the field-of-view (FOV) for the exact illumination of a target object for dynamic stereo depth map. The variable structured illumination module provides a new route for advanced imaging applications such as high-quality depth map, super-resolution, or motion recognition.

Editorial for the Special Issue on Optical MEMS

Optical micro-electro-mechanical systems (MEMS), micro-opto-electro-mechanical systems (MOEMS), or optical microsystems are devices or systems that interact with light through actuation or sensing at a micron or millimeter scale [...].