In order to avoid falling into the local optimal solution in the process of linear optimization, a calibration method for calculating the radial and tangential distortion coefficients is proposed. Firstly, the initial internal and external parameters of the camera are obtained by using the coordinates of the characteristic points in the central region of the calibration image. Camera imaging procedure is dividedinto two separate steps in which ideal image point is moved to actual image point through radial distortion and tangential distortion in sequence. With the distortion model and crossratio invariance, the distortion coefficient of the camera can be solved.. It is assumed that the point after distortion correction conforms to perspective projection principle, and the exact value is approximated step with the distortion model and iteration. Compared with the traditional nonlinear optimization method, the average back-projection error of the corrected image coordinates is significantly reduced and has better robustness and accuracy.
In this paper, carbon fiber reinforced polymer (CFRP) is tested using lock-in thermography, which is excited by an induction coil. A impact-damaged CFRP plate was available and measured as a function of the angle between a linear excitation coil and the main fiber directions. The amplitude and phase images of high signal-to-noise ratio (SNR) was obtained by Fast Fourier Transform (FFT). The relationship between angle sensitivity of included angle and crack shape is analyzed. The results show that the circular symmetry is basically maintained when the defect is linear. When the structure of defects is similar to that of bifurcated trees, the angular distribution is quite different, which are consistent with the results of X-ray imaging.
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