浑水中非均匀光场探测图像方法

分析了传统的水下观测和激光观测技术在能见度约为0.5~1.5 m的浑浊水域和0.2 m的特浑水域观测遇到的理论和技术上的局限性.叙述了一种非均匀光场集束光水下目标图像探测方法和系统，并在各种浑浊水质中进行其特性的试验.试验结果表明该系统基本上解决了浑浊水中的观测问题.     

集束光水下图像系统

报道了一种新型的适用于浑水条件下的集束光水下图像系统.分析了目前水下观察采用的两种技术—同步扫描技术和距离选通技术,介绍了该水下图像系统的构成原理以及系统组成.通过水池实验,验证了系统的有效性.给出了集束光水下图像系统与水下激光图像系统在探测距离及其它性能方面的比较结果.该系统有望在海洋工程及军事上得到广泛应用.     

Three-dimensional visualization of objects in scattering medium using integral imaging and spectral analysis

This paper presents a three-dimensional visualization method of 3D objects in a scattering medium. The proposed method employs integral imaging and spectral analysis to improve the visual quality of 3D images. The images observed from 3D objects in the scattering medium such as turbid water suffer from image degradation due to scattering. The main reason is that the observed image signal is very weak compared with the scattering signal. Common image enhancement techniques including histogram equalization and contrast enhancement works improperly to overcome the problem. Thus, integral imaging that enables to integrate the weak signals from multiple images was discussed to improve image quality. In this paper, we apply spectral analysis to an integral imaging system such as the computational integral imaging reconstruction. Also, we introduce a signal model with a visibility parameter to analyze the scattering signal. The proposed method based on spectral analysis efficiently estimates the original signal and it is applied to elemental images. The visibility-enhanced elemental images are then used to reconstruct 3D images using a computational integral imaging reconstruction algorithm. To evaluate the proposed method, we perform the optical experiments for 3D objects in turbid water. The experimental results indicate that the proposed method outperforms the existing methods.     

MAP-regularized robust reconstruction for underwater imaging detection

In order to enhance the visual quality of underwater images, applications such as enhancement and restoration can be applied, but the resolution is still limited. Super-resolution reconstruction is a widely used technique for improving resolution beyond the limit of imaging system. With knowledge of the point spread function and techniques of regularization, the performance of reconstruction can be further enhanced. The presented effort proposed a robust image super-resolution reconstruction method under maximum a posteriori framework with regularization by the point spread function for underwater imaging detection. Objective image quality metrics are used to quantify the effectiveness of the reconstruction. Experimental results showed that the proposed method can effectively improve the resolution and quality of underwater imaging detection.     

Integrated method for the detection and location of underwater pipelines

Five underwater pipelines with various outer diameters off the south-western coast of Taiwan were investigated with an integrated surveying system which included a dual-frequency side-scan sonar, a high resolution sub-bottom profiler, and a magnetometer. The pipelines consist of one water transport pipeline (with outer diameter of 0.2 m), one crude oil transmission pipeline (1.0 m) and three industrial waste water outflow pipelines (1.5 m, 1.7 m and 1.8 m). This study was aimed at discussing the performance of the integrated surveying system in searching for, monitoring and locating underwater proud and buried pipelines. Specific items associated with this research goal include: route of the pipeline, burial depth of the pipeline, composition and structure of surficial sediments, and artificial objects on and around the pipeline. This research demonstrated and confirmed the need for the multiple sensor approach to supplement an engineering evaluation of underwater pipelines.     

Testing of a scintillator and fibre optic based radiation sensor

We describe the optimisation of RadLine^®; a small, real time, remotely operated radiation detector, which consists of an inorganic scintillation crystal coupled to a fibre optic cable transporting produced photons to a CCD camera some distance away. RadLine^® is tested in a beta and gamma narrow radiation field of 2.4 GBq, from a Caesium-137 (662 KeV) source, at doses rates between 0.125 mSvhr⁻¹ and 10 mSvhr⁻¹. Our results establish that the lower limit of the device corresponds to a dose rate of 0.2 mSvhr⁻¹, constrained by the signal to noise ratio of the instrument. We also demonstrate the process of characterising the RadLine^® for utilisation underwater due to its partial electrical inactiveness; and to consider how the instrument might perform in aquatic environments and ultimately in a First Generation Magnox Storage Ponds (FGMSP). The RadLine^® brings a marked difference to actual underwater radiation monitoring devices such as; HPGe, CZT and GM detectors, which not only incorporate the whole electronics within and are more bulky, only perform over a short range. The RadLine^®’s design offers signification value for intermediate (>100 m) and long range detection.     

一个新型的水下实况微光高速电视记录系统

介绍了一个新型的水下实况微光高速电视记录系统在船上或岸上使用该系统的控制系统,可对该系统的水下成像系统进行远程控制,并通过该水下成像系统得到水下运动目标的水下常速电视图像、水下高速电视图像和水下微光高速电视图像该系统的工作水深可达水下50m.     

Precise Underwater Distance Measurement by Dual Acoustic Frequency Combs

Underwater acoustics is of fundamental importance for marine science and technology. However, acoustic waves transmitted by state‐of‐the‐art underwater acoustic systems are not inherently phase locked, which hinders the development of underwater acoustic technology. For example, the precision of underwater distance measurement can only achieve centimeter level. As a versatile tool, optical frequency combs have enabled revolutionary progress in optical metrology and precision measurement. In parallel with optical frequency combs, here, the generation of fully stabilized, underwater acoustic frequency combs is reported, in which equidistant acoustic modes are produced via a hydroacoustic transducer. The precision of each individual acoustic mode is measured to be 10^?9 at 1 s and 10^?12 at 1000 s averaging times. Underwater distance measurements are carried out in an anechoic pool using a dual‐comb scheme. Comparison with reference values shows consistency within 50 µm (7 × 10^?6 in relative). The relatively long‐duration experiments at 7 m distance yield an Allan deviation of 1.8 µm (2.6 × 10^?7 in relative) at 1 s and further 480 nm (6.8 × 10^?8 in relative) at 40 s averaging times. The approach to acoustic frequency comb generation offers a promising and powerful platform for future underwater distance measurement, positioning, and navigation.     

An open-access, very-low-field MRI system for posture-dependent 3He human lung imaging

We describe the design and operation of an open-access, very-low-field, magnetic resonance imaging (MRI) system for in vivo hyperpolarized ³He imaging of the human lungs. This system permits the study of lung function in both horizontal and upright postures, a capability with important implications in pulmonary physiology and clinical medicine, including asthma and obesity. The imager uses a bi-planar B₀ coil design that produces an optimized 65 G (6.5 mT) magnetic field for ³He MRI at 210 kHz. Three sets of bi-planar coils produce the x, y, and z magnetic field gradients while providing a 79-cm inter-coil gap for the imaging subject. We use solenoidal Q-spoiled RF coils for operation at low frequencies, and are able to exploit insignificant sample loading to allow for pre-tuning/matching schemes and for accurate pre-calibration of flip angles. We obtain sufficient SNR to acquire 2D ³He images with up to 2.8 mm resolution, and present initial 2D and 3D ³He images of human lungs in both supine and upright orientations. ¹H MRI can also be performed for diagnostic and calibration reasons.     

大视场大相对孔径水下专用摄影物镜的设计

分析了大视场大相对孔径水下专用摄影物镜的设计特点.基于反摄远结构引入一个高次非球面设计了相对孔径为1/1.4,水下全视场66°,焦距11.85 mm,光谱响应范围0.48～0.60 μm,采用平面水密壳窗的水下专用摄影物镜.全视场MTF在空间频率42 lp/mm时高于0.4.与相同技术要求下全部采用球面透镜的设计进行比较,表明该摄影物镜结构更简单,成像质量也更优异,能够满足深水微光摄影物镜对大视场、大相对孔径、小型化、轻量化的需求.     

基于蒙特卡洛法的水下无线光传输特性分析

水下无线光传输面临功率衰减和时域宽度变大的问题,蓝绿光长距离传输特性更难以测试。利用蒙特卡洛法对水下蓝绿高斯光束传输过程进行仿真,分析海水类型、传输距离和发散角等因素对接收功率及脉冲响应的影响,数值对比小发散角情况下不同海水类型和传输距离的接收面上功率分布。结果表明,随着海水衰减系数、传输距离和发散角的增加,接收功率逐渐减小,时域宽度扩展变大。其中浑浊港湾海水在10 m以上传输距离时,接收功率受发散角的影响微乎其微,而时域宽度随发散角的增加由0.32 ns变为0.8 ns。虽然传输距离相差一倍,纯海水和清澈海水功率相似度依然高达99%。基于海水衰减系数的差异性,利用功率相似度和面积比来研究水下长距离的传输,可高效快速地获得长距离传输特性。     

一种分焦平面偏振成像系统光源标定方法北大核心CSCD

水下偏振成像技术是目前水下成像研究的热点,由于自然光在水中衰减大,水下成像系统多采用主动照明方式。针对分焦平面偏振成像系统中偏振照明光源与偏振探测像元偏振方向不匹配引起采集图像偏振信息存在的偏差,进而影响目标图像增强质量的问题,提出了一种分焦平面偏振成像系统光源标定方法。阐述了偏振光源的标定原理,然后给出偏振光源标定的实施步骤,最后采用偏振去雾算法和图像质量评价方法对标定前后的水下目标图像进行了图像增强和图像质量评价。评价结果表明,标定后的增强图像质量优于未标定的增强图像质量,平均梯度最大提升了2.48倍。该标定方法简单有效,实用性强,适用于分焦平面偏振成像系统偏振光源标定。     

国家自然科学基金(40776059)资助

为了克服水下探测中后向散射光背景噪音的影响,提出了非均匀光场水下探测方法,推导得出非均匀光场的照度分布函数与接收口径、目标与接收器的距离以及海水的体积衰减系数几个因素有关.通过搭建集束光水下图像系统,对其产生的非均匀光场的分布特性和水下探测特性进行了水池实验.观测结果为,在0.6倍能见度下,可分辨1 mm细节,在1倍能见度时,可分辨目标轮廓,在1.5倍能见度时,可探测到目标.结果证明,该系统具有宽视角、全景深、图像清晰度高等特点.     

Experiments of ocean surface waves and underwater target detection imaging using a slit Streak Tube Imaging Lidar

This letter describes a flash Lidar imaging technique that uses a streak tube camera as a receiver to detect short scale surface wave fields for oceanographic, underwater target and coastal measurements. The range and intensity imaging of short scale sea-waves and underwater targets can be supplied by this system through light reflected from the surface of the sea and targets. Through a surface scattering model, more accurate range and intensity information can be extracted. The technique can be applied to surface wave measurements and underwater target detection with high resolution. The feasibility of this technique is demonstrated by applying the technique to data acquired by shipborne Streak Tube Imaging Lidar (STIL) in the Yellow Sea and also the East and South China Seas.     

浅海被动水下偏振成像探测方法

针对传统被动水下偏振成像方法忽略水体对光的吸收效应,成像结果中存在严重的色彩失真,且并未深入发掘利用背景散射光中包含的场景信息的问题.提出浅海被动水下偏振成像探测方法,该方法从水体中背景散射光的传输特性出发,分析场景深度信息与散射光的物理关系,建立基于深度信息的水下Lambertian反射模型,实现无色彩畸变的水下目标场景清晰成像探测.实验结果表明,该方法能够提供接近水下目标真实色彩、符合人眼视觉特性的清晰探测结果,提高水下成像探测能力.     

基于优势特征图像融合的水下光学图像增强

针对水下光学图像颜色失真、非均匀光照、对比度低的问题,提出基于优势特征图像融合的水下光学图像增强算法.首先,提出改进的暗通道先验算法去除退化图像中的不均匀浑浊并均衡色彩;其次,对颜色校正图像分别使用基于加权分布的自适应伽玛校正算法和限制对比度自适应直方图均衡-同态滤波算法,增强颜色校正图像对比度并使其亮度均衡;最后,定义三幅融合图像即颜色校正图像、亮度均衡图像、对比度增强图像的关联权重图,通过多尺度融合算法获得融合图像.与单一预处理算法只能解决对应的退化现象相比,该算法对单幅退化图像进行多算法处理,得到三幅优势特征图像,通过不同权重的组合最大程度地将各优势特征相结合,得到的综合效果远超各单一算法优化效果,不再局限于解决颜色失真等单一问题.将本文算法与现有算法在主观评价和客观评价两方面进行实验对比,结果表明,该算法可以有效平衡水下图像的色度、饱和度及清晰度,视觉效果接近自然场景下的图像.     

Remote sensing estimation of Chlorophyll a and suspended sediment concentration in turbid water based on spectral separation

Chlorophyll a and suspended sediment are important indicators of water quality, and remote sensing estimation of them is difficult due to the optical complexity of turbid water. The spectrum above water surface is influenced by phytoplankton, suspended sediment and colored dissolved organic material in water, thus spectral separation is important before estimating one specific component. Based on the field experiment of pond water and Taihu lake, China, this study calculated the Gaussian parameters of Chlorophyll a (Chla) and suspended sediment (SS) through spectral decomposition, and then these parameters were used to separate the mixed spectrum of water samples from pond water and Taihu lake. After spectral separation, the Chla estimation model based on the peak height at 650 nm has high accuracy (R² = 0.78, RMSE = 4.80 mg/m³), better than the band-ratio model; the SS estimation model based on the peak height at 811 nm (R² = 0.82, RMSE = 6.80 mg/L) performs better than the single-band model. Results in this study indicate that spectral separation based on Gaussian parameters is a good method for Chla and SS estimation in turbid lake water.     

Spatial and temporal mapping of water content across Nafion membranes under wetting and drying conditions

Water transport and water management are fundamental to polymer electrolyte membrane fuel cell operation. Accurate measurements of water content within and across the Nafion layer are required to elucidate water transport behavior and validate existing numerical models.We report here a direct measurement of water content profiles across a Nafion layer under wetting and drying conditions, using a novel magnetic resonance imaging methodology developed for this purpose. This method, multi-echo double half k-space spin echo single point imaging, based on a pure phase encode spin echo, is designed for high resolution 1D depth imaging of thin film samples. The method generates high resolution (<8 μm) depth images with an SNR greater than 20, in an image acquisition time of less than 2 min. The high temporal resolution permits water content measurements in the transient states of wetting and drying, in addition to the steady state.     

Underwater target localization using long baseline positioning system

The absolute position of an underwater target is difficult to pinpoint because the global positioning system (GPS) cannot penetrate water bodies. The long baseline (LBL) positioning system can extend GPS using high-precision calibrated underwater beacons as references. While traditional LBL systems give the target position without considering calibration error of deployed beacons. To solve this problem, we propose a method different from previous works, that combining the errors of observations together. We use GPS outputs as true values to evaluate the localization performance. An LBL system with four beacons was installed in deep sea to test the results. The positioning accuracy in deep sea improves nearly 5 m. The results suggest that beacon positioning errors have a great impact on localization precision, that is significant in high-precision positioning tasks.     

MWIR imaging experiments with large F-number optics on LEO spacecraft

To determine the feasibility of spacecraft imaging in the mid-infrared region (3.0–5.0 μm) utilizing large F-number optics, we create a radiometric simulation of a low-Earth orbit (LEO) extended object, imaging background and noise, in order to predict the signal-to-noise ratio of such objects as viewed from a telescope during the daytime and terminator, etc. We find that the observation of Space Station and other extended targets utilizing large F-number optics for MWIR imaging is possible, given the right conditions. We have developed a large F-number infrared camera with cold stop matched, which is successfully applied to acquire LEO extended object images taken by a 1.23 m telescope. We get clear images of International Space Station, Tian-gong, etc. and our proposed the infrared imaging technology can be demonstrated to greatly improve the imaging ability of current ground-based Optoelectric (OE) telescope for the detection of space activities.