Electro-optical Imaging Systems Research Articles

Experimental Study on Damage Effect of Mid-Infrared Pulsed Laser on Charge Coupled Device (CCD) and HgCgTe Detectors.

As the weak link in electro-optical imaging systems, photodetectors have always faced the threat of laser damage. In this paper, we experimentally investigated the damage mechanism of the photodetector induced by the out-of-band laser. The damage thresholds of the mid-infrared pulsed laser for Charge Coupled Device (CCD) and HgCdTe detectors were determined through damage experiments. The analysis of the damage phenomena and data for both CCD and HgCdTe detectors clearly demonstrated that out-of-band mid-infrared pulsed lasers could entirely incapacitate CCD and HgCdTe detectors. Our analysis of the damage process and data revealed that the primary mechanism of damage to CCD and HgCdTe detectors by mid-infrared pulsed lasers was primarily thermal. This study serves as a reference for further research on the mid-infrared pulsed laser damage mechanisms of CCD and HgCdTe detectors, as well as for laser protection and performance optimization in imaging systems.

Millimeter-Wave Band Electro-Optical Imaging System Using Polarization CMOS Image Sensor and Amplified Optical Local Oscillator Source.

In this study, we developed and demonstrated a millimeter-wave electric field imaging system using an electro-optic crystal and a highly sensitive polarization measurement technique using a polarization image sensor, which was fabricated using a 0.35-µm standard CMOS process. The polarization image sensor was equipped with differential amplifiers that amplified the difference between the 0° and 90° pixels. With the amplifier, the signal-to-noise ratio at low incident light levels was improved. Also, an optical modulator and a semiconductor optical amplifier were used to generate an optical local oscillator (LO) signal with a high modulation accuracy and sufficient optical intensity. By combining the amplified LO signal and a highly sensitive polarization imaging system, we successfully performed millimeter-wave electric field imaging with a spatial resolution of 30×60 µm at a rate of 1 FPS, corresponding to 2400 pixels/s.

Exploring the impact of high-power infrared lasers on electro-optical systems performance: A field study with different wavelengths

The performance of different electro-optical (EO) and infrared (IR) imaging systems under the impact of high-power pulsed infrared lasers has been well evaluated and studied. The infrared lasers operating at 1.06 µm, 3.7–4.2 µm, and 9.6 μm wavelengths, including short-wave infrared (SWIR), mid-wave infrared (MWIR), and long-wave infrared (LWIR) sensors used in television, infrared imaging, and tracking systems. Our research is focused on the intended waveband of the infrared imagers at significant distances relevant to tactical security operations, with a range of 6 km–10 km. The study compares the effects of both dazzling and damaging laser radiation on EO systems using a straightforward theoretical framework supported by atmospheric modeling. It was observed that increased humidity and temperature diminish both dazzling and damage distances, with less impact at 1.06 µm and more significant at 9.6 µm, compared to 3.7–4.2 µm. The effect of atmospheric conditions on IR laser energy was studied using MODTRAN software to determine the effective laser parameters for EO imaging systems at different distances. The study also investigates the critical parameters of practical high-power pulsed infrared laser wavelengths to dazzle and damage imaging systems at different distances. Field investigations revealed that dazzling on TV seekers can extend beyond 10 km by 1.06 µm, with damages up to 8 km, depending on the sensor field of view, laser source pulse frequency, and environmental conditions. Lasers with wavelengths of 3.7–4.2 μm can produce high-dazzling spots at the center of images of MWIR seekers up to 6 km. Lasers with wavelengths of 9.6 μm can rapidly damage LWIR seekers’ pixels due to their varied energy outputs and pulse frequency-range responses up to 8 km. System functions were lost under the dazzling and damaging effects. The study revealed a strong correlation between experimental results and predictions, with a strong agreement between experimental and theoretical effective distances under similar environmental and atmospheric conditions.

Laser safety for electro-optical imaging systems: exposure limits and hazard distances

Laser safety for electro-optical imaging systems: exposure limits and hazard distances

Laser Safety-What Is the Laser Hazard Distance for an Electro-Optical Imaging System?

Laser safety is an important topic. Everybody working with lasers has to follow the long-established occupational safety rules to prevent people from eye damage by accidental irradiation. These rules comprise, for example, the calculation of the Maximum Permissible Exposure (MPE), as well as the corresponding laser hazard distance, the so-called Nominal Ocular Hazard Distance (NOHD). At exposure levels below the MPE, laser eye dazzling may occur and is described by a quite new concept, leading to definitions such as the Maximum Dazzle Exposure (MDE) and to its corresponding Nominal Ocular Dazzle Distance (NODD). In earlier work, we defined exposure limits for sensors corresponding to those for the human eye: The Maximum Permissible Exposure for a Sensor, MPES, and the Maximum Dazzle Exposure for a Sensor, MDES. In this publication, we report on our continuative work concerning the laser hazard distances arising from these exposure limits. In contrast to the human eye, unexpected results occur for electro-optical imaging systems: For laser irradiances exceeding the exposure limit, MPES, it can happen that the laser hazard zone does not extend directly from the laser source, but only from a specific distance to it. This means that some scenarios are possible where an electro-optical imaging sensor may be in danger of getting damaged within a certain distance to the laser source but is safe from damage when located close to the laser source. This is in contrast to laser eye safety, where it is assumed that the laser hazard zone always extends directly from the laser source. Furthermore, we provide closed-form equations in order to estimate laser hazard distances related to the damaging and dazzling of the electro-optical imaging systems.

Dual-antenna GPS-based UAV Attitude Determination Method and Accuracy Analysis

Accurate attitude information of small-sized unmanned aerial vehicles (UAVs) is required to obtain absolute azimuth information of a target via an airborne electro-optical imaging system. The direct attitude solution method of dual-antenna Global Position System (GPS) is investigated in this paper to address this need. We analyzed the attitude measurement principle of the direct method, gave the error estimation model, and estimated the inner and outer precision through simulation. The simulation results demonstrated that to achieve the requirement of UAV yaw angle error within 1°, the baseline length should be no less than 1 m. The applicability of the model and algorithm was confirmed.

Improvement of on-pixel polarizer with 0.35 μm CMOS process for electro-optic imaging systems

In this paper, we propose an improved on-pixel polarizer structure for an image sensor, which was fabricated using the 0.35 μm standard CMOS process with dual metal layers (Metal 1 and Metal 2), i.e. two layers of gratings with metal-wiring layers were stacked. By designing the line/space ratio as 0.7/0.7 μm, the extinction ratio was significantly improved at a wavelength of 780 nm, which was over 1.4 times higher than that of a single-layer grating, and the detection sensitivity of the polarization change was enhanced by more than 1.8 times. We also demonstrated electro-optical imaging at radio frequency using this image sensor.

Motion profile and control design of fast steering mirror for backscanning step and stare imaging.

Fast steering mirrors (FSMs) have been used for decades to improve the performance of electro-optical imaging systems, such as airborne imaging systems and space-based optical surveillance systems. With the advantage of increasing the accuracy of image motion compensation and the efficiency of scanning imaging, backscanning step-and-stare imaging has become the main approach to realizing wide-area surveillance for airborne imaging systems. According to the operating mode and motion profile of the FSM in the imaging system, a combined optimized profile (COP) is designed to avoid abrupt changes in the velocity and acceleration of the FSM. The angular position sensor based on a four-quadrant detector is used in FSMs to expand the measuring range and cut the cost at the expense of larger measurement noise. Combining a Kalman filter with a disturbance observer and zero-phase error tracking control, a control method is proposed to improve the control precision and bandwidth while suppressing measurement noise. Simulation and experimental results show that the profile designed by COP is smooth enough to meet the special requirements of FSM's backscanning image motion compensation and that the Kalman filter-based FSM control method can significantly improve the control accuracy.

Efficient method for real-time range enhancement of electro-optical imaging system

Electro-optical (EO) imaging sensors are employed for object detection, recognition, identification, and tracking. The observation range of EO systems is defined as the maximum distance at which a target of interest can be seen and perceived. The performance of these systems deteriorates under degraded weather conditions, which severely affects the image quality and thus the observation range of EO surveillance systems. An improved image quality results in an enhanced observation range. Image quality can be improved by applying suitable digital image enhancement techniques by considering target/background signature knowledge. Under bad weather conditions, these digital techniques become ineffective due to the presence of scattered background signals. To reduce the effect of scattering, optical methods are also used to mitigate degradation caused by the atmosphere. It is observed that the optical method alone cannot produce the desired result. Observation range enhancement in surveillance system is achieved using the proposed hybrid method, which utilizes the optimal combination of optical and digital techniques in real-time. The proposed method not only improves human visual perception but also reveals more image content for better image analysis. The performance of the proposed range enhancement technique is compared with the predicted observation range model. The processed images using the hybrid method show more than 30% observation range enhancement under bad weather conditions. In addition to the subjective evaluation, qualitative and quantitative evaluations of image quality achieved through the proposed method are also presented. It is observed that the proposed hybrid method significantly improves the observation range and image quality of EO sensors under degraded weather in real time.

Adaptive position calibration technique for an optical micro-scanning thermal microscope imaging system

In order to improve the spatial resolution of an optical micro-scanning thermal microscope system, the micro-scanning position must be accurately calibrated. An adaptive calibration method based on image registration and plane coordinate system is proposed. The meaning of calibration is given, and the principle and method of point calibration are introduced in detail and experiments using the real system were done. Different reconstruction methods were applied to reconstruct the visible light image and the real thermal microscope image, and the evaluation scores are given. Results of simulation and real thermal imaging processing show that the method can successfully calibrate the micro-scanning position. The method can significantly improve the oversampled reconstructed image quality, thus enhancing the spatial resolution of the system. This method can also be used in other electro-optical imaging systems.

基于激光测距的机载光电成像系统目标定位

基于激光测距的机载光电成像系统目标定位

Modulation transfer function of an imaging system with a hexagonal pixel array detector

The modulation transfer function (MTF) is the tool most commonly used for quantifying the performance of an electro-optical imaging system. Hexagonal sampling strategy for detector arrays can provide an improvement over rectangular sampling, thus the hexagonal pixel array detector is designed with hexagonal pixel and hexagonal sampling. In this paper, firstly, two dimensional MTF model of a hexagonal pixel array detector is deduced according to its definition. Then, the MTF of an imaging system with such a detector is established. Additionally, the MTFs of detectors with other shaped pixel geometry, but with the same photosensitive areas are compared. The results show that the values of hexagonal MTF are obviously larger than those of rectangular and triangular pixel array detectors at the same frequencies. In addition, with the increase of the atmospheric coherence length, the MTF values of the system become larger, and the system imaging capability will be improved.

Theoretical investigations of infrared optical wave modulation transfer function models in anisotropic marine turbulence.

The previously derived marine turbulence modulation transfer function (MTF) adopts the classical assumption of isotropic and Kolmogorov turbulence. However, more theoretical research shows that marine turbulence demonstrates anisotropic non-Kolmogorov properties. In this study, new long-exposure plane and spherical wave MTF models under anisotropic non-Kolmogorov marine turbulence will be investigated. Comparative analyses are performed for the new models and the existing ones that have been developed under the isotropic marine turbulence and the anisotropic terrestrial turbulence cases. It turns out that the anisotropic marine turbulence influences the electro-optical imaging system less than the isotropic marine turbulence does. Also, under the same turbulence strength condition, due to the humidity fluctuations of marine atmosphere, the anisotropic marine turbulence will reduce more severely the quality of electro-optical imaging systems than the anisotropic terrestrial turbulence. Investigations conducted in this paper are beneficial to understand better the electro-optical imaging mechanism in marine atmospheric turbulence media.

Error correction based on micro-scanning preprocessing for an optical micro-scanning thermal microscope imaging system

In recent years, various thermal microscope imaging systems have been developed to meet the demands of micro-thermal analysis for large-scale integrated circuits, biomedical, science, and research fields. However, conventional thermal microscope imaging systems, which use cooled infrared detectors are heavy and expensive. In order to solve this problem, we developed a thermal microscope imaging system based on an uncooled infrared detector. However, the spatial resolution of the thermal microscope imaging system based on an uncooled infrared detector is low. With optical micro-scanning technology, the spatial resolution of the thermal microscope imaging system can be increased without increasing the detector dimension or reducing the detector unit size. In order to improve its spatial resolution, a micro-scanning system based on optical plate rotation was developed, and an optical microscanning thermal microscope imaging system was obtained after the integrated design. Due to environmental factors, mechanical vibration, alignment error and other factors, there is micro-scanning error in the designed micro-scanning thermal microscope imaging system. The four low-resolution images collected by micro-scanning thermal microscope imaging system are not standard down-sampled images. The quality of the image interpolated directly by four collected images is reduced and the performance of the micro-scanning system isn’t fully exploited. Therefore, based on the proposed second-order oversampling reconstruction micro-scanning error correction algorithm and the new edge directed interpolation algorithm, a new micro-scanning error correction technique is proposed. Simulations and experiments show that the proposed technique can effectively reduce optical micro-scanning error, improve the systems spatial resolution and optimize the effect of the imaging system. It can be applied to other electro-optical imaging systems to improve their spatial resolution.

Imaging quality evaluation method of pixel coupled electro-optical imaging system

With advancements in high-resolution imaging optical fiber bundle fabrication technology, traditional photoelectric imaging system have become “flexible” with greatly reduced volume and weight. However, traditional image quality evaluation models are limited by the coupling discrete sampling effect of fiber-optic image bundles and charge-coupled device (CCD) pixels. This limitation substantially complicates the design, optimization, assembly, and evaluation image quality of the coupled discrete sampling imaging system. Based on the transfer process of grayscale cosine distribution optical signal in the fiber-optic image bundle and CCD, a mathematical model of coupled modulation transfer function (coupled-MTF) is established. This model can be used as a basis for following studies on the convergence and periodically oscillating characteristics of the function. We also propose the concept of the average coupled-MTF, which is consistent with the definition of traditional MTF. Based on this concept, the relationships among core distance, core layer radius, and average coupled-MTF are investigated.Results show that the coupled-MTF oscillation converges to a fixed value when the deviation between a spatial frequency of input signal and Nyquist frequency is 1% and when the total number of pixels in the coupled system is more than 1000. The minimal frequency deviation corresponds to the slow velocity of the oscillational convergent. The oscillation amplitude of coupled-MTF differs in tangential and sagittal directions in a manner related to the corresponding pixel coupling error. The coupled-MTF periodically oscillates with the alignment error between the coupled pixel. One cycle is equivalent to the diameter of fiber cladding. The observation from the simulations further reveal that the distance between adjacent fiber cores and the dimension of core layer are directly related to the system imaging quality and the signal-to-noise ratio. Moreover, the average coupled-MTF can be used to quantitatively describe the function of related parameters and the imaging quality of the system.

Optimization method of electro-optical imaging system based on information distortion

Optimization method of electro-optical imaging system based on information distortion

High-resolution over-sampling reconstruction algorithm for a microscanning thermal microscope imaging system

Due to environmental factors, mechanical vibration, alignment error and other factors, the micro-displacement of four collected images deviates from the standard 2×2 micro-scanning images in our optical micro-scanning thermal microscope imaging system. This influences the quality of the reconstructed image and the spatial resolution of the imaging system cannot be improved. To solve this problem and reduce the optical micro-scanning errors, we propose an image reconstruction method based on the principle of the Second-order Taylor series expansion. The algorithm can obtain standard 2×2 microsanning under-sampling images from four non-standard 2×2 microsanning under-sampling images and then can obtain high spatial oversample resolution image. Simulations and experiments show that the proposed technique can reduce the optical micro-scanning errors and improve the systems spatial resolution. The algorithm has low computational complexity, and it is simple and fast. Furthermore this technique can be applied to other electro-optical imaging systems to improve their resolutions.

Modulation transfer function of a trapezoidal pixel array detector

The modulation transfer function (MTF) is the tool most commonly used for quantifying the performance of an electro-optical imaging system. Recently, trapezoid-shaped pixels were designed and used in a retina-like sensor in place of rectangular-shaped pixels. The MTF of a detector with a trapezoidal pixel array is determined according to its definition. Additionally, the MTFs of detectors with differently shaped pixels, but the same pixel areas, are compared. The results show that the MTF values of the trapezoidal pixel array detector are obviously larger than those of rectangular and triangular pixel array detectors at the same frequencies.

基于改进倾斜刃边法的光电成像系统MTF高准确度测试

基于改进倾斜刃边法的光电成像系统MTF高准确度测试

Q Selection for an electro-optical earth imaging system: theoretical and experimental results

This paper explores practical design considerations for selecting Q for an electro-optical earth imaging system, where Q is defined as (λ FN) / pixel pitch. Analytical methods are used to show that, under imaging conditions with high SNR, increasing Q with fixed aperture cannot lead to degradation of image quality regardless of the angular smear rate of the system. The potential for degradation of image quality under low SNR is bounded by an increase of the detector noise scaling as Q. An imaging test bed is used to collect representative imagery for various Q configurations. The test bed includes real world errors such as image smear and haze. The value of Q is varied by changing the focal length of the imaging system. Imagery is presented over a broad range of parameters.