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该红外系统采用中波640×512制冷型焦平面阵列探测器,其像元尺寸为24 μm×24 μm,红外光学系统的基本参数如表1所示。
表 1 光学系统的基本参数
Table 1. Basic parameters of the optical system
Parameter Value Wavelength/μm 3.7−4.8 Aperture/mm 20 Focal length/mm 40 F number 2 Full field of view 21.74°×17.46°(27.88°) -
为保证红外系统具有100%冷光阑效率,即探测器的冷屏与系统的出瞳重合。由基本参数可知,该系统具有中等焦距、中等视场、大相对孔径的结构特点,采用二次成像的折射式结构形式,以提高系统的孔径利用率。
相对孔径较大的红外系统存在着多种高级像差,视场较大的系统轴外像差(彗差、像散、场曲、倍率色差、畸变)需要进行校正。通过Si、Ge两种材料的搭配以校正色差,并采用非球面来校正大相对孔径引起的球差、彗差与控制畸变,可有效校正和平衡初级及高级像差[6-10]。
利用光学设计仿真软件CodeV对初始结构进行优化,将光学结构复杂化,并将3个Ge材料的表面非球面化,以达到平衡像差的目的,最终优化结果如图4所示,系统由8片透镜组成,其中3个表面为高次非球面,透镜材料为Ge和Si,系统采用二次成像设计,压缩第一片透镜的口径,总长145 mm,结构紧凑,系统结构参数如表2所示。
表 2 系统结构参数表
Table 2. System structure parameters
Serial number Radius Thickness Surface type Glass 1 24.595 9.72 Sphere Si 2 39.335 3.36 Sphere 3 56.817 10 Sphere Ge 4 14.881 10.49 Asphere 5 1 620.865 6.17 Sphere Si 6 −40.232 23.58 Sphere 7 −54.727 9.86 Asphere Ge 8 −102.979 3.19 Sphere 9 −63.085 7.54 Sphere Si 10 −28.932 9.79 Sphere 11 −43.21 4 Sphere Ge 12 −134.589 2 Asphere 13 1 051.776 6.66 Sphere Si 14 −49.914 5.9 Sphere 15 302.679 4 Sphere Ge 16 Infinity 5 Sphere 设计完成的中波红外光学系统的像质接近衍射极限,系统MTF和点列图分别如图5、图6所示,系统在奈奎斯特频率(21 lp/mm)处轴外视场的MTF>0.69,边缘视场的弥散斑的均方根值(RMS)为3.3 μm,小于爱里斑直径(39.04 μm),具有较好的成像质量。
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基于外视场拼接原理,于系统前依次放置多孔旋转反射镜和平面反射镜,利用单镜头实现2×1的视场扩大。为有效利用光学系统的视场资源,采用两视场边缘光线平行的方式(
$\theta = 2\omega $ )进行拼接,在保证各自的机械支撑结构的同时,尽量缩短两平行光线的间隔,减小视场盲区。根据中波红外系统的视场角确定平面反射镜的放置角度及位置、多孔旋转反射镜的放置角度及位置、多孔旋转反射镜中的扇形反射镜和平面反射镜的尺寸,即要保证其中波红外系统的视场不被遮挡,又要避免因扇形反射镜的有效反射区域过大而造成的鬼像。合理控制曝光位置,以达到和孔/反射镜曝光同步。
将原理样机对着室外进行成像,实物图如图7所示,其采集图像如图8所示,图8(a)、(b)的左右图像为对应不同视场下的拍摄图形,下图为拼接图像,由拼接图像可看出拼接处过渡平滑,无缝衔接,既不存在视场重叠[11],也没有出现视场盲区。
和其他物方拼接相比,如“横1”,利用2个镜头对应2个探测器,每个相机按照一定的角度实现2×1的视场拼接,如图9所示。采用“横1”拼接方式也可得到如图8的拼接效果。
两种拼接方式的对比分析如表3所示。由表3分析可知,前置多孔旋转反射镜和反射镜的拼接方式具有一定的优势,拼接方式简单,成本低。
表 3 两种拼接方式的对比分析
Table 3. Comparative analysis of two splicing methods
Object space splice Advantages Disadvantages Same points "Cross 1" No loss of frame frequency, no motion mechanism Two optical systems,two thermal imagers,two processing systems;relatively complex structure Exist of blind areas or overlapping field of view Front rotating mirror and mirror One optical system,one thermal imager,one processing system;simple splicing Loss of frame frequency, need motion mechanism
Realized large field of view infrared imaging system of single lens with external field splicing
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摘要: 基于外视场拼接原理,研制一套大视场中波红外原理样机。设计一制冷型中波红外光学系统。采用光学系统出瞳与冷屏重合的二次成像结构形式,保证系统具有100%冷光阑效率。工作波段为3.7~4.8 μm,焦距为40 mm,相对孔径为1:2,全视场角为21.74°×17.46°(27.88°),系统总长145 mm。采用孔/反射镜分时成像的外视场拼接,实现2×1视场扩展。设计结果表明:在空间频率21 lp/mm处,轴外视场MTF>0.68,接近衍射极限。系统结构紧凑,成像质量较高。利用原理样机完成可行性和合理性验证。Abstract: Based on the principle of external field splicing, a principle prototype of large-field mid-wave infrared was developed. A cooled mid-wave infrared optical system was designed. The secondary imaging structure of the optical exit aperture coincides with the cold screen was adopted to make sure 100% cold shielding efficiency of the system. The working wave band was 3.7-4.8 μm, the focus was 40 mm, the relative aperture was 1:2, the full field of view was 21.74°×17.46°(27.88°), the total length of the system was 145 mm. The principle of external field splicing with time-sharing exposure imaging of holes/mirrors was adopted, 2×1 field of view expansion was realized. The design result show that MTF>0.68 of the off-axis field of view at spatial frequency of 21 lp/mm, which closes to the diffraction limited. The system has compact constructure and has high imaging quality. The feasibility and rationality of the splicing principle were verified using prototype .
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Key words:
- time-sharing exposure /
- external field splicing /
- field of view expansion
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表 1 光学系统的基本参数
Table 1. Basic parameters of the optical system
Parameter Value Wavelength/μm 3.7−4.8 Aperture/mm 20 Focal length/mm 40 F number 2 Full field of view 21.74°×17.46°(27.88°) 表 2 系统结构参数表
Table 2. System structure parameters
Serial number Radius Thickness Surface type Glass 1 24.595 9.72 Sphere Si 2 39.335 3.36 Sphere 3 56.817 10 Sphere Ge 4 14.881 10.49 Asphere 5 1 620.865 6.17 Sphere Si 6 −40.232 23.58 Sphere 7 −54.727 9.86 Asphere Ge 8 −102.979 3.19 Sphere 9 −63.085 7.54 Sphere Si 10 −28.932 9.79 Sphere 11 −43.21 4 Sphere Ge 12 −134.589 2 Asphere 13 1 051.776 6.66 Sphere Si 14 −49.914 5.9 Sphere 15 302.679 4 Sphere Ge 16 Infinity 5 Sphere 表 3 两种拼接方式的对比分析
Table 3. Comparative analysis of two splicing methods
Object space splice Advantages Disadvantages Same points "Cross 1" No loss of frame frequency, no motion mechanism Two optical systems,two thermal imagers,two processing systems;relatively complex structure Exist of blind areas or overlapping field of view Front rotating mirror and mirror One optical system,one thermal imager,one processing system;simple splicing Loss of frame frequency, need motion mechanism -
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