Spatial and temporal bilateral filter for infrared small target enhancement

This paper presents a spatial and temporal bilateral filter (BF) to detect target trajectories, by extracting spatial target information using a spatial BF and temporal target information using a temporal BF. Background prediction when it is covered by targets is the key to small target detection. In order to apply the BF to a small target detection field for this purpose, this paper presents a novel spatial and temporal BF with an adaptive standard deviation to predict spatial background and temporal background profiles, based on analysis of the blocks surrounding a spatial and temporal filter window. In order to discriminate between the edge or object regions with a flat background and the target region spatially and temporally, spatial and temporal variances of the blocks surrounding the filter window are calculated in a spatial infrared (IR) image and temporal profile. The spatial and temporal variances adjust standard deviations of the spatial and temporal BF. Through this procedure, spatial background and temporal background profiles are predicted, and then small targets can be detected by subtracting the predicted spatial background (and temporal background profile) from the original IR image (and original temporal profile) and multiplying spatial and temporal target information. To compare existing target detection methods and the proposed method, signal-to-clutter ratio gain (SCRG) and background suppression factor (BSF) are employed for spatial performance comparison and receiver operating characteristics (ROC) is used for detection-performance comparison of the target trajectory. Experimental results show that the proposed method has a superior target detection rate and a lower false-alarm rate.     

Effects of Temporal Frequency and Contrast on Spatial Frequency Characteristics for Disparity Threshold

We measured disparity threshold for identifying the depth direction as a function of spatial frequency with various temporal frequencies and stimulus contrasts using drifting gratings. The results showed that disparity threshold depended little on temporal frequency with the exception of high temporal frequencies (≥ 10Hz) independently of stimulus contrast. Contrary to temporal frequency, contrast substantially influenced spatial frequency characteristics. The disparity threshold was approximately constant with change in spatial frequency with slight increase at high spatial frequencies for contrasts higher than 0.2 when the threshold is expressed by phase difference between the left and right eye images (phase disparity). The phase disparity threshold had a negative peak at a spatial frequency between 1 and 5c/deg (band-pass) for contrasts lower than 0.2. We discuss possible differences in the underlying mechanisms to determine disparity threshold below and above temporal frequency of 10 Hz.     

Optical Time-to-Three-Dimensional (3-D) Space Interconversion System

We propose a new technique for an interconversion between a three dimensional (3-D) spatial signal and a temporal signal. It is realized by combination of a holographic technique, a 2-D spatial filtering technique and a time-space conversion technique. Using this method, we can physically utilize the multiplicity between a temporal domain parameter and all of the spatial domain parameters of light. To demonstrate the proposed technique, we achieved experimentally conversion from different three point light sources as 3-D spatial signals into a temporal signal and vice versa.     

Eulerian spatial and temporal autocorrelations: assessment of Taylor's hypothesis and a model

We present measurements of Eulerian longitudinal velocity autocorrelations in homogeneous, isotropic, high-intensity (～9%) free-stream turbulence behind an active grid. Spatial correlations are measured using particle image velocimetry as well as with two-point hot-wire anemometry (HWA), while temporal correlations are measured using HWA. The temporal correlations are transformed into spatial correlations by using Taylor's ‘frozen’ hypothesis with both the mean as well as instantaneous velocities. A model relating Eulerian spatial and temporal autocorrelations is also used for this purpose. The differences from the measured spatial correlation resulting from the use of Taylor's hypothesis on the temporal correlation is quantified; even at this moderately high level of turbulent intensity, the result from the use of the instantaneous velocity as convection velocity is practically indistinguishable from that obtained using the mean velocity. Use of the model produces a good agreement between the estimates of the spatial correlation function. A relation between Eulerian spatial and temporal integral scales is also derived.     

The Effect of Exposure Duration on Stereopsis and Its Dependency on Spatial Frequency

To investigate the effect of exposure duration on stereopsis and its spatial frequency dependency, we measured disparity threshold for the depth discrimination varying stimulus exposure duration between 0.05 and 2 s for three spatial frequencies (0.23, 0.94 and 3.75 c/deg). The results showed that disparity threshold decreased with increase in exposure duration up to a certain duration, beyond which it was approximately constant (the duration is called critical duration). The critical duration was about 150 ms for gratings with low and middle spatial frequencies (0.23 and 0.94 c/deg) while the duration was about 750 ms for gratings with high spatial frequency (3.75 c/deg). This suggests that temporal integration property varies dependently on stimulus spatial frequency. We also attempted to relate the spatial frequency dependency of the temporal integration property to the differences in temporal frequency tuning to different spatial frequency stimuli.     

光场时空对称性及其在光学技术中的应用

本文论述光波场的时问、空间对称性,包括拍频导致的“空间放大”与“时间放大”;多束光干涉提高条纹细锐度“空间压缩”、锁模技术中脉冲的“时间压缩”;空间调制与时间调制;非线性效应中的时间倍频与空间倍频;空间相干性与时间相干性;时域测不准与空域测不准。     

The effects of longitudinal spatial coherence of light in interference experiments

The physical conditions needed to observe the effects of longitudinal spatial coherence of emission of an extended thermal source are considered. Results of experiments on observation of interference pulses of longitudinal spatial coherence, performed using a scanning longitudinal shearing Michelson interferometer, are presented. Distinctions between the effects of temporal and longitudinal spatial coherence of light are established. It is shown that the presence of a noncompensated optical layer in one of the arms of the interferometer makes the interference pulses of the temporal and spatial coherence of light shift relative to each other (“recession”).     

Tactile pattern perception and its perturbations

One of the problems encountered in conveying information to the skin by means of vibratory patterns is that presenting patterns, such as letters of the alphabet, in close spatial and temporal proximity, may result in considerable masking. Generally, there is more interference when the masking stimulus follows the target (backward masking) than when it precedes it (forward masking). Masking has also been demonstrated with vibrotactile patterns derived from speech signals. Masking may be reduced by increasing the temporal and spatial separation between patterns; however, other problems may arise when such separations are made. Because stimulus onset asynchrony appears to be the critical temporal dimension for masking, it may prove difficult to reduce masking by increasing the temporal separation between patterns without also reducing the rate at which the patterns are presented. Increasing the spatial separation between patterns may lead to problems in combining the patterns from spatially distinct sites on the skin. This latter difficulty may be due to problems in attending to several sites of stimulation simultaneously.     

Small target detection using bilateral filter and temporal cross product in infrared images

We introduce a spatial and temporal target detection method using spatial bilateral filter (BF) and temporal cross product (TCP) of temporal pixels in infrared (IR) image sequences. At first, the TCP is presented to extract the characteristics of temporal pixels by using temporal profile in respective spatial coordinates of pixels. The TCP represents the cross product values by the gray level distance vector of a current temporal pixel and the adjacent temporal pixel, as well as the horizontal distance vector of the current temporal pixel and a temporal pixel corresponding to potential target center. The summation of TCP values of temporal pixels in spatial coordinates makes the temporal target image (TTI), which represents the temporal target information of temporal pixels in spatial coordinates. And then the proposed BF filter is used to extract the spatial target information. In order to predict background without targets, the proposed BF filter uses standard deviations obtained by an exponential mapping of the TCP value corresponding to the coordinate of a pixel processed spatially. The spatial target image (STI) is made by subtracting the predicted image from the original image. Thus, the spatial and temporal target image (STTI) is achieved by multiplying the STI and the TTI, and then targets finally are detected in STTI. In experimental result, the receiver operating characteristics (ROC) curves were computed experimentally to compare the objective performance. From the results, the proposed algorithm shows better discrimination of target and clutters and lower false alarm rates than the existing target detection methods.     

Relation between the spatial growth rate of a traveling wave and the temporal growth rate of a standing wave

The relation between the spatial growth rate of a traveling wave and the temporal growth rate of the corresponding standing wave is examined. It is shown that, when radiation propagates in a gain medium with a sufficiently narrow gain line and a high amplification coefficient in the line center, the frequency dependences of the spatial and temporal growth rates of the field amplitude can differ significantly. In particular, at a fixed population inversion, the unbounded narrowing of the gain line, which results in an unbounded increase in the spatial growth rate and the narrowing of its frequency profile, is accompanied by neither an unbounded increase in the maximum value of the temporal growth rate nor an unbounded narrowing of the frequency profile of this growth rate.     

节律性时间期待和空间期待对听觉目标辨别的影响

合理的推断和预期是加快辨别听觉目标的关键。通过听觉目标辨别的反应时任务,研究了听感知过程中节律性时间期待和空间期待对声音目标辨别的影响。实验中,听觉序列中各声音以规律或不规律的时间节奏、沿有序或随机的空间方位呈现,形成四种期待条件:时间和空间期待兼备、只有时间期待、只有空间期待和没有期待。被试对声音序列末尾随机呈现的听觉目标尽快做按键反应。结果发现:（1）节律性时间期待显著加快被试对目标声音的反应速度;（2）空间期待对目标辨别速度的促进作用仅在时间非规律条件下发生。可见,在辨别听觉目标时,听者优先使用时间期待,并在无有效时间预测信息时使用空间期待,以促成对声音目标的最优知觉加工。     

Necessary conditions for mode interactions in parametrically excited waves

We study the spatial and temporal structure of nonlinear states formed by parametrically excited waves on a fluid surface (Faraday instability), in a highly dissipative regime. Short-time dynamics reveal that 3-wave interactions between different spatial modes are only observed when the modes' peak values occur simultaneously. The temporal structure of each mode is functionally described by the Hill's equation and is unaffected by which nonlinear interaction is dominant.     

Unified microscopic-macroscopic formulation of high-order difference-frequency mixing in plasmas

We investigate high-order difference-frequency mixing in plasmas, taking into account the microscopic rescattering physics and propagation effects for the first time. We show that phase matching can occur over a broad frequency range, up to very high photon energies, and that it is confined to specific temporal and spatial windows. This gated phase matching mechanism is driven by the continuous phase slip between two driving fields and can be employed for manipulating the temporal, spatial, and spectral properties of high harmonic emission.     

克尔介质内飞秒光脉冲时空耦合特性的光束脉冲矩阵

完整地将光脉冲的时间分量和空间分量耦合到一起,使得时间矩阵元素内有空间变量,空间矩阵元素中也有时间变量,从而推导出描述克尔介质内飞秒光脉冲时空耦合特性的光束脉冲矩阵。     

Time averaging of the spatial structure of scattered radiation

The paper describes a theoretical and experimental investigation of the relationships between the spatial and temporal statistics of the background of scattered coherent radiation. It is shown that temporal and spatial recording of the scattered radiation background are equivalent in studies of its statistical properties and an investigation of the influence of the recording time is reported.Translated fron Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 56–62, June, 1980.     

超高速碰撞产生等离子体云的粒子能量时空分布

为了研究超高速碰撞产生等离子体的粒子能量对航天器电路中元器件的毁伤,获得超高速碰撞产生等离子体粒子能量的时空分布特性是十分必要的。基于超高速碰撞产生稀薄等离子体中带电粒子的运动速度、等离子体的扩散特点,推导出等离子体的粒子能量密度与带电粒子密度及带电粒子运动速度的关系式。进而通过对超高速碰撞2024-T4铝靶实验采集的原始数据分析,利用Matlab编程得到了超高速碰撞2024-T4铝靶产生膨胀等离子体云物理过程中,等离子体的粒子能量密度与带电粒子持续时间及被测点到碰撞点距离的时空分布规律。     

双板电极条纹变像管设计

为满足惯性约束聚变（ICF）实验等离子体诊断中对X射线条纹相机大动态和高时空分辨能力的需求,设计了一款利用板状电极对加电四极透镜聚焦方式的变像管。此款变像管最主要的特点为时空方向聚焦方式不同,在时间方向上采用两对平板电极聚焦电子,而在空间方向上采用电四极透镜对电子进行聚焦,其空间分辨能力在阴极边缘处达到了40 lp/mm,时间分辨能力在10 ps左右,阴极有效长度为20 mm。由于变像管采用时空方向分别聚焦方式,相对传统轴对称管型来说,电子不会聚焦成点,空间电荷效应更弱,所以具有大动态能力。     

双板电极条纹变像管设计

为满足惯性约束聚变（ICF）实验等离子体诊断中对X射线条纹相机大动态和高时空分辨能力的需求,设计了一款利用板状电极对加电四极透镜聚焦方式的变像管。此款变像管最主要的特点为时空方向聚焦方式不同,在时间方向上采用两对平板电极聚焦电子,而在空间方向上采用电四极透镜对电子进行聚焦,其空间分辨能力在阴极边缘处达到了40 lp/mm,时间分辨能力在10 ps左右,阴极有效长度为20 mm。由于变像管采用时空方向分别聚焦方式,相对传统轴对称管型来说,电子不会聚焦成点,空间电荷效应更弱,所以具有大动态能力。     

A Novel Technique to Measure the Spatial–Temporal Intensity of an Ultrashort Pulse

Based on a modified cross-correlation technique, we experimentally demonstrate a technique for measuring the spatial–temporal intensity of laser pulses. Pulse widths at different spatial positions of the ultrashort pulse are measured by changing the scan position of the probe beam. Due to the existence of residual chirp in the transverse position, pulse widths at the center of the beam are less than that at the edge. By measuring the temporal evolution in the fastest growth area of spatial intensity during small-scale self-focusing, we find that its pulse width decreases as power increases because of the spatial–temporal coupling effect. The results show that this method not only can be used to accurately measure the pulse width at any one spatial position of the beam, but can also be useful for real-time monitoring of the spatial–temporal evolution.     

Exploring space-time structure of human mobility in urban space

Understanding of human mobility in urban space benefits the planning and provision of municipal facilities and services. Due to the high penetration of cell phones, mobile cellular networks provide information for urban dynamics with a large spatial extent and continuous temporal coverage in comparison with traditional approaches. The original data investigated in this paper were collected by cellular networks in a southern city of China, recording the population distribution by dividing the city into thousands of pixels. The space-time structure of urban dynamics is explored by applying Principal Component Analysis (PCA) to the original data, from temporal and spatial perspectives between which there is a dual relation. Based on the results of the analysis, we have discovered four underlying rules of urban dynamics: low intrinsic dimensionality, three categories of common patterns, dominance of periodic trends, and temporal stability. It implies that the space-time structure can be captured well by remarkably few temporal or spatial predictable periodic patterns, and the structure unearthed by PCA evolves stably over time. All these features play a critical role in the applications of forecasting and anomaly detection.