湍流大气成像系统长曝光传递函数的研究

采用傅里叶变换法对在Kolmogorov谱大气湍流下望远镜的长曝光成像进行了模拟,给出了湍流大气成像系统长曝光传递函数的数值模拟方法,求出了在此情况下的长曝光传递函数。给出了大气湍流在Kolmogorov谱假设下成像系统长曝光传递函数的理论值。基于空间目标为点目标的假设,提出了一种湍流大气成像系统长曝光传递函数的测量方法。对由三种方法获取的大气湍流长曝光传递函数进行了比对分析。结果表明,由三种方法获取的大气湍流长曝光传递函数基本上是一致的,由傅里叶变换法模拟和实际测量得到的传递函数是合理的。     

大望远镜系统衍射极限倍数的确定

利用大气湍流的光学相关函数确定湍流光学相关时间，并在此时间内通过成像系统的光学传递函数修正大气湍流效应后定标大望远镜(500mm口径，500m焦距)系统衍射极限倍数。     

大气湍流对长曝光和短曝光条件下分辨率与像斑半径的影响

基于湍流中调制传递函数的数学模型,从理论上研究了大气相干长度和系统光瞳直径对积分分辨率和像斑半径的影响.采用内插值法,分别给出长曝光和短曝光条件下分辨率和像斑半径的近似代数解.结果表明,在系统光瞳直径小于大气相干长度时,分辨率主要受光瞳直径限制,增大光瞳直径可提高分辨率,然而随着其值的继续增大,对分辨率的改善不再明显,因为当光瞳直径大于大气相干长度时成像性能逐渐由湍流限制.光瞳直径与大气相干长度的比值趋于无穷大时,分辨率和像斑半径受大气相干长度的限制有一个极限值.短曝光条件下,去除了大尺度漩涡所产生的倾斜效应,系统的积分分辨率和像斑半径性能要好于长曝光条件.对于短曝光近场,与长曝光相比,其积分分辨率的最大值提高了3.5倍,像斑半径最小值减小了1/2.     

大气湍流对航空光电侦察图像质量影响因素分析北大核心CSCD

大气湍流效应是严重影响航空光电侦察系统图像质量的重要因素之一。从大气湍流参数描述出发,研究了大气湍流对光学系统成像质量的影响机理,分析了大气湍流影响光学系统调制传递函数(modulation transfer function,MTF)的影响因素,建立了大气湍流影响光学系统成像MTF的理论模型。仿真结果表明,在大气湍流影响下,光学系统光瞳口径与大气相干直径的比值对光学系统成像MTF影响较大。通过光学系统地面外场实验图像测试对比,验证了实际光学系统受到大气湍流影响后的成像MTF理论模型。研究结果可为大气湍流影响下的航空光电侦察系统设计及提升其成像质量提供理论支持。     

远海海面大气光学湍流实验测量

将光纤湍流测量系统搭载于我国的“远望号”航天测量船, 首次实现了远海海面大气光学湍流的空间多点同步测量, 初步获得了远海海面大气光学湍流的基本特征和定量数据. 测量共进行了37天, 测量数据处理包括大气光学湍流强度的统计分析、采用Greenwood湍流空间相关函数模型对实测的光学湍流空间相关函数进行非线性拟合从而获得湍流的空间外尺度, 以及采用分段拟合算法获取光学湍流的功率谱标度指数. 结果表明: 海面大气光学湍流强度中等偏弱, 且无明显规律性的日变化趋势; 空间外尺度较小, 约为0.2-0.3 m; 湍流谱标度指数符合-5/3次方的概率均为25%左右, 低于相应的近地面概率.     

相干场成像技术分辨率研究

根据光学传递函数的相关理论, 推导了相干场成像技术(又称傅里叶望远镜)的光学传递函数和点扩散函数, 给出了T型、O型两种发射镜阵列布局相干场成像系统的分辨率计算公式, 为分析相干场成像系统能实现的极限角分辨率提供了理论依据. 在此基础上研究了T型、O型两种发射镜阵列布局相干场成像系统的分辨率之间的关系. 关键词： 相干场成像 傅里叶望远镜 分辨率 光学传递函数     

大气湍流对光学系统图像分辨力的影响

讨论了大气湍流对空 地和地 空光学系统图像分辨力的影响。大气相关长度r0是评定大气湍流是否影响光学系统图像分辨力的主要依据。采用不同的大气湍流模型,通过计算相关长度得到了大气湍流对系统图像分辨力的影响,并对空 地与地 空系统的大气相关长度进行了计算比较。结果表明:对空 地观测系统来说,大气相关长度是大气层高度和反映大气湍流特征的C2n的函数。而对地 空观测系统来说,大气相关长度与高度几乎无关。分析了大气湍流对机载和星载光学遥感系统成像分辨力的影响。     

大气湍流下合成孔径激光雷达成像数值模拟及PGA补偿

为研究大气湍流对合成孔径激光雷达(SAL)的成像影响,采用谱反演法对符合Kolmogorov谱的大气随机相位屏进行了数值模拟,给出了单层及多层叠加的随机相位屏图像;然后以多层随机相位屏叠加方式模拟实际大气湍流,计算了无湍流、弱湍流、中等湍流和强湍流等情况下理想机载单站聚束式SAL对点目标以及扩展目标的成像,模拟成像表明大气湍流对SAL成像的方位分辨率有严重影响,湍流越强,图像方位向分辨率越差;针对湍流造成的SAL图像方位向分辨率降低,采用相位梯度自聚焦(PGA)算法对SAL图像进行了方位向补偿,结果有效地改善了图像的聚焦效果,提高了成像质量。     

自适应光学对大气扰动波前的补偿效果研究：I.理论推导

在采用冻结湍流假设和几何光学近似的条件下，得到了经自适应光学系统校正后的剩余波前相位扰动的结构函数，进而根据剩余相位结构函数与系统Ｓｔｒｅｈｌ比的关系，求出自适应光学系统的补偿效果受系统的时空传递函数，大气湍流，光波传播路径上的横向风，观察目标的旋转角速度以及系统时间延迟影响情况的解析表达式。     

基于辐射状靶标的高分辨率光学卫星传感器像质评价方法研究

空间分辨率与调制传递函数(MTF)是高分辨率光学卫星传感器像质评价的重要参数,直接客观反映遥感器成像系统的成像质量。针对空间分辨率检测的辐射状靶标,提出一种与相同反射率大面积靶标相结合的方法,依据靶标调制度、传感器入瞳的物方调制度与像方调制度之间的关系,准确获取不含大气的星载遥感器成像系统在轨MTF与大气MTF值。实验结果表明:采用辐射状靶标法可同时获取星载遥感器成像系统空间分辨率与在轨MTF值,实时的大气MTF值为0.7519,基于辐射状靶标的在轨MTF检测结果与刃边法在轨MTF检测结果差异小于5%,该方法可用于实现高分辨率光学卫星传感器在轨像质评价。     

Analytic modeling of the angular sensitivity function and modulation transfer function of ultrathin multichannel imaging systems

We propose what we believe to be a novel, refined model of the angular sensitivity function of artificial apposition compound eyes. Compared with the formerly used Gaussian approximation that was derived for natural compound eyes, our model is better suited to describe the resolution capacity of artificial compound eyes accounting for the cylindrical sensitivity function of technical receptors. It is shown that this analytic model is valid over a broad range of parameters of the optical system, which was not fulfilled by one of the previous models. Finally, an analytic approach is used to derive the modulation transfer function of these multichannel imaging systems.     

激光在湍流大气中的干涉仿真

通过研究激光在湍流大气中的传输情况，旨在为光学高分辨率干涉成像提供理论依据。采用相位屏的近似处理方法对激光通过大气湍流传输进行模拟，研究激光通过湍流大气的干涉情况。试验结果表明：孔径间距越大，干涉条纹越密；在传输距离为2km，大气湍流折射率结构常数C2n为10-14，即接近强湍流的情况下，可以形成比较好的干涉条纹。孔径间距较小时，干涉的明暗条纹清晰可见。在实际干涉成像中，可以选择适当的孔径间距来获得较好的干涉条纹，从而获得目标的高分辨率像。     

Integration of Chandrasekhar's integral equation

We solve Chandrasekhar's integration equation for radiative transfer in the plane-parallel atmosphere by iterative integration. The primary thrust in radiative transfer has been to solve the forward problem, i.e., to evaluate the radiance, given the optical thickness and the scattering phase function. In the area of satellite remote sensing, our problem is the inverse problem: to retrieve the surface reflectance and the optical thickness of the atmosphere from the radiance measured by satellites. In order to retrieve the optical thickness and the surface reflectance from the radiance at the top-of-the atmosphere (TOA), we should express the radiance at TOA “explicitly” in the optical thickness and the surface reflectance. Chandrasekhar formalized radiative transfer in the plane-parallel atmosphere in a simultaneous integral equation, and he obtained the second approximation. Since then no higher approximation has been reported. In this paper, we obtain the third approximation of the scattering function. We integrate functions derived from the second approximation in the integral interval from 1 to ∞ of the inverse of the cos of zenith angles. We can obtain the indefinite integral rather easily in the form of a series expansion. However, the integrals at the upper limit, ∞, are not yet known to us. We can assess the converged values of those series expansions at ∞ through calculus. For integration, we choose coupling pairs to avoid unnecessary terms in the outcome of integral and discover that the simultaneous integral equation can be deduced to the mere integral equation. Through algebraic calculation, we obtain the third approximation as a polynomial of the third degree in the atmospheric optical thickness.     

INNER AND OUTER SCALE EFFECTS ON SCINTILLATION INDEX FOR AN INFRARED LASER PROPAGATING ON EARTH-SPACE PATHS

The effects of the inner-and outer-scale of turbulent atmosphere on the scintillation index for an infrared laser beam propagating through atmospheric turbulence are discussed under the assumption that small-scale irradiance fluctuation is modulated by large-scale irradiance fluctuation on Earth-space paths. A model about the scintillation index with the inner-and outer-scale is developed. A numerical analysis is done by using this model. It is shown that the effect of the inner scale on scintillation index is larger than the outer scale effect for the lesser wavelength wave at visible and infrared band. From moderate to saturation regime, the inner scale effect becomes gradually small; however, the outer scale effect becomes gradually obvious. Under moderate to strong regime, therefore, the effects of the inner-and outer-scale on scintillation index must be considered for theoretical prediction scintillation of an infrared laser beam propagating through turbulent atmosphere on Earth-space paths.     

Slant path average intensity of finite optical beam propagating in turbulent atmosphere

The average intensity of finite laser beam propagating through turbulent atmosphere is calculated from the extended Huygens Fresnel principle. Formulas are presented for the slant path average intensity from an arbitrarily truncated Gaussian beam. The new expressions are derived from the modified von Karman spectrum for refractive-index fluctuations, quadratic approximation of the structure function,and Gaussian approximation for the product of Gaussian function and Bessel function. It is shown that the form of average intensity is not a Gaussian function but a polynomial of the power of the binomial function, Gaussian function, and the incomplete gamma function. The results also show that the mean irradiance of a finite optical beam propagating in slant path turbulent atmosphere not only depends on the effective beam radius at the transmitting aperture plane, propagation distance, and long-term lateral coherence length of spherical wave, but also on the radius of emit aperture.     

光电成像系统动态调制传递函数测量装置

在光电成像系统中,动态调制传递函数受到光学系统波像差、探测组件信号分辨率及传递特性以及载体运动引起图像模糊等环节对图像质量的综合影响,成为光电成像系统的重要参数之一。论文基于光电成像系统动静态调制传递函数的测试原理,研制一种动态调制传递函数测量装置,其中光学准直系统焦距为10 000 mm,运动目标速度控制范围达到30 mm/s～5 000 mm/s,满足长焦距光电成像系统的动态调制传递函数测试。利用该装置进行了一系列的实验研究,结果表明,动态调制传递函数测量重复性优于0.01,测量不确定度达到U=0.05（k=2）。     

The impact on the X-ray phase contrast imaging clarity of the light source area

Based on the partially coherent optical theory, a new theoretical model is established about the X-ray light source radius of X-ray phase contrast imaging system. Through the integral phase contrast modulation transfer function, a comprehensive analysis about the light source radius is made. Then the light source radius selection methods are investigated. Finally, an actual imaging experiment is shown to confirm the choice method of light source radius in X-ray phase contrast imaging system.     

空间高层大气遥感远紫外成像光谱仪的光学系统

空间高层大气遥感远紫外成像光谱仪主要用于观测高层大气中的远紫外辐射和实现对其内部中尺度现象成像的功能。目前我国该类的相关仪器研究基础还比较薄弱,针对这种情况,在光学系统设计的角度上给出了一种适用于130～180nm波段探测的光学系统方案。该成像光谱仪光学系统以离轴抛物镜为物镜,串联Wadsworth结构为成像光谱系统;这种串联Wadsworth成像光谱系统采用离轴抛物镜做准直镜,分光器件为平面光栅和球面光栅串联,实现二次色散,同时球面光栅起到聚焦成像作用;在像差理论的基础上,对该结构的光程函数和各像差进行了分析,获得了改进结构的宽波段完善成像条件。针对低轨空间探测应用要求设计了相关改进型Wadsworth结构成像光谱仪光学系统,设计结果证明系统像差得到了充分校正,在奈奎斯特频率(20lp/mm)下全视场全波段调制传递函数值在0.6以上。该优化结构同时具备高空间分辨率和高光谱分辨率,性能优越。     

Axial intensity distribution of truncated Bessel-Gauss beams in a turbulent atmosphere

By means of the extended Huygens-Fresnel integral formula in the paraxial approximation and based on the fact that a hard aperture function can be expanded into a finite sum of complex Gaussian functions, an approximate analytical expression for Bessel-Gauss beams propagating in a turbulent atmosphere truncated by an aperture has been derived. The result is more convenient for studying the axial intensity distribution properties of Bessel-Gauss beams with different topological charges in a turbulent atmosphere than the usual way by using diffraction integral directly. Utilizing the analytical expression, the axial intensity distribution of such kind of beam in a turbulent atmosphere is investigated numerically in detail. Results show that the intensity distribution on the axis changes with the variation of the beam topological charge m, the wavelength and beam waist width of the initial beam and the turbulent strength.     

Some experiments in partially coherent imaging and modulation transfer function evaluation

A relatively simple and inexpensive scanning optical system is described as a versatile tool for the demonstration and analysis of a variety of optical imaging properties. Some examples of partially coherent imaging and modulation transfer function (MTF) evaluation are presented.