Traditional binocular stereo vision measurement system based on high resolution area array camera has many defects, such as limited acquisition speed, insufficient sensitivity under high frame rate and overlarge data volume. To overcome these defects, this paper proposes a precision vision measurement method which is based on the optimized combination of pixelbinning cameras. This method transforms the traditional area array camera into an equivalent narrow area array (ENAA) camera through single-dimensional binning. Then, multiple ENAA cameras are used to achieve precise 3D measurement of target points through reasonable combination. However, single-dimensional binning will reduce the image resolution in one direction, while the image resolution in another direction (maximum image resolution) remains unchanged. Therefore, this paper studies the typical spatial layouts of two even three ENAA cameras and analysis their measurement errors. Finally, an optimal spatial layout of three ENAA cameras was obtained, and it can reach the same level of measurement accuracy as the binocular stereo vision measurement system based on area array camera. Comparative experiments verify the effectiveness and measurement accuracy of the above method, which can reduce the data amount to 3/8 and increase the acquisition rate by 4 times while ensuring the measurement accuracy.
The global calibration of multi-camera vision systems with non-overlapping views has been widely studied. However, traditional global calibration methods that using planar targets with invariant relative positions are easily limited by the high cost of manufacturing high-precision large planar targets, and the difficulty of rigid connection between them. This paper proposes a low-cost and convenient global calibration method for non-overlapping multiple cameras using double mobile phone screens in defocus scene. During calibration, phase-shifting circular grating (PCG) arrays are displayed on the high-resolution mobile phone screens, and by phase-shifting methods, the PCG centers as feature points can be extracted accurately even if the phone is located at the defocus part of camera. Then the global calibration can be carried out based on the invariance of the relative positions between the two mobile phones. Experiment results show that this method is effective and has high accuracy.
Femtosecond control of electron spin not only promises the capability of satisfying the ever-increasing demand of storage information and ultrafast manipulation of magnetization in mediums, but also delivering controllable, highlyefficient, cost-effective and compact terahertz sources. Femtosecond spin dynamics have been extensively investigated these years with the methods of ultrafast magnetic-optical Kerr effect, inverse Faraday effect, inverse spin Hall effect and so on. Recently emerged coherent terahertz emission spectroscopy can also be employed to study this ultrafast spin dynamics with its unique advantages. For example, terahertz emission spectroscopy is a coherent, time-resolved, contactless Ampere-meter, which can be used to deduce the spin-charge conversion. However, femtosecond laser interaction with magnetic mediums is a rather complex process, there are still lots of physical mechanisms waiting to be unveiled. Here, we systematically investigate the femtosecond spin dynamics in ferromagnetic materials via polarization-resolved terahertz emission spectroscopy. We obtain detectable electromagnetic field radiation with its polarization parallel to the external magnetic field direction, which was not observed in the same materials in previous work. Inverse spin-orbit torque tilting is responsible for the observed phenomenon. Based on this mechanism, the efficiency and polarization of the generated terahertz waves can be coherently controlled and manipulated not only by the external magnetic fields, but also by the sample structures and the pumping femtosecond laser pulses. Our work not only helps further deepen understanding of the physical mechanism of all-optical magnetization reversal, boosting future spin recording technology, but also offers a very promising way for developing novel and efficient terahertz functional sources and devices.
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