Shun Wang

Statistical behavior analysis and precision optimization for the laser stripe center detector based on Steger's algorithm.

Triangulation laser range scanning, which has been wildly used in various applications, can reconstruct the 3D geometric of the object with high precision by processing the image of laser stripe. The unbiased line extractor proposed by Steger is one of the most commonly used algorithms in laser stripe center extraction for its precision and robustness. Therefore, it is of great significance to assess the statistical performance of the Steger method when it is applied on laser stripe with Gaussian intensity profile. In this paper, a statistical behavior analysis for the laser stripe center extractor based on Steger method has been carried out. Relationships between center extraction precision, image quality and stripe characteristics have been examined analytically. Optimal scale of Gaussian smoothing kernel can be determined for each laser stripe image to achieve the highest precision according to the derived formula. Flexible three-step noise estimation procedure has been proposed to evaluate the center extraction precision of a typical triangulation laser scanning system by simply referring to the acquired images. The validity of our analysis has been verified by experiments on both artificial and natural images.

Gaussian pulse gated InGaAs/InP avalanche photodiode for single photon detection

The capacitive response noise has been problematic for high-speed single photon detection based on gated InGaAs/InP avalanche photodiodes. Traditionally, the noise must be suppressed by complex electronic circuit if low afterpulse probability is desired. In this Letter, we propose a compact and flexible method for noise cancellation, which gates the photodiode with a Gaussian pulse. Because of the differential effect of junction capacitor, the shape of the capacitive response output in our method is the first-order derivative of the Gaussian function that can be matched by the rising edge of a delayed and attenuated version of the gating pulse itself. With matching signal, the avalanche pulse is raised onto a flat platform that can be easily discriminated from the background. For 1550 nm optical signal, the detection efficiency could reach 10.2% with 9.7×10(-6) per gate dark count probability and 3.4% afterpulse probability at 80 MHz gating frequency. Experimental results have shown that the proposed method can decrease the afterpulse probability sharply while maintaining the detection efficiency and dark count performance.

Freight train gauge-exceeding detection based on three-dimensional stereo vision measurement

We present a method for freight train gauge-exceeding detection based on three-dimensional (3D) stereo vision measurement. To reach high measurement accuracy under large-scale situation, the factors which influence the 3D measurement error are analyzed in detail. Algorithm to accurately extract the laser stripe feature projected by the measurement system is described. With the obtained stripe features, the 3D structure of the freight train can be reconstructed with nonlinear optimization procedure. Specially designed targets are used to identify the global coordinate system for gauge-exceeding detection. A prototype has been developed and the reliability and accuracy have been demonstrated by external field experiment.

Quality inspection guided laser processing of irregular shape objects by stereo vision measurement: application in badminton shuttle manufacturing

Abstract. The quality inspection process is usually carried out after first processing of the raw materials such as cutting and milling. This is because the parts of the materials to be used are unidentified until they have been trimmed. If the quality of the material is assessed before the laser process, then the energy and efforts wasted on defected materials can be saved. We proposed a new production scheme that can achieve quantitative quality inspection prior to primitive laser cutting by means of three-dimensional (3-D) vision measurement. First, the 3-D model of the object is reconstructed by the stereo cameras, from which the spatial cutting path is derived. Second, collaborating with another rear camera, the 3-D cutting path is reprojected to both the frontal and rear views of the object and thus generates the regions-of-interest (ROIs) for surface defect analysis. An accurate visual guided laser process and reprojection-based ROI segmentation are enabled by a global-optimization–based trinocular calibration method. The prototype system was built and tested with the processing of raw duck feathers for high-quality badminton shuttle manufacture. Incorporating with a two-dimensional wavelet-decomposition–based defect analysis algorithm, both the geometrical and appearance features of the raw feathers are quantified before they are cut into small patches, which result in fully automatic feather cutting and sorting.

Electro-optic modulator based gate transient suppression for sine-wave gated InGaAs/InP single photon avalanche photodiode

Abstract. Capacitive gate transient noise has been problematic for the high-speed single photon avalanche photodiode (SPAD), especially when the operating frequency extends to the gigahertz level. We proposed an electro-optic modulator based gate transient noise suppression method for sine-wave gated InGaAs/InP SPAD. With the modulator, gate transient is up-converted to its higher-order harmonics that can be easily removed by low pass filtering. The proposed method enables online tuning of the operating rate without modification of the hardware setup. At 250 K, detection efficiency of 14.7% was obtained with 4.8×10−6u2009u2009peru2009gate dark count and 3.6% after-pulse probabilities for 1550-nm optical signal under 1-GHz gating frequency. Experimental results have shown that the performance of the detector can be maintained within a designated frequency range from 0.97 to 1.03 GHz, which is quite suitable for practical high-speed SPAD applications operated around the gigahertz level.

Error analysis and system implementation for structured light stereo vision 3D geometric detection in large scale condition

Stereo vision based 3D metrology technique is an effective approach for relatively large scale object’s 3D geometric detection. In this paper, we present a specified image capture system, which implements LVDS interface embedded CMOS sensor and CAN bus to ensure synchronous trigger and exposure. We made an error analysis for structured light vision measurement in large scale condition, based on which we built and tested the system prototype both indoor and outfield. The result shows that the system is very suitable for large scale metrology applications.