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红外热成像是指利用目标的自身热辐射,通过红外光学系统对热辐射信息的采集而获取图像的过程。红外热成像具有穿云透雾、抗干扰和作用距离远等优点,在军事和民用产品中得到了广泛的应用,特别是针对带有发动机的动目标以及人脸等温升异常目标。
对于长波红外热成像系统,噪声等效温差(NETD)是其灵敏度指标。NETD的理论公式如下[6]:
$$NETD = \dfrac{{4\sqrt {A \cdot \Delta f} }}{{{{\rm{M}}_T} \cdot {\varOmega } \cdot {D^2} \cdot {t_o} \cdot {D^*} \cdot \delta }}$$ (1) 式中:Ω为成像系统瞬时视场;A为探测器接收面积;Δf为等效噪声带宽;D为成像系统口径;
$ {t}_{o} $ 为光学系统光学效率;D*为探测器探测率;MT为表面光谱辐射函数;δ为过程因子。 -
激光主动照明成像的工作原理如图1所示,采用照明激光器主动照射的方式,照明光束覆盖被测目标,成像系统用于收集目标表面反射后的激光能量并最后成像至探测器[11]。根据作用距离和目标成像区域大小不同,可调节照明光束的发散角,将目标特定区域照亮,满足成像系统信噪比要求,实现对目标精确成像和特征点跟踪。
激光主动照明成像系统的作用距离与信噪比、激光照射功率、光路效率等相关。假定目标表面为朗伯分布,照明光束覆盖区域为目标探测区域。根据激光雷达方程,可得激光主动照明成像系统的作用距离表达式[12]:
$$P = \dfrac{{4{R^2}A{\rm{E}}}}{{{K\rm{\rho }}{t_a}{t_o}{D^2}}}\dfrac{{{{\rm{\theta }}^2}}}{{{{{\Omega }}^2}}}$$ (2) 式中:P为连续激光照射平均功率;R为作用距离;E为探测器照度灵敏度阈值;K为光源的光视效能;θ为照射视场角度;ρ为目标表面反射率;ta为大气传输光路效率。
Imaging experiments for weak small target in low-light-level background
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摘要: 无人机等弱小目标具有飞行高度低、反射截面小、热信息弱等特点,尤其在低照度背景下,单一成像方式难以实现对弱小目标的探测和识别。实验对比了可见光成像、微光夜视成像、长波红外成像、激光主动照明成像等对弱小目标的成像效果,目的是研究适用于不同照度背景下弱小目标的探测识别方法。外场实验中,被测目标为小型四旋翼无人机,尺寸为290 mm×290 mm×196 mm,实验环境照度在10−2~10−4 lx之间,作用距离范围0.5~2 km。成像实验结果表明:普通可见光系统无法在环境照度低于10−2 lx时成像;环境照度为10−3 lx时,微光夜视和长波红外热系统的识别距离仅为0.5 km;近红外激光(中心波长808 nm)主动照明与微光夜视结合的主动成像方式可增加对弱小目标的识别距离,同等条件下主动照明成像作用距离是被动成像的3倍。Abstract: Weak small targets, such as unmanned aerial vehicles, have the characteristic of low flight height, small reflection cross-section and weak thermal information, which are difficult to detect and recognize in low-light-level background by a single imaging method. The visible light, low light night vision, long wave infrared and laser active illuminated imaging experiments were carried out, to study the suitable detection and recognition methods by comparing of weak small target images in different low-light-level background. In the field experiment, the target was a small four rotor UAV with the size of 290 mm×290 mm×196 mm, and the environmental illumination was between 10−2 lx and 10−4 lx. The working distance was from 0.5 km to 2 km. The experimental results show that simple visible light system cannot image under 10−2 lx environment illumination; the recognition distance of low-light night vision and long-wave infrared system is only 0.5 km when the environment illumination is 10−4 lx; the active imaging method combining near infrared (central wavelength 808 nm) laser illumination and low-light night vision can increase the recognition distance of weak small targets; under the same condition, the active illuminated imaging working distance is 3 times of the dynamic imaging.
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