Talbot効果による傾斜角度格子投影手法とリニアファイバアレイを用いた位相シフトによる3次元計測

3-D shape measurement systems by contactless method are required in the quality inspections of metal molds and electronic parts in industrial fields. A grating projection method with phase-shifting method has advantages of high precision and high speed. Recently, the size of a BGA (ball grid array) becomes smaller. So the pitch of a grating pattern projected onto the specimen should be smaller. In conventional method, fringe pattern is projected using an imaging lens. The focal depth becomes smaller in the case of reduced projection. It is therefore difficult to project a grating pattern with small pitch onto an object with large incident angles. Authors recently proposed a light source stepping method using a linear LED device. It is easy to shrink the projected grating pitch with a lens because this projection method does not use an imaging lens. The pitch of the projected grating depends on the width of the light source. There is a limit to shrink the projected grating pitch according to the size of the LED chip. In this paper, a small pitch fringe projection method with multiple linear fiber arrays for 3D shape measurement is proposed. The width of the fiber array is 30μm. It is one digit smaller than the width of the LED chip. The experimental result of 3-D shape measurement with small pitch projection with large incident angles is shown.

In-situ 3-D shape measurements with submicron shape uncertainty of fast rotating objects in a cutting lathe are expected, which can be achieved by simultaneous distance and velocity measurements. Conventional tactile methods, coordinate measurement machines, only support ex-situ measurements. Optical measurement techniques such as triangulation and conoscopic holography offer only the distance, so that the absolute diameter cannot be retrieved directly. In comparison, laser Doppler distance sensors (P-LDD sensor) enable simultaneous and in-situ distance and velocity measurements for monitoring the cutting process in a lathe. In order to achieve shape measurement uncertainties below 1 μm, a P-LDD sensor with a dual camera based scattered light detection has been investigated. Coherent fiber bundles (CFB) are employed to forward the scattered light towards cameras. This enables a compact and passive sensor head in the future. Compared with a photo detector based sensor, the dual camera based sensor allows to decrease the measurement uncertainty by the order of one magnitude. As a result, the total shape uncertainty of absolute 3-D shape measurements can be reduced to about 100 nm.

Development of underwater 3D shape measurement system with improved radiation tolerance

A prototype of a 3D shape measurement device with two cameras was developed, and the prototype was used to confirm the effectiveness of the 3D shape measurement method that does not require camera parameters. For 3D shape measurement using a fringe projection method, generally the pixel coordinate in the image and phase information of the projected fringe pattern are used; however, 3D coordinates can be obtained from only three fringe phase values. Recently, authors proposed this method as a feature quantity type whole-space tabulation method. There were no camera parameters required because pixel coordinates were not used, and thus a camera calibration-free 3D shape measurement can be realized. Moreover, the experimental evaluation was performed using the prototype having two cameras. Although these cameras were located at different positions and had lenses with different focal lengths, their measured shapes of an object were almost identical. An experiment of 3D shape measurement using an uncalibrated camera was performed. The effectiveness of the proposed method was quantitatively validated from experimental result obtained using the developed prototype.

Three-dimensional (3D) shape measurement systems based on diffuse reflection of projected structured light do not deliver reliable data when measuring glossy, transparent, absorbent, or translucent objects. In recent years, we have developed a method based on stereo recording with infrared cameras and projection of areal aperiodic sinusoidal thermal patterns to detect such objects. However, the measurements took longer than 10 s, up to minutes; moreover, the measurement accuracy was improvable. Now, we have succeeded in both drastically reducing measurement time and significantly increasing measurement quality. This finally provides a technique for reliably measuring transparent objects, e.g., in series production. We demonstrate measurement examples achieved within 1 s and with 3D standard deviations less than 10 µm.

In this paper, a three-dimensional (3D) shape measurement method based on structured light field imaging is proposed, which contributes to the biomedical imaging. Generally, light field imaging is challenging to accomplish the 3D shape measurement accurately, as the slope estimation method based on radiance consistency is inaccurate. Taking into consideration the special modulation of structured light field, we utilize the phase information to substitute the phase consistency for the radiance consistency in epi-polar image (EPI) at first. Therefore, the 3D coordinates are derived after light field calibration, but the results are coarse due to slope estimation error and need to be corrected. Furthermore, the 3D coordinates refinement is performed based on relationship between the structured light field image and DMD image of the projector, which allows to improve the performance of the 3D shape measurement. The necessary light field camera calibration is described to generalize its application. Subsequently, the effectiveness of the proposed method is demonstrated with a sculpture and compared to the results of a conventional PMP system.

This paper presents a patented three-dimensional (3D) shape measurement system, ReCreator, which is mainly composed of two CCD cameras and a random pattern projector with fixed geometry. It can work in the binocular mode, where the stereo image pair is utilized to reconstruct the 3D point cloud in the common field of view of the two cameras; while in the monocular mode, each single camera and the random pattern projector constitute a measuring unit respectively to alleviate the highlight and occlusion effect. The design and the calibration of the random pattern projector, which is the key ingredient for enabling the system, are discussed in detail. The system algorithms are partially implemented with NVIDIA Graphics Processing Units (GPU) in the Compute Unified Device Architecture (CUDA) programming environment, which proves effective to accelerate the computation. Experiments are conducted to validate the precision, rapidity and data integrity of the measurement system.

Rapid and automatic optimal exposure control for digital fringe projection technique

Color-coded fringe projection profilometry (FPP) is one of the most important single-frame three-dimensional (3D) shape measurement techniques. Transverse chromatic aberration (TCA) in a color-coded FPP system yields imaging deviations, which will result in 3D shape measurement error. In this work, a TCA compensation method for color-coded FPP is proposed. The proposed method establishes the relationship among phase, pixel deviation, and imaging position for decreasing the 3D shape measurement error introduced by TCA. Compared to existing methods that use the relationship among depth, pixel deviation, and imaging position or polynomial fitting for each pixel, the proposed method offers the merits of precise and simple implementation, since it voids the cumulative error in-depth calculation and removes the time-consuming process of polynomial fitting for each pixel. For validation, a color-coded FPP system is established and calibrated. Two verification experiments, including a translation test and 3D shape measurement of a regular spherical object, are conducted with the calibrated system. Experimental results demonstrate that the proposed TCA compensation method is capable of effectively reducing the 3D shape measurement error caused by TCA.

Phase analysis of fringe pattern technique has been widely used for three-dimensional shape and deformation measurement by fringe projection method, precise inspection of optical components by various interferometry, etc. To analyze the phase distribution of fringe pattern, more than 80 phase analysis methods have been proposed by many researchers in this research field [1]. For instance, a Fourier transform [2], a wavelet transform, a windowed Fourier transform, and a sampling Moire method [3] have been developed from a single-shot image by using the intensity information in the spatial domain. In addition, the phase-shifting method (PSM) [4], which uses multi-frame phase-shifted fringe patterns is mostly used to analyze the phase distribution by using the intensity information in the temporal domain. However, these techniques only use one-dimensional intensity information in the spatial- or temporal-domains. For this reason, the analyzed phase distribution for noisy fringe pattern remains large measurement error due to the low reflectance of the object and the random noise of the camera.

The abilities to increase precision of surgical procedures by tracking real-time motions, accurately measuring the mechanical properties of complex 3D geometries, and tracking deformations of components over time, among many other applications, depend on the availability of robust, high-speed, full-field-of-view, 3D shape measurement systems. In this paper, we present advances in our development of a high-speed 3D shape measurement system based on fringe projection. The system consists of a high-speed projector, with speeds up to 20,000 frames per second, that is integrated with a CCD camera to provide full-field-of-view information. By using high-speed projection of sinusoidal fringe patterns with varying spatial densities together with temporal phase unwrapping algorithms that we are developing, we are able to compute and display unwrapped phase maps at video rates, which enable the capability to perform absolute shape measurements of components. We present representative results obtained with our system as we have applied it to art conservation and to biomedical imaging. Results validate system capabilities as a high-speed method of dynamically gathering, analyzing, and displaying shape information.

Non-sinusoidal phase error is common in structured light three-dimensional (3D) shape measurement system, thus we perform theoretical and experimental analyses of such error. The number of non-sinusoidal waveform errors in a 2\pi phase period is the same as the number of steps of the phase-shifting algorithm; no errors occur within the one-phase period. Based on our findings, a new structured light method, the linear sinusoidal phase-shifting method (LSPS), that is resistant to non-sinusoidal phase error is proposed. Experiments show that the non-sinusoidal waveform error is reduced to an almost negligible level (0.001 rad) using the proposed LSPS.

A pure rotational Raman lidar has become one of valid methods of profiling atmospheric temperature. However, its proper operation generally needs a certain collocated device of atmospheric temperature to calibrate three retrieval coefficients. This fact seriously restricts the applications of pure rotational Raman lidar in the meteorology and environment fields. In order to execute the detection technique of atmospheric temperature without calibration, we present and design a pure rotational Raman lidar based on the dependence of atmosphere molecular rotational Raman spectral envelope on temperature. It is configured with a laser having a pulse energy of 300 mJ, a pulse repetition rate of 20 Hz, and a Cassegrain telescope with a clear aperture of 250 mm. A two-stage multi-channel pure rotational Raman spectroscopic filter is proposed to extract efficiently the rotational Raman spectral lines with more than 70 dB suppression to the elastic-scattering optical signals. It is configured with one blazed diffraction grating, one convex lens, one linear fiber array and seven groups of fiber Bragg gratings. The blazed diffraction grating and fiber Bragg grating are separately utilized as the primary and secondary spectroscope. The tailor-made fiber array, which is composd of ten single mode fibers of 460-HP type and one multi-mode fiber, is designed to transfer the spectral signals. One end face of multi-mode fiber lies in the focal point of telescope, and then it transfers the lidar echo signals to the pure rotational Raman spectroscopic filter. The other end face of multi-mode fiber lies in the focal point of convex lens. The ten single mode fibers are used to transfer the optical signals from the primary spectroscope to the secondary, and their end faces lie in the focal plane of convex lens. Six pure rotational Raman spectral lines of nitrogen molecule in the anti-Strokes branch are chosen under the condition of the 0.09-nm forbidden band, with the consideration of the relationship between the pure rotational Raman spectral lines of nitrogen and oxygen molecules. While the excited laser wavelength is 532 nm, their central wavelengths are 530.76 nm, 529.86 nm, 529.41 nm, 528.51 nm, 527.62 nm, and 527.17 nm, respectively. Their corresponding positions of fiber end faces on fiber array are 156 m, 407 m, 532 m, 782 m, 1031 m, and 1156 m. Compared with these pure rotational Raman spectral lines, the elastic scattering signal lies on the other side of the focal point of convex lens, which improves the spectral purity of pure rotational Raman spectral lines. A retrieval algorithm of absolute detection technique is presented based on the least square principle. The performance of this lidar is simulated based on the U. S. standard atmospheric model. Simulation results show that this designed lidar can achieve the extraction of the pure rotational Raman spectral lines of nitrogen molecules, and that the atmospheric temperature profile obtained from absolute retrieval algorithm within a measurement time of 17 min can be up to 2.0 km with less than 0.5-K deviation. This pure rotational Raman lidar without calibration will provide a new detection method and retrieval scheme for atmospheric temperature profile.

Blade is the key component of the aero-engine. In generally, it requires precise size and accurate shape, so the three-dimensional shape measurement of the blade is very important. Fringe projection profilometry is generally used to measure the 3D shape of an object, because of the advantages of non-contact operation, full-field acquisition, high resolution, and fast data processing. In this paper, 3D shape measurement method based on surface structured light that combination of gray-code and phase-shift projection is proposed. The wrapped phase of the blade is got by the four-step phase-shift method. In the projection process of the gray-code, due to the ambient light, different surface reflectance and surface physical discontinuous, the edge of gray-code becomes smooth, leading to decode error, so that the gray-code pattern must be binaried before decode. Through the accurate binarization of the blade gray-code pattern, the decoding cycle of the blade is achieved, and the unwrapped phase is achieved by phase unwrapping. Then the unwrapped phase difference between the blade and reference plane is got, the height of the blade can be obtained by the relationship between the phase difference and the height. The experimental and simulation results show that the proposed method can achieve a high precision, high speed and low cost 3D shape measurement of the blade. The measurement accuracy reaches 0.03 mm. The proposed method extends the reliability and practicality of the fringe projection profilometry.

Three-dimensional (3D) shape measurement system with binary defocusing technique can perform high-speed and flexible measurements if binary fringe patterns are defocused by projector properly. However, the actual defocusing degree is difficult to set, and the fringe period is difficult to determine accordingly. In this study, we present a square-binary defocusing parameter selection framework. First, we analyze the fringe formation process mathematically. The defocusing degree is quantified and manipulated by using the focusing distance of projector, which is calibrated by point spread function measurement. To optimize parameter selection, single-point sinusoidal error is modeled as the objective function for the evaluation of the defocusing effect. We verify the correctness by using different parameter combinations and object measurements in our experiments. The appropriate defocusing parameters can be easily obtained according to the analysis of practical system setup, which improves the quality and robustness of the system.