The experimental dispersion curves of laser-shock-peened aluminum samples were obtained from the phase spectra of the laser induced surface acoustic waves (SAWs) and the results indicated that the more the peening times, the greater the dispersion. The change in phase velocity resulting from the surface roughness was obtained by comparing the dispersions of the samples before and after being polishing. It was found that the surface roughness decreased the phase velocity, and the change in velocity depended on the frequency of the SAW as expected from the theoretical analysis of the case when the lateral correlation length is much smaller than the wavelength of the SAWs. Furthermore, the experimental dispersion results of the SAWs for three samples with different peening times showed that the more the peening times, the smaller the roughness, which was verified by the values measured with a roughmeter.
With the development of optoelectronic technologies, color cameras have been widely exploited in space remote sensing, earth observations from space, environmental monitoring, urban construction, and many other fields. Currently, most commercial color cameras use a single charge coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) sensor that has a Bayer color filter array (CFA) on its pixel surface to obtain red (R), green (G), or blue (B) samples. As a way of evaluating imaging quality, modulation transfer function (MTF) can provide a comprehensive and objective metric for camera imaging performance. In the conventional knife-edge method for color camera MTF measurement, a linear uniform sampling of the edge spread function (ESF) must be completed before a fast Fourier transform (FFT) can be applied. As the sampling rate becomes large, the number of pixel points on the line which is parallel to the knife-edge become less. So taking average of the pixel points to obtain ESF can be strongly affected by the noise of sensor. Therefore it is necessary to balance the influences of sampling rate and sensor noise on the MTF measurement, and the recommended sampling rate is 4-6. When the tilt angle of knife-edge has an error, the non-uniform sampling ESF can be obtained by the slanted knife-edge method. This leads to a variation in the results of the camera MTF on a spatial frequency scale and early cut-off. The best MTF results of camera can be obtained by rotating knife-edge, calculating MTF power under different tilt angles of knife-edge, and finding the maximum MTF power. And we propose an algorithm for Bayer filter color camera MTF measurement. The algorithm processing includes extracting R, G, B colors of knife-edge images; projection; differential operation; Hanning window filtration; FFT; correction; weighting combination of R, G, B colors MTF; MTF power calculation; optimal tilt angle of knife-edge estimation. To verify the accuracy of the proposed method, the weighting response factors of R, G, B colors are calibrated and an experimental setup for color camera MTF measurement is established. The knife-edge target is rotated in angle steps of 0.02, and the MTF results are calculated under different tilt angles of knife-edge within0.1 surrounding the estimate position by the proposed algorithm. The maximum differences of MTF results between the proposed method and fringe target method are 0.061 (Nyquist frequency fc) and 0.043 (fc/2), respectively. The results show that by searching the optimal tilt angle of knife-edge, the effect of non-uniform sampling on MTF result of color camera can be eliminated. Compared with the conventional method, the proposed method is superior for the measurement of the super-sampled MTF of color camera. Meanwhile, this method can also be applied to MTF measurements of radiographic systems, such as X-ray imaging system and other systems.
Traditional analytical algorithm needs to combine the transmission functions of grating and lens to generate a computer generated hologram (CGH), so as to realize the distribution of three-dimensional (3D) multi-focal points in space, but the grating phase will inevitably produce high-order diffraction focus, resulting in energy loss, and the traditional analytic algorithm is more suitable for generating array multi-focal distribution with equal spacing. To solve this problem, this paper simplifies the traditional analytical algorithm, and proposes a method that only uses multi-lens phase and random phase superposition to generate the CGH required by the target light location, by changing the focal length of the lens phase, the multi-focus distribution along the z-axial direction of multiple independent focal planes is realized. Then the phase of these different focal planes is superimposed, and a 0~2π random phase modulation is added, which can quickly generate 3D multi-focus distribution with controllable number and position. The simulation results show that the energy uniformity of focal spot on each focal plane is between 89.45% and 98.08%. The experimental results show that the energy uniformity of focal spots on each focal plane is between 88.40% and 96.13%, which is consistent with the simulation results. Compared with traditional analytical algorithm, the proposed method is more universal for multi-focus distribution in 3D space without special requirements of array distribution with equal spacing, and has potential application value in laser processing, holographic optical tweezers, optical communication and other fields.
The angular method (AS) cannot be used in long-distance propagation because it produces severe numerical errors due to the sampling problem in the transfer function. Two ways can solve this problem in AS for long-distance propagation. One is zero-padding to make sure that the calculation window is wide enough, but it leads to a huge calculation burden. The other is a method called band-limited angular spectrum (BLAS), in which the transfer function is truncated and results in that the calculation accuracy decreases as the propagation distance increases. In this paper, a new method called modified scaling angular spectrum (MSAS) to solve the problem for long-distance propagation is proposed. A scaling factor is introduced in MSAS so that the sampling interval of the input plane can be adjusted arbitrarily unlike AS whose sampling interval is restricted by the detector’s pixel size. The sampling interval of the input plane is larger than the detector’s pixel size so the size of calculation window suitable for long-distance field propagation in the input plane is smaller than the size of the calculation window required by the zero-padding. Therefore, the method reduces the calculation redundancy and improves the calculation speed. The results from simulations and experiments show that MSAS has a good signal-to-noise ratio (SNR), and the calculation accuracy of MSAS is better than BLAS.
Shack-Hartmann sensor is widely used in adaptive optics systems, and laser beam quality measurements. The traditional method separates measures and calculations, and the wavefront reconstruction algorithm is slow to implement on the host computer. In this paper, the embedded GPU is introduced to Shack-Hartmann sensors' wavefront phase reconstruction. A parallel calculation method is proposed to speed up the wavefront phase reconstruction process. The experiment result shows the algorithm speed improves 50× with the image size of 2592×2048 pixels.
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