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该系统以人体为辐射源,利用光机扫描和波束聚焦系统将视场中人体的各个位置逐点成像 [13-15]。光机扫描系统可以将视场中的每一个点逐点扫描到探测器上,波束聚焦系统则用于把太赫兹波束聚焦到探测器上,最后探测器把采集到的太赫兹信号转换为电信号,从而在计算机系统中获得太赫兹图像。具体来说,人体上每一个点辐射出的太赫兹波,首先通过一个平面转镜,再反射到一个三面体聚焦扫描镜上,该扫描镜同时实现了太赫兹波束的一维扫描和波束聚焦功能,最后探测器将太赫兹波段的电磁波信号转换为电信号,计算机处理接受到的电信号形成太赫兹图像。
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该系统主要由太赫兹信号扫描模块,太赫兹波束聚焦模块,太赫兹信号接收与处理模块组成。其光路示意图如图1所示。被测物体辐射出太赫兹波,经过第一面反射镜反射至下面的聚焦扫描镜。系统中光学元器件为经过抛光处理的纯铝制作,以提高太赫兹信号反射率。第一面反射镜为方形反射镜,可绕平行于纸面的中心轴转动,实现横向扫描;下面的聚焦扫描镜为一个三面体扫描镜,可绕垂直于纸面的中心轴转动,实现纵向扫描,太赫兹波通过两面扫描镜即可实现对物体的二维扫描,组成太赫兹信号扫描模块。三面体扫描镜同时还是一面凹面镜,将信号汇聚至后面的信号接收器,这样组成太赫兹聚焦系统。系统的信号接收器为250 GHz的太赫兹探头,通过光纤通信系统连接到计算机,计算机处理接收到太赫兹信号形成图像,组成太赫兹信号接收与处理模块。
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为了获取系统优化的成像距离,成像时间和成像分辨率等具体性能指标,需要对实验所得图像进行图像质量评价。图像评价分为两种,一种为对无噪声的参考图像和带噪声的待测图像进行评价,另一种为对没有参考图像的待测图像进行评价。太赫兹被动成像系统所成图像为无参考图像,因此,可以采用信息熵[16]对实验所得图像进行图像质量评价。信息熵为:
$$ H = \mathop \sum \nolimits_{i = 0}^{255} {p_i}log{p_i} $$ (1) 式中:
${p_i}$ 表示灰度值为i的像素所占的概率;H为信息熵,反映了图像中平均信息量的多少。图像灰度值i的级别范围为0~255,0表示图像显示颜色为黑色,255表示图像显示颜色为白色,灰度值越高,表明图像越亮。首先,依次统计每一级灰度值为i的像素在待测图像像素矩阵中出现的个数n,再用n除以待测图像总的像素数量,得出每一级灰度值为i的像素出现的概率${p_i}$ ,再将${p_i}$ 代入公式(1),计算结果即为待测图像的信息熵H,信息熵越高代表图像包含的信息越多,图像质量越好。
Study on the performance of terahertz passive imaging system
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摘要: 太赫兹被动成像技术具有很多独特的优势,已经成为安全检查领域的重要研究方向。为了进一步提高太赫兹被动成像系统的成像性能和实用性,研究了实验室自研的一套基于光机扫描的太赫兹被动成像系统的性能。通过探索不同系统参数和实验条件,包括成像距离、成像速度以及探测器状态,对背景噪声以及分辨率等图像性能的影响,获得了系统优化的性能参数,同时探究了该系统探测人体隐藏物品的能力,说明该系统可以有效地对人体进行安全检查。实验结果表明:该太赫兹被动成像系统的成像距离在机器前面1.5~2.5 m的范围内,即在大约1 m的景深内,能够显示清晰的太赫兹图像;该系统的成像速度为每帧1~2 s;探测器工作电平和增益对图像清晰度和系统噪声有较大的影响,存在一个优化的探测器工作状态,在该条件下可以获得清晰的太赫兹图像。在各项参数优化的条件下,该太赫兹成像系统可以达到的目标分辨率为1.5~2 cm,能够清晰地探测隐藏在人体衣服之下的金属物品和对太赫兹波有较强吸收的物质。Abstract: Terahertz passive imaging technology has many unique advantages and has become an important research direction in the field of security inspection. In order to further improve the imaging performance and practicability of the terahertz passive imaging system, the performance of a terahertz passive imaging system based on opto-mechanical scanning developed by our laboratory was studied. By exploring the influence of different system parameters and experimental conditions including its imaging speed, distance and the status of detector on the performance of imaging system such as the image resolution and background noise, the optimized system parameters were obtained. At the same time, the performance of imaging system to detect the human body hiding the metal objects was explored. It was shown that the system was suitable for the security inspection of human body. The experimental results show that the imaging distance of the terahertz passive imaging system is within the range of 1.5–2.5 m in front of imager, that is, its depth of field is about 1 m. The imaging speed of system is 2–4 seconds per frame. Moreover, the operating level and gain of detector have a great impact on image sharpness and system noise. There exists an optimized operating state of detector. Under the optimized condition, the clear terahertz images can be achieved and the resolution of target was 1.5–2 cm. Our imaging system can clearly detect metal objects hidden under human body’s clothes and substances with strong absorption on terahertz waves.
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Key words:
- terahertz /
- passive imaging /
- opto-mechanical scanning /
- security inspection
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