Optimized stereo matching in binocular three-dimensional measurement system using structured light

Generating an M2 × N2 spot array with a dual-period hybrid Dammann grating fabricated using maskless projection lithography

The Dammann grating (DG), which redistributes a collimated laser beam into a spot array with a uniform intensity, is a widely adopted approach for profile measurement. Conventional DGs for dense spot projection are binary phase gratings with precisely designed groove structures, which suffer from low efficiency, poor uniformity, and a hard-to-fabricate fine feature size when utilized for a large field of view (FOV). Here, we propose a new, to the best of our knowledge, hybrid DG architecture consisting of two different grating periods which effectively generates an engineering M² × N² spot array with a non-complex structural design. As a proof-of-concept, a dual-period hybrid DG with a two-scale grating period ratio of 11.88 μm/95.04 μm (∼1/8) is designed and fabricated as a means to generate a dense 7²× 7² diffraction spot array with a FOV of 17° × 17°. In addition, the DG exhibits superior performance, with a high efficiency (>60%) and a low non-uniformity (<18%) at a wavelength of 532 nm. This kind of hybrid DG constructed from photoresist patterns with a minimum feature size of ∼1.2 μm can be perfectly fabricated by maskless projection lithography for large-scale and low-cost production. The proposed dual-period hybrid DG can pave the way for depth-perception-related applications such as face unlocking and motion sensing.

Visible light waveband Dammann grating based on all-dielectric metasurface

Dammann gratings (DGs) can generate a spot array in a particular arrangement. In recent years, DGs have been used in many fields such as laser beam splitting and optical coupling. Nanograting encoding technology can achieve a high signal-to-noise ratio and high-efficiency diffraction distribution; it also provides new design ideas for realizing the miniaturization and deviceization of DGs. In this work, we have comprehensively studied the DG based on an all-dielectric metasurface, which can produce a 5×5 diffraction spot array with a diffraction angle of 20^∘×20^∘. In an operation waveband from 650 to 690 nm, the DG has superior performance with high efficiency ≥60%; meanwhile, it achieves a relative low contrast ratio ≤0.33. Owing to high efficiency, wide waveband performance, and polarization insensitive property, the all-dielectric metasurface DG can provide possibilities for various application, including laser technology and optical information processing.

Two-dimensional picometer comb for three-dimensional reconstruction

A two-dimensional (2D) picometer comb, a novel optical element made by picometer-differential four times exposed in two perpendicular directions, is proposed to generate the dot array projection pattern for three-dimensional (3D) shape reconstruction and other applications. Not only does a 2D picometer comb generate a stable light field distribution with extremely long depth of field and small divergence angle as a one-dimensional picometer comb, it also has new properties, such as periodicity of diffraction field in two perpendicular directions and high concentration of energy of points, which is particularly suitable for providing dot array structured light. We demonstrate that the diffraction field of a 2D picometer comb provides a solution for non-defocusing 3D reconstruction with a dot array. In fabrication of a 2D picometer comb, we can modulate the holography by changing the angle of two beams slightly, so its period can be measured at picometer accuracy. A 2D picometer comb can be made to any scale, then it can be integrated to mobile devices, such as a mobile phone, for 3D shape reconstruction. Furthermore, the concept of a 2D picometer comb would be applied to generate a picometer light field for opening the door of pico-optics in the future.

Picometer-differential twice-exposed element for three-dimensional measurement with extremely long depth of field

The accuracy of optical three-dimensional (3D) shape measurement is always influenced by the defocusing of a projection or imaging system. In this paper, a novel optical element made by picometer-differential twice-exposed holography, called a picometer comb, is proposed to generate the projection pattern for 3D shape measurement. Two interference fields with picometer-scale period difference are recorded on a substrate to fabricate the picometer comb by twice-exposed laser holography; this element reconstructs the diffraction field, which is essentially the interference between the holograms of two object waves with a slight angle. This picometer comb has the advantage of the generation of a stable light field distribution with extremely long depth of field and small divergence angle. We demonstrate that this diffraction field provides a solution for non-defocusing 3D shape measurement.

3-D face registration solution with speckle encoding based spatial-temporal logical correlation algorithm

3-D information acquisition (registration) of whole face plays a significant role in 3-D human face recognition application. In this paper, we develop a prototype of 3-D system consisting of two binocular measurement units that allows a full 3-D reconstruction by utilizing the advantages of a novel correlation algorithm. In this system, we use optical modulation to produce temporally and spatially varying high-density binary speckle patterns to encode the tested face, then propose a spatial-temporal logical correlation (STLC) stereo matching algorithm to fast determine the accurate disparity with a coarse and refined strategy. Finally the 3-D information of whole face from left- and right ear (~180°) can be obtainable by fusing the data from two measurement units. Comparative researches are performed to test a plastic model and a real human face by simulating real application situations. The results verify the feasibility and good performances of our computational frameworks and experimental configuration in terms of accuracy and time cost, which show a good application prospect in our future 3-D human face recognition research.

Binocular vision profilometry for large-sized rough optical elements using binarized band-limited pseudo-random patterns

In this paper, a non-contact binocular vision profilometry method is proposed to measure a rough lens with aperture of around 300mm. A series of binarized band-limited pseudo-random patterns (BBPPs) are projected onto the rough lens, we utilize the temporal encoding method so that each pixel in the captured images has its specific code word. Homologous points could be matched via stereo matching procedure, then the surface of the rough lens will be reconstructed based on triangulation method according to the previous calibration data. Compared with the three coordinate measuring machine (CMM), this method achieves a fast and cheap measurement of the large-sized rough lens, which might be highly interesting for fast and overall measurement of metre-sized rough elements in the future.

Two-dimensional gold matrix method for encoding two-dimensional optical arbitrary positions

In this study, a novel two-dimensional spatial coding pattern called two-dimensional Gold matrix method is proposed for general two-dimensional positioning. Considering the difficulty in representing a two-dimensional position in a single binary matrix, constructing a matrix while each submatrix refers to its location is a challenging mathematical problem. The general two-dimensional signal can be labeled by the two-dimensional Gold matrix, which results from a preferred pair of two m-sequences. For a pseudorandom m-sequence, the span-n property of the two-dimensional Gold matrix states that every n×n submatrix is unique and the decoding is fast and convenient. Numerical simulation and a proof-of-principle experiment are performed, and experimental results verified that the two-dimensional Gold matrix method is effective for high resolution and large range two-dimensional measurements.

Optimization reconstruction method of object profile using flexible laser plane and bi-planar references

An optimization method to reconstruct the object profile is performed by using a flexible laser plane and bi-planar references. The bi-planar references are considered as flexible benchmarks to realize the transforms among two world coordinate systems on the bi-planar references, the camera coordinate system and the image coordinate system. The laser plane is confirmed by the intersection points between the bi-planar references and laser plane. The 3D camera coordinates of the intersection points between the laser plane and a measured object are initially reconstructed by the image coordinates of the intersection points, the intrinsic parameter matrix and the laser plane. Meanwhile, an optimization function is designed by the parameterized differences of the reconstruction distances with the help of a target with eight markers, and the parameterized reprojection errors of feature points on the bi-planar references. The reconstruction method with the bi-planar references is evaluated by the difference comparisons between true distances and standard distances. The mean of the reconstruction errors of the initial method is 1.01 mm. Moreover, the mean of the reconstruction errors of the optimization method is 0.93 mm. Therefore, the optimization method with the bi-planar references has great application prospects in the profile reconstruction.

Binocular vision measurement using Dammann grating

In this paper, we propose a novel three-dimensional (3D) profilometry using a binocular camera and a 64 × 64 Dammann grating for generation of a regular square laser array. A new constraint called a "ray constraint," taking advantage of the splitting characteristic of Dammann grating, is proposed for binocular matching. Binocular matching is realized by using ray constraint and precalibration of a laser array. Point clouds without outliers are obtained with binocular matching results according to triangulation. The experimental apparatus weighs less than 170 g with a width of less than 14 cm. We used this apparatus to scan a statue of Apollo under indoor illumination (at 450 lux). Its 3D model with complex profile was reconstructed by more than 150,000 points. This 3D profilometry has advantages of low cost, low power, and small size and should be useful for practical applications.