Real-time Three-dimensional Measurement Research Articles

Real-time three-dimensional measurement for color object based on HSI multi-level Gray code demodulation temporal phase unwrapping

A real-time three-dimensional(3D) measuring method for color object based on hue-saturation-intensity (HSI) multi-level Gray code demodulation temporal phase unwrapping is proposed. In order to reduce the number of auxiliary fringe patterns to be more adopted to real-time measurement, the multi-level Gray code is used to assist temporal phase unwrapping. While measuring color object, the fringe order may be disorder by color-to-gray algorithm(C2G). In order to improve the characteristic of anti-color interference in real-time measurement, a multi-level Gray code demodulation method using HSI color space is proposed. Intensity in HSI color space instead of the gray in RGB space is used in multi-level Gray code decoding, at the same time, the color texture of the object surface can also be reconstructed without projecting additional auxiliary color texture patterns. Experimental results show that this method can effectively correct the fringe order confusion caused by color object chroma, and reduce errors in fringe order.

Real-time computer-generated frequency-carrier Moiré profilometry with three-frequency heterodyne temporal phase unwrapping

A real-time computer-generated frequency-carrier Moiré profilometry with three-frequency heterodyne temporal phase unwrapping is proposed. While three sinusoidal gratings shared the same direct current(DC) component and the same amplitude but with designed different frequencies are projected onto the measured object, three corresponding deformed patterns can be captured by camera. Then, the shared DC component is extracted from the highest-frequency pattern. And based on it, three AC components of the captured deformed patterns can also be effectively extracted by normalization. From this way, the number of the needed gratings can be reduced from six to three, and the disturbance of the object’s nonuniform reflectivity can be avoided efficiently. By using computer-generated frequency-carrier Moiré profilometry(CGFCMP), the carrier frequency designed as pure sinusoidal and cosinusoidal gratings is optimized in order to effectively extract the computer-generated frequency-carrier Moiré fringes, and finally-three wrapped phases at different frequencies can be obtained. Then the wrapped phase with the highest-frequency grating can be unwrapped by three-frequency heterodyne temporal phase unwrapping algorithm. The simulation and experimental results verify the feasibility and flexibility of the proposed method. It is suitable to measure the complex surface with abrupt slope and multiple isolated objects, and has prospective application in real-time three-dimensional measurement.

1-to-100-micron surface height full-field 3D topography by use of the external reflectance versus height conversion method

We propose “the external reflectance versus height conversion (ERHC) method” for measuring the full-field three-dimensional surface topography of a sample height from one micron to 100 micrometers. It is similar to the camera method, capturing images reflected and/or not from a prism by using a lens and a CCD. The reflectance of a point in the image can be converted to the height of the point. The method can obtain large-area full-field real-time three-dimensional measurement results and has the advantages of simple operation, low cost, and easy assembly. The measurement area is in the range of a few square millimeters for each time. The lateral and vertical resolutions are 2 and 0.1 micrometers, respectively, and the error is about 1% compared with the confocal microscope.

Grayscale imbalance correcting method based on fringe normalization in RGB tricolor real-time three-dimensional measurement

In real-time phase measuring profilometry based on color-encoded sinusoidal three-step phase-shifting algorithm, three frames of red (R), green (G), and blue (B) individual channel color-encoded sinusoidal gratings with an equivalent shifting phase of 2π/3 are combined to be a color fringe pattern. While this color fringe pattern is projected onto the measured object rapidly and sequentially by a special digital light projector, a high-speed monochrome camera synchronized with the projector signal is used to capture the corresponding deformed patterns in R, G, and B channels. Due to the monochrome camera’s different sensitivity to R, G, and B light, there will be a grayscale imbalance among the captured phase-shifting deformed patterns from R, G, and B channels. So, a grayscale imbalance correcting method based on fringe normalization is proposed. The experimental results verify the feasibility of the proposed method. The three-dimensional (3-D) shape information can be obtained in real-time. It is suitable for real-time 3-D measurement.

结合极线约束和散斑相关的实时三维测量方法

结合极线约束和散斑相关的实时三维测量方法

Graphics processing unit-assisted real-time three-dimensional measurement using speckle-embedded fringe.

This paper presents a novel two-frame fringe projection technique for real-time, accurate, and unambiguous three-dimensional (3D) measurement. One of the frames is a digital speckle pattern, and the other one is a composite image which is generated by fusing that speckle image with sinusoidal fringes. The contained sinusoidal component is used to obtain a wrapped phase map by Fourier transform profilometry, and the speckle image helps determine the fringe order for phase unwrapping. Compared with traditional methods, the proposed pattern scheme enables measurements of discontinuous surfaces with only two frames, greatly reducing the number of required patterns and thus reducing the sensitivity to movements. This merit makes the method very suitable for inspecting dynamic scenes. Moreover, it shows close performance in measurement accuracy compared with the phase-shifting method from our experiments. To process data in real time, a Compute Unified Device Architecture-enabled graphics processing unit is adopted to accelerate some time-consuming computations. With our system, measurements can be performed at 21 frames per second with a resolution of 307,000 points per frame.

Depth fingerprint map of active range-gated imaging for real-time three-dimensional measurement of foreground objects

We present a depth fingerprint map to obtain three-dimensional (3D) spatial information from objects in a large deterministic background by active range-gated imaging, for night remote surveillance. This method first gives the depth fingerprint map of the region of interest in the background by gate viewing in the form of contour bands in range. The map is then embedded in the range-gated laser surveillance system. Finally, 3D spatial information such as target scale and location can be estimated by segmenting the target from the background and matching them with the depth fingerprint map. The measurement is performed by computer background processing. Therefore, the method has no influence on the frame rate of surveillance systems and can realize real-time surveillance. In this paper, the approach to acquisition of the depth fingerprint map is also demonstrated without an echo-broadening effect.

一种提高复合光栅实时三维测量精度的方法

一种提高复合光栅实时三维测量精度的方法

Direct Visualization of Molecularly Thin Lubricant Films Using Low-Coherence Phase-Shifting Interferometry

We have developed a method utilizing low-coherence phase-shifting interferometry (LCPSI) to directly visualize molecularly thin lubricant films coated on diamond-like carbon (DLC) surfaces. The measurement accuracy is guaranteed by the use of a low-coherence light source, a feedback-controlled phase-shifting system, and an image subtraction method. By measuring nanometer-thick perfluoropolyether films coated on DLC surfaces, we demonstrated that the LCPSI is capable of providing real-time three-dimensional measurements at a sub-nanometer thickness resolution and a spatial resolution of several micrometers.

Three-Dimensional Measurements of Nanometer-Thick Lubricant Films Using Low-Coherence Phase-Shifting Interferometry

We have developed a method utilizing low-coherence phase-shifting interferometry (LCPSI) to directly visualize molecularly thin lubricant films coated on diamond-like carbon (DLC) surfaces. In contrast to conventional point-by-point scanning methods, this method straightforwardly measures lubricant distributions in three dimensions using phase-shifted two-dimensional interferograms which are generated by a Michelson interferometer and a robust feedback controlled phase-shifting system. To effectively suppress the image noise caused by unwanted interference, we employed a low-coherence light source rather than a laser. Moreover, an image subtraction method, in which the pseudo lubricant thickness attributed to surface roughness and system noise is first measured on a non-lubricated disk and is then subtracted from the result obtained on the lubricated disk, was introduced to eliminate the influence of surface roughness and system noise. By measuring nanometer-thick perfluoropolyether films coated on DLC surfaces, we demonstrated that the LCPSI is capable of providing real-time three-dimensional measurements with a thickness resolution of sub-nanometer and a spatial resolution of several micrometers.

Development of a real-time three-dimensional spinal motion measurement system for clinical practice

This paper presents an inertial based sensing system for real-time three-dimensional measurement of human spinal motion, in a portable and non-invasive manner. Applications of the proposed system range from diagnosis of spine injury to postural monitoring, on-field as well as in the lab setting. The system is comprised of three inertial measurement sensors, respectively attached and calibrated to the head, torso and hips, based on the subject's anatomical planes. Sensor output is transformed into meaningful clinical parameters of rotation (twist), flexion-extension and lateral bending of each body segment, with respect to calibrated global reference space. Modeling the spine as a compound flexible pole model allows dynamic measurement of three-dimensional spine motion, which can be animated and monitored in real-time using our interactive GUI. The accuracy of the proposed sensing system has been verified with subject trials using a VICON optical motion measurement system. Experimental results indicate an error of less than 3.1 degrees in segment orientation tracking.