Based on the phase-shifting fringe scanning projection technique using MEMS galvanometer, the threedimensional shape measurement of the object can be realized. However, we found that the experimental results were more sensitive to some experimental parameters. To avoid it, we introduced a light plane calibration model in the threedimensional measurement system. This calibration model avoids the physical measurement of some experimental parameters, and at the same time makes the operation more convenient and faster, and improves the stability and operability of the experiment. Through the detection experiment of the plane, the relative root mean square relative error of the obtained result is 9.155mm, which is greatly improved compared with the previous experimental results, but the result is still not ideal. We discuss some factors affecting the accuracy of the system which can be improved in the future. The experiment proved its feasibility and potential, and the Powell prism and MEMS galvanometer selected are small in size. The dimensions of Powell prism and MEMS galvanometer are 9.0mm 8.0mm and 2.5mm 3.0mm respectively. The miniaturization of equipment is very advantageous for achieving integration and maintenance of the device.
Lidar is short for light detection and ranging, which is a tool to help measuring some useful information of atmosphere. In the recent years, more and more attention was paid to the research of wind measurement by lidar. Because the accurate wind information can be used not only in weather report, but also the safety guarantee of the airplanes. In this paper, a more detailed signal model of wind measurement lidar is proposed. It includes the laser transmitting part which describes the broadening of the spectral, the laser attenuation in the atmosphere, the backscattering signal and the detected signal. A Voigt profile is used to describe the broadening of the transmitting laser spectral, which is the most common situation that is the convolution of different broadening line shapes. The laser attenuation includes scattering and absorption. We use a Rayleigh scattering model and partially-Correlated quadratic-Velocity-Dependent Hard-Collision (pCqSDHC) model to describe the molecule scattering and absorption. When calculate the particles scattering and absorption, the Gaussian particles model is used to describe the shape of particles. Because of the Doppler Effect occurred between the laser and atmosphere, the wind velocity can be calculated by the backscattering signal. Then, a two parameter Weibull distribution is used to describe the wind filed, so that we can use it to do the future work. After all the description, the signal model of coherent wind measurement lidar is decided. And some of the simulation is given by MATLAB. This signal model can describe the system more accurate and more detailed, so that the following work will be easier and more efficient.
Streak tube imaging lidar (STIL) is an active imaging system that has a high depth resolution with the use of a pulsed laser transmitter and streak tube receiver to produce three-dimensional (3-D) range images. This work investigates the optimal signal width of the lidar system, which is helpful to improve the depth resolution based on the centroid algorithm. Theoretical analysis indicates that the signal width has a significant effect on the depth resolution and the optimal signal width can be determined for a given STIL system, which is verified by both the simulation and experimental results. An indoor experiment with a planar target was carried out to validate the relation that the range error decreases first and then increases with the signal width, resulting in an optimal signal width of 8.6 pixels. Finer 3-D range images of a cartoon model were acquired by using the optimal signal width and a minimum range error of 5.5 mm was achieved in a daylight environment.
Based on the work on scattering properties of laser beam in the atmosphere, we designed and made a prime coherent lidar using laser light at 1.55 μm. By using this equipment, we detected the reflected signal from a wall and scattering signal from the air. Then we tried to measure the backscattering coefficient of air in our local. However, after analyzing and calculating on the data we got, the result is not very good. We did some quantum calculation about our equipment, and gave some suggestions about how to improve it to a practical device.
We have developed two range gating laser imaging systems, which base on modulated gain method and centroid algorithm respectively. The system based on centroid algorithm uses a pulse laser as transmitter and CCD with image intensifier, through delaying gated receiver repetitiously to acquire a series of 2D time-slicing image to restore the three-dimensional rang image. The imaging system based on modulated gain mainly adopts the same pulse laser is used and CCD with image intensifier with constant and modulated gain over time, which can capture two different intensity images of echo pulse respectively in order to restore the three-dimensional range image. We conduct a series of experiments with the tow system, and the results are consistent with the actual practical ones. By comparing result of laser imaging based on the two systems, we summarize the different strength and weaknesses, requirements, and challenges to lay a solid foundation for later experiments.
Gain modulation imaging technique is one of the prominent schemes for scannerless lidar. By controlling the gate width, it’s easy to suppress backscatter noise and make the image more accurately. Imaging range and accuracy of gain modulation laser imaging become a research focus at present. According to the principle of imaging, the signal energy and the noise energy reaching the imager can be found. Further signal-to-noise ratio can be obtained. Previous theoretical models consider only linear gain condition. However the influence of laser pulse width and other factors are less taken into consideration. These models will have a certain deviation with the actual one. By simulating the nonlinear gain with consideration of the laser pulse width and lambert spherical radiation, more accurate SNR model of gain modulation laser imaging is obtained. On this basis, the established SNR model can be used to estimate the experimental distance with good imaging effect. It provides the theoretical basis for subsequent experiment system parameter selection and image processing.
We have developed a whole set of range gated laser imaging system with ~3km maximum acquisition distance, the
system uses a Nd:YAG electro-optical Q-switched 532nm laser as transmitter, a double micro channel plate as gated
sensor, all the components are controlled by the a trigger control unit with accuracy of subnanosecond. A imaging
scheme is designed for imaging the large building ~500m away, and a sequence of images are obtained in the
experiment, which are the basic data for 3D reconstruction; to improve the range resolution, we study the temporal
distribution of intensity of the received signal, and use centroid algorithm for data processing. We compare the 3D image
with the theoretical model, and the results are corresponding.
A range gated laser imaging system has been designed and developed for high precision three-dimensional imaging. The system uses a Nd:YAG electro-optical Q-switched 532nm laser as transmitter, a double microchannel plate as gated sensor, and all the components are controlled by a trigger control unit with accuracy of subnanosecond. An experimental scheme is also designed to achieve high precision imaging; a sequence of 2D “slice” images are acquired in the experiment, and these images provide the basic data for 3D reconstruction. Basing on the centroid algorithm, we have developed the 3D reconstruction algorithm, and use it to reconstruct a 3D image of target from the experimental data. We compare the 3D image with the system performance model, and the results are corresponding.
Laser radar can simultaneously produce the intensity and range images, and the space resolution is high, so the
recognition performance is well, and it can choose the aim point of target. Laser radar is applied to many fields, such as
guidance, navigation, and becomes the research hot point in recent years. In the vertical detection of laser radar, the
algorithm is required not only solving in-plane rotation-invariant problem, also the distortion-invariant problem, and it
must satisfied the real-time. Correlation algorithm is a parallel processing procedure, detecting many targets at one time,
and its design can be implemented on the high speed digital signal processor. In the paper, a new filter named
CHF-MACH filter is presented, which combine multiple circular harmonic expansions into one filter through MACH
criteria. Because of the filter having the characters of the two filters, it can solve the problems of in-plane
rotation-invariance and distortion-invariance simultaneously, and meet the real-time requirement. The simulated range
image of laser radar is regarded as research target, and computing the PSR (peak to sidelobe ratio) values of correlation
output of the different objects, and plotting the PSR curves of the different angles. Simulating the scene of laser radar
which includes multiple objects, CHF-MACH filter performance is validated through testing with the different angles for
the objects, and the non-training images can obtain the well correlation output.
The target recognition of laser radar is a hot research because laser radar can produce the intensity and range imagery. Laser radar has high space resolution, and can obtain rich target information. Correlation recognition has been used to many fields, such as infrared as well as synthetic aperture radar (SAR). In this paper, the two filters are used in experiment of laser radar. MACH filter is used to detect the target, and DCCFs are used to recognize the unknown target. The samples are generated by OpenGL technology, and the filters are designed using the simulated ladar images. The test samples are added noise according to the imaging principle of laser radar. Two sample sets, one adding noise, another filtering the noise, are used in order to contrast the different performance. At last, the experiment results are given.
In recent years, with the rapid development of laser radar technology both in military and civil fields, the research on laser radar has come to the stage of application throughout the world. This paper studies the offset frequency locking system used for a short-pulsed laser outdoors. It is often the case that if the received IF signal bandwidth of coherent laser radar becomes more narrow in coherent detection, the signal noise ratio will be improved greatly. Therefore the technique is needed for offset locking the heterodyne frequency between two lasers. However the unstable ambient temperature will produce an effect on the behavior of a gas laser, and the offset locking technique should have such characterized features as in a frequency offset locking loop used for pulsed lasers. The heterodyne frequency between two lasers is necessitated to have a wide enough bandwidth so as to prevent itself from a loss of locking and to have a high enough discriminating frequency accuracy so as to make the radar possess a high accuracy of measuring the target speed. In order to attack the problems mentioned above, this paper designs an offset frequency locking system with two frequency discrimination loops. One has an all-digital frequency discriminator with a bandwidth of 600MHz and accuracy of 5MHz in discriminating frequency to an optical pulse width 30 ; the other has a coincidence gate circuit discriminator with a bandwidth of 20MHz and an accuracy of less then 500kHz in discriminating frequency to an optical pulse width 30 ns.
Laser radar has been widely used these years and the hardware-in-the-loop (HWIL) testing of laser radar become important because of its low cost and high fidelity compare with On-the-Fly testing and whole digital simulation separately. Scene generation and projection two key technologies of hardware-in-the-loop testing of laser radar and is a complicated problem because the 3D images result from time delay. The scene generation process begins with the definition of the target geometry and reflectivity and range. The real-time 3D scene generation computer is a PC based hardware and the 3D target models were modeled using 3dsMAX. The scene generation software was written in C and OpenGL and is executed to extract the Z-buffer from the bit planes to main memory as range image. These pixels contain each target position x, y, z and its respective intensity and range value. Expensive optical injection technologies of scene projection such as LDP array, VCSEL array, DMD and associated scene generation is ongoing. But the optical scene projection is complicated and always unaffordable. In this paper a cheaper test facility was described that uses direct electronic injection to provide rang images for laser radar testing. The electronic delay and pulse shaping circuits inject the scenes directly into the seeker's signal processing unit.