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在4孔径视场部分重叠热成像系统中,各子孔径的光轴呈发散状态,上下左右探测器的视场存在约50%的重叠率,各子孔径图像对应的空间点表现为中心对称,且在视场重叠区域目标位置的变化总存在探测器成像位置的差异。
经测试可得4孔径分布式仿生热成像实验系统实际的总视场大小为48.0°×36.5°,中心视场为15.9°×12°,系统采集的图像如图11所示。
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由于各子孔径图像成像特点,在对子孔径的“并集”视场拼接成大视场前需要进行视场的投影和平移变换。
针对4孔径视场部分重叠热成像系统多视轴呈一定夹角的大视场特点,充分利用系统设计的先验条件,采用SIFT算法进行了图像的旋转、平移和投影变换,实现了大视场拼接,并在图像采集软件的SDK端进行了处理,实现了大视场拼接的准实时化,帧频为25 Hz。
在图像拼接阶段所采取的拼接策略是:已配准的各子孔径图像,乘以权重再相加求和,实现图像融合拼接。对于各子孔径的图像,在重叠区域具有相同的融合权重,且权重和为1。这样设置的融合权重保证了各子孔径图像对最终融合图像的贡献都是平等的,且最终融合图像的像素值与各子孔径保持相同数量级,且避免在各不同重叠度区域加入新的不均匀因素。
具体步骤为:选取其中1个子孔径图像为基准,对于基准图像上某点P,视场重叠子孔径为N (N≤4)个;则子孔径n在P点的权重
${W_n}$ 为:$${W_n} = {1/N}$$ (1) $$\sum\limits_{i = 1}^N {{W_n}} = 1$$ (2) P点经过拼接处理后的像素值
$\overline I $ 为:$$\overline I = \sum\limits_{i = 1}^N {{W_n}{I_n}} $$ (3) 式中:
${I_n}$ 为重叠子孔径n经配准后在P点的灰度。大视场拼接图像如图12所示。子孔径之间存在一些拼接缝痕迹,主要是因为4个独立的红外探测器响应存在一定差异及在图像投影变换时对边界像素处理引入像素偏差,通过探测器的先行标定处理及进一步的算法处理可消除。
Theory and technology of bionic thermal imaging with multi-aperture distributed and partially overlapped field of view
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摘要: 热成像系统视场与空间分辨率(作用距离)的矛盾是常规成像模式难以解决的问题。多孔径热成像技术主要分为低重叠率、高重叠率和中度重叠率部分重叠成像模式。文中研究了一种视场部分重叠仿生热成像理论,利用4组红外物镜及IRFPA机芯构成了中心变分辨率4孔径分布式热成像系统,各子孔径的“并集”视场构成系统成像大视场,“交集”视场特别是中心重叠视场具有超分辨能力,从而构成空间变分辨率视觉模式,可减缓传统单孔径热成像问题;利用重叠视场可构成4目和2目近场目标场景的体视成像;通过子孔径检偏偏振片,中心视场可构成全偏振热成像模式;对角探测器分别采用长波或中波红外焦平面探测器,则可构成双色热成像模式。分析表明:这种多孔径分布式视场部分重叠仿生热成像具有仿生智能的特性,可针对感兴趣目标进行智能观测,提高复杂背景条件下的目标探测和识别能力,具有广泛的应用前景。Abstract: The contradiction between the field of view (FOV) and the spatial resolution (operating range) of the thermal imaging system is difficult to solve in the conventional imaging mode. Multi-aperture imaging technology is mainly divided into low overlapping rate, high overlapping rate and moderate partial overlapping imaging mode. In this paper, a theory of partially overlapped FOV bionic thermal imaging was studied. Four sets of infrared objective lens and infrared IRFPA cores were used to form a 4-aperture distributed thermal imaging system with central variable resolution. The "union" FOV of each sub-aperture constituted a large FOV for the system, and the "intersection" FOV of each sub-aperture, especially the central overlapped FOV, had super-resolution capability, thus forming a spatially variable resolution visual mode, which could alleviate the traditional single aperture thermal imaging problem. The overlapped field of view could be used to construct 4-eye and 2-eye stereo imaging for near-field target scene. Through polarizers on the sub-apertures, the central FOV could form a fully polarized thermal imaging mode. The detectors on the diagonal with LW or MW infrared focal plane detectors respectively could form a dual-color thermal imaging mode. Theoretical analysis shows that this bionic thermal imaging system with multi-aperture distributed and partially overlapped FOV has the characteristics of bionic intelligence, which can intelligently observe the interested targets, improve the target detection and recognition capabilities in complex background conditions and has extensive application prospects.
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