Junhui Huang

Calibration of a camera–projector measurement system and error impact analysis

In the camera–projector measurement system, calibration is a key to the measurement accuracy; especially, it is more difficult to obtain the same calibration accuracy for projector than camera due to the inaccurate corresponding relationship between its calibration points and imaging points. Thus, based on stereo vision measurement models of the camera and the projector, a calibration method with direct linear transformation (DLT) and bundle adjustment (BA) is introduced to adjust the corresponding relationships for better optimization purpose in this paper, which minimize the effect of inaccurate calibration points. And an integral method is presented to improve the precision of projection patterns to compensate the projector resolution limitation. Moreover impacts of system parameter and calibration points errors are evaluated when the calibration points positions change, which not only provides theoretical guidance for the rational layout of the calibration points, but also can be used for the optimization of system structure. Finally, the calibration of the system is carried out and the experiment results show that better precision can be achieved with those processes.

Adaptive spatial filtering based on region growing for automatic analysis in digital holographic microscopy

Abstract. The adaptive spatial filtering method is commonly adopted to extract the +1 term spectrum in digital holography for real-time dynamic analysis. However, the typical filtering method is not satisfactory for automatic analysis, because the reset of the filtering window is needed to extract the area of the +1 term spectrum. Therefore, an adaptive spatial filtering method based on region growing and the characteristic of the spectrum separation is proposed. Its filtering window is automatically formed by region growing. The key parameters, including threshold and seed point, are set by the intensity distribution of the hologram spectrum. Then the adaptive filtering extracting the +1 term spectrum is realized by multiplying the hologram spectrum by the filtering window. Compared to the typical filtering method, the experimental results of a microhole array and a phase step show that the proposed method has better adaptability and a higher precision. Moreover, the applicability of this method for different uses is also demonstrated by experiments with a microhole array and a phase step.

Projector calibration with error surface compensation method in the structured light three-dimensional measurement system

Abstract. In the structured light three-dimensional measurement system, calibration is the key to the measurement accuracy; however, conventional calibration methods for the projector are either too complex or inaccurate. Therefore, we propose a simple and accurate method to calibrate the projector. In this method, the calibration points in the camera image plane can be mapped to the projector according the homography of the planar projection. In addition, an error surface compensation method is developed to minimize mapping errors caused by lens distortion of the camera and projector. As a result, the projector can be calibrated with the same method as the camera. Experiments are conducted to verify the effectiveness of the proposed method.

A novel color coding method for structured light 3D measurement

This paper presents a novel color coding method which is applied to structured light measurement using fringe-pattern projections. The method is based on time coding, and reduces the projection times of fringe patterns with the same number of fringes by adding colors into the conventional fringe patterns. But the introduction of colored fringes also brings a difficulty in image processing, that is color crosstalk. To address this problem, sinusoidal fringe patterns with four colors of white, red, green and blue are used to project to the measured object surface to reduce the edge effect. Based on those ideas and methods, a three-dimensional measurement system is built, and the direct linear transform (DLT) method of calibration with lens distortion corrected by cross ratio invariance principle is used to calibrate the internal and external parameters of the system. Then the parameters of the system are optimized by the bundle adjustment method. Finally, a standard metal hemisphere with spraying is measured, and the experimental results show that with the same times of projection, the lateral measurement resolution is higher than the traditional method, and the measurement accuracy of the depth direction is about 0.27mm.

Analysis and Compensation for Lateral Chromatic Aberration in a Color Coding Structured Light 3D Measurement System

While color-coding methods have improved the measuring efficiency of a structured light three-dimensional (3D) measurement system, they decreased the measuring accuracy significantly due to lateral chromatic aberration (LCA). In this study, the LCA in a structured light measurement system is analyzed, and a method is proposed to compensate the error caused by the LCA. Firstly, based on the projective transformation, a 3D error map of LCA is constructed in the projector images by using a flat board and comparing the image coordinates of red, green and blue circles with the coordinates of white circles at preselected sample points within the measurement volume. The 3D map consists of the errors, which are the equivalent errors caused by LCA of the camera and projector. Then in measurements, error values of LCA are calculated and compensated to correct the projector image coordinates through the 3D error map and a tri-linear interpolation method. Eventually, 3D coordinates with higher accuracy are re-calculated according to the compensated image coordinates. The effectiveness of the proposed method is verified in the following experiments.

Total aberrations compensation for misalignment of telecentric arrangement in digital holographic microscopy

Abstract. The telecentric arrangement in digital holographic microscopy (DHM), considered to be a pure-physical compensation for defocus aberration introduced by microscope objective (MO), shows shift-invariant behavior. Its optical arrangement requires precise adjustment of the distance between MO aperture stop and collimated lens. However, it is difficult to measure and quantify the distance even by monitoring the spatial frequency spectrum of recorded hologram in the absence of object. Thus the misalignment results in the residual defocus aberration in the telecentric arrangement. The total aberrations compensation for misalignment of telecentric arrangement in DHM is presented, in which a posteriori surface fitting method based on Zernike polynomials is performed to eliminate the residual defocus aberration as well as other primary aberrations. The approach reduces the difficulty in precise alignment of the telecentric arrangement and decreases the measurement error caused by aberrations in construction. Three-dimensional retrieval of the height for micro-hole arrays with high-spatial-frequency content demonstrates the feasibility of the method.

Modeling and Analysis of Phase Fluctuation in a High-Precision Roll Angle Measurement Based on a Heterodyne Interferometer.

Heterodyne interferometry is a high-precision method applied in roll angle measurements. Phase metering is essential for high precision. During a high-precision measurement, a phase fluctuation appears even when the roll angle does not vary, which has never been analyzed before. Herein, the reason for the phase fluctuation is revealed, which results from the frequency-difference fluctuation and time difference between measurement and reference beams. A mathematical model of that phase-fluctuation mechanism is established, and that model provides a theoretical basis for analyzing and reducing the phase fluctuation. The impact that the main factors have on the phase metering is analyzed quantitatively, and experiments are carried out to validate the model. Finally, the phase fluctuation decreases to 0.02° by frequency reduction, which conversely verifies the theoretical model.

High-precision registration of point clouds based on sphere feature constraints

Point cloud registration is a key process in multi-view 3D measurements. Its precision affects the measurement precision directly. However, in the case of the point clouds with non-overlapping areas or curvature invariant surface, it is difficult to achieve a high precision. A high precision registration method based on sphere feature constraint is presented to overcome the difficulty in the paper. Some known sphere features with constraints are used to construct virtual overlapping areas. The virtual overlapping areas provide more accurate corresponding point pairs and reduce the influence of noise. Then the transformation parameters between the registered point clouds are solved by an optimization method with weight function. In that case, the impact of large noise in point clouds can be reduced and a high precision registration is achieved. Simulation and experiments validate the proposed method.

Improving the measuring accuracy of structured light measurement system

Abstract. A method for improving the measuring accuracy of structured light measurement system, which adopts projecting stripe pattern to measure the three-dimensional profile, is presented. Based on the evaluation of the reliability of center extraction results, the improvement of accuracy is achieved by identifying and rejecting the stripe center extraction results with large error. Two parameters are used to evaluate the reliability of center extraction results. The first parameter is the average energy of the stripe, which is used to analyze and establish the relationship between the extraction accuracy and the signal-to-noise ratio through a statistical method. The second parameter is the asymmetric degree of the stripe gray distribution which introduces error into the center extraction, and a new method is proposed for measuring the asymmetric degree. Then, the criteria of the data rejection defined by the thresholds are presented, and large error data with low reliability are identified according to the thresholds. Higher measuring accuracy is achieved by rejecting the identified data. The validity of the method has been proved by experiments.

Coherent noise reduction of reconstruction of digital holographic microscopy using a laterally shifting hologram aperture

Abstract. The coherent noise appears in the constructed image of digital holographic microscopy due to the laser source; thus, the imaging quality is degraded. A method of coherent noise reduction using a laterally shifting hologram aperture is presented. An original hologram with coherent noise is captured by a camera first. A series of holograms are sampled by laterally shifting the digital aperture in the original hologram. Instead of extracting the specimen’s part information, each sampled hologram, which includes the whole specimen, is reconstructed. The coherent noise is reduced by averaging the different reconstructed images. The experiment demonstrates the feasibility of the approach. The presented approach with a single recorded hologram realizes the coherent noise reduction without loss of spatial resolution, which is useful for real-time measurement.