Computational imaging technology for high-sensitivity space-image acquisition

Acquiring deep-space images with high spatial resolution and sensitivity is important for space-debris surveillance and early warning. We propose a novel computational imaging （CI） method for high-sensitivity image acquisition in this letter. The proposed approach introduces CI into image formation. The proposed capturing process conducts minor modifications for cameras to encode more desirable information during capture, which is practical for hardware implementation. The latent image is reconstructed by formulating a recovery problem into an optimization problem, which is solved with iteratively reweighted least square technique. The experimental results clearly show the effectiveness of the proposed method.     

A simulation method of aircraft plumes for real-time imaging

Real-time infrared simulation technology can provide a large number of infrared images under different conditions to support the design, test and evaluation of a system having infrared imaging equipment with very low costs. By synthesizing heat transfer, infrared physics, fluid mechanics and computer graphics, a real-time infrared simulation method is proposed based on the method of characteristics to predict the infrared feature of aircraft plumes, which tries to obtain a good balance between simulation precision and computation efficiency. The temperature and pressure distribution in the under-expansion status can be rapidly solved with dynamically changing flight statuses and engine working states. And a modified C–G (Curtis–Godson) spectral band model that combines the plume streamlines with the conventional C–G spectral band model was implemented to calculate the non-uniformly distributed radiation parameters inside a plume field. The simulation result was analyzed and compared with the CFD++, which validates the credibility and efficiency of the proposed simulation method.     

Handheld, battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumes

A handheld, battery-powered tunable-diode-laser sensor platform is described. The sensor is based on frequency modulation (FM) spectroscopy using near-IR diode lasers and passive topographic backscatter from common environmental targets such as buildings, ground and foliage. A specific application to the detection of methane using a 1.65-micron diode laser is described in detail, showing a detection sensitivity sufficient to identify typical leaks from buried residential natural gas service lines at stand-off distances up to 30 m. Signal and noise sources are analyzed in detail, along with laboratory and field-test data, including known service leaks. Received: 15 April 2002 / Revised version: 4 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-978/689-3232, E-mail: allen@psicorp.com     

Retroemission by a glass bead monolayer for high-sensitivity, long-range imaging of upconverting phosphors

We introduce a retroemitter (REM) device comprising a planar glass bead set placed on a luminescent material substrate, which converges an excitation beam into a set of foci (voxels). The in-voxel emission is collimated by the beads, and propagates upstream over the long range, unlike the out-of-voxel emission spreading in all angles. The REM signal contrast is characterized as a function of incidence and observation angles and propagation distance. REM signal contrasts of approximately 20 and 1600 were found for the organic fluorescent dye and upconverting phosphor substrates, respectively. In the latter case, nonlinear optical signal enhancement plays a role in addition to the retroemission effect. This allows centimeter-scale REM patterns to be read out at the meter-scale distance using eye-safe sub-mW/cm(2) excitation intensities.     

Spectral signature of single-grain quartz using a high-sensitivity TL imaging system

The spectral signature of individual quartz grains were measured using a high-sensitivity thermoluminescence imaging system based on a charge-coupled device (CCD) camera and custom optics. Luminescence emission behaviour was characterised for individual quartz grains (180–212 μm), with single grains shown to emit luminescence strongly across multiple signal bands. The spectral profiles of three quartz samples from contrasting geological contexts were then compared, with clear distinction in the spectral signatures of individual grains originating from single deposits within each provenance.     

Background gas collisional effects on expanding fs and ns laser ablation plumes

The collisional effects of a background gas on expanding ultrafast and short pulse laser ablation plumes were investigated by varying background pressure from vacuum to atmospheric pressure levels. For producing Cu ablation plumes, either 40 fs, 800 nm pulses from a Ti: Sapphire laser or 6 ns, 1,064 nm pulses from a Nd:YAG laser were used. The role of background pressure on plume hydrodynamics, spectral emission features, absolute line intensities, signal to background ratios and ablation craters was studied. Though the signal intensities were found to be maximum near to atmospheric pressure levels, the optimum signal to background ratios are observed ~20–50 Torr for both ns and fs laser ablation plumes. The differences in laser–target and laser–plasma couplings between ns and fs lasers were found to be more engraved in the crater morphologies and plasma hydrodynamic expansion features.     

Laser relaxation-oscillation frequency imaging

We describe a new imaging technique based on modification of laser relaxation frequency induced by coherent optical feedback from an external target. A direct comparison (both theoretical and experimental) is made with laser feedback interferometry techniques, in which there is a modification of the laser's steady state. We show that, for a laser with a cavity damping rate gamma(c) higher than the population damping rate, gamma(1) , the modification of the laser relaxation frequency can be several orders of magnitude more sensitive than the perturbation of the laser's output power. Application of this technique to imaging is reported.     

Bipolar-power-transistor-based limiter for high frequency ultrasound imaging systems

High performance limiters are described in this paper for applications in high frequency ultrasound imaging systems. Limiters protect the ultrasound receiver from the high voltage (HV) spikes produced by the transmitter. We present a new bipolar power transistor (BPT) configuration and compare its design and performance to a diode limiter used in traditional ultrasound research and one commercially available limiter. Limiter performance depends greatly on the insertion loss (IL), total harmonic distortion (THD) and response time (RT), each of which will be evaluated in all the limiters. The results indicated that, compared with commercial limiter, BPT-based limiter had less IL (−7.7 dB), THD (−74.6 dB) and lower RT (43 ns) at 100 MHz. To evaluate the capability of these limiters, they were connected to a 100 MHz single element transducer and a two-way pulse-echo test was performed. It was found that the −6 dB bandwidth and sensitivity of the transducer using BPT-based limiter were better than those of the commercial limiter by 22% and 140%, respectively. Compared to the commercial limiter, BPT-based limiter is shown to be capable of minimizing signal attenuation, RT and THD at high frequencies and is thus suited for high frequency ultrasound applications.     

基于Vega的红外图像仿真

介绍了红外成像仿真的基本理论,讨论了利用Vega传感器模块将输入的可见目标图像生成红外图像的技术。该技术使用Creator软件进行三维几何建模和纹理映射,使用TMM软件进行红外材质建模;然后,用Mat软件进行大气衰减的计算;最后,用SensorWorks软件进行传感器特性建模,通过光电转换生成红外图像。以阿帕奇AH6直升机为例进行了红外图像仿真,结果表明,利用Vega传感器模块进行传感器建模,建模效果较好,成本低,易于工程实现。     

离体猪淋巴结组织的高频成像研究

用C扫的超声显微镜系统对猪的淋巴结组织进行了研究,得到了猪的正常淋巴结和出血性淋巴结组织的高频超声图像和声参量值,结果表明:高频超声图像能反映组织结构的变化;猪的出血性淋巴结组织的声散射系数比正常淋巴结组织的小,且随频率的变化而变化,背向散射技术是在高频段进行组织定征的有效手段。     

基于Vega的红外图像仿真

介绍了红外成像仿真的基本理论,讨论了利用Vega传感器模块将输入的可见目标图像生成红外图像的技术。该技术使用Creator软件进行三维几何建模和纹理映射,使用TMM软件进行红外材质建模;然后,用Mat软件进行大气衰减的计算;最后,用SensorWorks软件进行传感器特性建模,通过光电转换生成红外图像。以阿帕奇AH6直升机为例进行了红外图像仿真,结果表明,利用Vega传感器模块进行传感器建模,建模效果较好,成本低,易于工程实现。     

Chaotic imaging in frequency downconversion

We analyze and recover, by means of spatial intensity correlations, the image obtained by a seeded frequency-downconversion process in which the seed field is chaotic and an intensity modulation is encoded in the pump field. Although the field generated is as chaotic as the seed field and does not carry any information about the modulation of the pump, one can extract an image of the pump by measuring the spatial intensity correlations between the generated field and one Fourier component of the seed.     

Fast frequency selective MR imaging

With the proposed fast frequency selective MR imaging (FFSMRI) method, we focused on the elimination of all off-resonance components from the image of the observed object. To maintain imaging speed and simultaneously achieve good frequency selectivity, MRI was divided into two steps: signal acquisition and postprocessing. After the preliminary phase in which we determine imaging parameters, MRI takes place; the signal from the same object is successively acquired M times. As a result, we obtain M partial signals in k-space, from which we calculate the image of the observed object in postprocessing phase, after signal acquisition has been completed. With proper selection of parameters, it is possible to exclude from the image a majority of off-resonance components present in the observed object. However, we can decide to keep only a chosen off-resonance component in the image and eliminate all other components, including the on-resonance component and thus producing a different image from the same acquisition. The experiments with Fe(OH)(3) and oil showed that signal-to-noise ratio (SNR) can be improved by about a factor of four. The proposed FFSMRI method is suitable for frequency selective MR imaging and quantitative measurements in dynamic MRI where exclusion of off-resonance components can improve the reliability of measurement.     

A high frequency amplitude-steered array for real-time volumetric imaging

Real-time three-dimensional acoustic imaging is difficult in water or tissue because of the slow speed of sound in these media. Conventional pulse-echo data collection, which uses at least one transmit pulse per line in the image, does not allow for the real-time update of a volume of data at practical ranges. Recently, a linear amplitude-steered array was presented that allows the collection of a plane of data with a single transmit pulse by spatially separating frequencies in the lateral direction. Later, by using a linear array with frequency separation in the vertical direction and rotating the array in the horizontal direction, volumetric data were collected with a small number of transmit pulses. By expanding the linear array to a two-dimensional array, data can now be collected for volumetric imaging in real time. In this study, the amplitude-steered array at the heart of a real-time volumetric sonar imaging system is described, giving the design of the array and describing how data are collected and processed to form images. An analysis of lateral resolution in the vertical and horizontal directions shows that resolution is improved in the direction of frequency separation over systems that use a broad transmit beam. Images from simulated data are presented.     

Estimation methods for flow imaging with high frequency ultrasound

This article proposes to estimate slow blood flow with high frequency ultrasound imaging. The proposed technique combines 2 methods. First, a statistical method, called Speckle Flow Imaging (SFI) based on the analysis of changes in the speckle pattern along time, gives an index directly related to the total velocity vector. Secondly, a block matching approach estimates the in-plane velocity components. Results on calibrated flow sequences of blood mimicking fluid have shown good agreement with the statistical model. The quantification of flow is achieved with pulsed flow and is also angle independent when the flow is perpendicular to the ultrasound beam. Speckle Tracking has been evaluated on the same data and has shown good estimation of the in-plane velocity vector when the component of velocity perpendicular to the imaging plane is inferior to 1mm/s. The results of these two methods permit the evaluation of the total 3D velocity field and the orthogonal velocity component relative to the imaging plane. This allows the quantification of blood flow (volumetric per time unit across the sequence).