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文中基于该结构类型的复眼,设计了一种六边形子眼紧密拼接形式的曲面微透镜阵列,该结构既能满足复眼对于大视场的需求,又可使整个结构紧凑,提高复眼球壳基底的空间利用率[10]。
在设计子眼透镜时,为了实现物空间的完整成像,相邻子眼间的视场要有一定的重叠比例,保证各个子眼在成像后,像面可以通过后期图像处理算法拼接在一起,组成一个新的完整的像面。同时在保证大视场的前提下,要使得子眼的成像区域不发生重叠,便于后期进行图像处理和图像拼接。综上所述,即要保证其大视场优势,又要有一定重叠比例,所以子眼视场角
$\Delta \theta $ 与子眼间夹角$\Delta \varphi $ 的关系应满足:$\Delta \varphi < $ $ \Delta \theta < 2\Delta \varphi $ 。子眼间夹角与复眼参数的关系为:
$$ \Delta \phi ={\arctan}\frac{2a}{R} $$ (1) 如图2所示,a为子眼的半口径,R为复眼所在的球壳基底的曲率半径。
根据设计参数的要求,取子眼的半口经a为0.25 mm,复眼的口径D为8.66 mm,微透镜阵列的角度为120°,根据几何关系可知,复眼所在的球壳基底的曲率半径R = 5 mm,由公式得出
$\Delta \varphi \;{\rm{ = 5}}{\rm{.7}}{{\rm{2}}^ \circ }$ ,取子透镜视场角$\Delta \theta = {{\rm{6}}^ \circ }$ 。常见的生物复眼类似于正透镜,设置子眼前后表面的曲率半径为
${r_1} = - {r_2}$ ,${r_1} > 0$ 。为保证复眼结构的小型化,设定子眼焦距${f'}$ 为0.93 mm,子眼厚度d为0.35 mm。子眼透镜的材料选择便于3D打印制造的PMMA光固化材料,其折射率为1.4918,根据公式(2):$${f'} = \frac{{n{r_1}{r_2}}}{{\left( {n - 1} \right)\left[ {n\left( {{r_2} - {r_1}} \right) + \left( {n - 1} \right)d} \right]}}$$ (2) 可得到子眼的曲率半径为
${r_1}$ = 0.97 mm,${r_2}$ = −0.97 mm。将上述计算得到的初始参数值输入ZEMAX软件中进行分析及优化,由于子眼的视场和口径均较小,像差也很小。优化后的子眼结构及成像质量如图3所示。
在ZEMAX中的非序列模式对曲面微透镜阵列进行建模,以中心子眼为对称中心,在三维空间内进行环形阵列,得到曲面微透镜阵列模型如图4所示。
文中采用六边形子眼紧密拼接的结构形式,为避免相邻子眼之间的串扰现象,设置光阑阵列。光阑阵列位于曲面微透镜阵列和转像系统之间,且微透镜阵列到曲面像间的距离为1.5 mm,即可确定光阑长度近似1.5 mm。考虑3D打印的精度为2 μm,光阑内壁厚度为0.05 mm。光阑孔与子眼一一对应且排列方式相同,由于光阑是有一定斜率的通孔,故将光阑设计为锥形结构。通过ZEMAX软件来确定光阑的参数,每个光阑采用相同的设计参数进行阵列,光阑前端与子透镜后表面相对应,其口径应略小于子眼口径,考虑内壁厚度,故将光阑前端口径设为0.4 mm,光阑后端口径大小应略大于子透镜的像面大小,避免光阑对成像光束的干扰,故将光阑后端口径设为0.25 mm。该结构可作为视场光阑来限制子眼的成像区域,能有效防止子眼间的成像串扰现象,保证不同子眼透镜间的成像独立。
根据上述分析和计算,通过ZEMAX软件对仿生复眼进行模拟,选取曲面复眼的最大视场(60°)进行光线追迹,因复眼结构是对称的,所以列出中心子眼组、过渡子眼组和边缘子眼组,每个子眼设置三个角度(0°、3°、−3°)的入射光线,如图5所示,加入光阑阵列后,每个子眼有着特定的成像通道,相互独立成像,互不干扰。
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复眼系统由微透镜阵列、光阑阵列、转像系统组成。根据光学系统的光瞳衔接准则,微透镜阵列的出瞳位置应与转像系统的入瞳位置相重合,将光阑设置在转像系统第二片透镜的后表面上,系统组合完成后 其成像质量并不理想。考虑到子眼透镜口径和视场都很小,对像差的影响也很小,综合分析影响组合系统成像质量的最大因素是转像系统,通过系统优化来平衡光阑移动给系统带来的像差影响。
优化完成后的组合复眼光路如图11所示,利用多重结构给出七个视场(0°、10.5°、21°、30°、40.3°、50°、60°)的光路图,整个系统长度为8.7987 mm。
由于篇幅限制,文中仅给出三个视场下(0°、40°、60°)的MTF曲线图,如图12所示,在90 lp/mm处,三个视场的MTF分别大于0.45、0.32、0.3,该系统已达到成像质量要求。
三个视场(0°、40°、60°)的点列图如表1所示,各个视场的RMS半径小于艾里斑半径,且在探测器的像元尺寸范围内(5.6 μm)满足该系统的成像质量要求,且无明显色差。
表 1 不同视场的点列图
Table 1. Spot diagrams of different fields of view
Angle 0° 40° 60° 0° RMS radius 2.283 μm 2.701 μm 2.939 μm 3° RMS radius 2.547 μm 2.678 μm 2.632 μm −3° RMS radius 2.541 μm 3.172 μm 4.156 μm Airy radius 4.365 μm 4.732 μm 5.034 μm 复眼系统由S130型的3D打印设备进行制作,在透镜间设计柱状支撑结构,用于固定透镜位置,同时在打印过程中柱状结构有利于打印液的流出,从而确保打印精度。为了避免杂散光对复眼成像的影响,复眼结构的外壳均为遮光材料,“镜筒”、光阑由黑色树脂材料整体打印。以转像系统的光阑为分界,前后两部分分别进行打印,镜片和镜筒之间通过垫片和压圈固定。光阑阵列由黑色树脂材料单独打印,以柱状结构为支撑,装配在复眼球壳后部,复眼球壳及前后部分均以卡扣的形式安装在系统两侧。仿生复眼系统的结构图,如图13所示。
文中对仿生复眼系统进行了公差分析,保证复眼系统加工的可行性。3D激光打印机的最高打印精度为2 μm,各公差参数设置范围如表2所示。选择“ MTF平均值”模式作为公差分析的评价方法,采用灵敏度法和蒙特卡洛法对1000组镜头数据进行公差分析,分析结果如表3所示。
表 2 公差参数范围
Table 2. Tolerance parameter range
Sort Data Radius/mm 0.002 Thickness/mm 0.002 Surface decenter/mm 0.002 Surface tilt 0.03° Element decenter 0.01 Element tilt 0.03 Abbe 0.5% Index 0.001 表 3 复眼系统公差分析结果
Table 3. Tolerance analysis results of compound eye system
Name Result Number of Monte Carlo 1000 MTF nominal value 0.348 542 13 MTF average 0.347 187 12 98% lens MTF value >0.336 334 57 80% lens MTF value >0.343 335 32 50% lens MTF value >0.348 132 40 20% lens MTF value >0.351 250 35 10% lens MTF value >0.352 350 71 2% lens MTF value >0.353 558 39 可以看出,系统MTF的名义值为0.34854213。透镜在98%和10%时的MTF值分别大于0.33633457和0.35235071。从以上数据可以看出,公差分析后MTF值的变化量很小,对仿生复眼系统的公差分析结果比较合理,可以满足系统成像质量的要求。
Design of bionic compound eye system based on hexagonal closely spliced structure
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摘要: 基于仿生复眼的视觉优势,分析了仿生复眼的研究进展,对复眼的成像原理及部分昆虫复眼的结构进行了研究。根据生物复眼的结构形态,设计了六边形紧密拼接形式的曲面微透镜阵列及转像系统。同时,为了防止相邻子眼间的成像光束串扰,设计了单个光阑长度为1.5 mm的光阑阵列,实现了各子眼的单通道成像。根据光瞳衔接原则,对微透镜阵列和转换系统进行组合并优化。整个复眼的口径为8.66 mm,视场角为121°,每个子眼的口径为500 μm,子眼视场角为6°,在90 lp/mm处,复眼系统的MTF值均大于0.3,其RMS半径均小于艾里斑半径,系统成像质量达到设计要求。为满足3D增材制造工艺需求,设计了复眼系统的机械结构。公差分析结果表明,3D增材制造工艺可以满足系统的像质要求。Abstract: Based on the visual advantages of bionic compound eyes, the research progress of bionic compound eyes was analyzed, and the imaging principle of compound eyes and the structure of some insect compound eyes were studied. According to the structure of the biological compound eye, a hexagonal closely spliced curved micro-lens array and image transfer system were designed. At the same time, in order to prevent the crosstalk of imaging beams between adjacent sub-eyes, an aperture array with a single aperture of 1.5 mm in length was designed to realize single-channel imaging of each sub-eye. According to the principle of pupil connection, the micro lens array and the conversion system were combined and optimized. The diameter of the entire compound eye was 8.66 mm, the field of view was 121°, the diameter of each sub-eye was 500 μm, and the field of view of the sub-eye was 6°. At 90 lp/mm, the MTF value of the compound eye system was greater than 0.3, and the RMS spot radius were less than the radius of the Airy disk, the imaging quality of the system meet the requirements of the design index. In order to met the requirements of 3D additive manufacturing process, the mechanical structure of the compound eye system was designed. The tolerance analysis result shows that the system structure meets the preparation requirements of 3D printing technology.
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表 1 不同视场的点列图
Table 1. Spot diagrams of different fields of view
Angle 0° 40° 60° 0° RMS radius 2.283 μm 2.701 μm 2.939 μm 3° RMS radius 2.547 μm 2.678 μm 2.632 μm −3° RMS radius 2.541 μm 3.172 μm 4.156 μm Airy radius 4.365 μm 4.732 μm 5.034 μm 表 2 公差参数范围
Table 2. Tolerance parameter range
Sort Data Radius/mm 0.002 Thickness/mm 0.002 Surface decenter/mm 0.002 Surface tilt 0.03° Element decenter 0.01 Element tilt 0.03 Abbe 0.5% Index 0.001 表 3 复眼系统公差分析结果
Table 3. Tolerance analysis results of compound eye system
Name Result Number of Monte Carlo 1000 MTF nominal value 0.348 542 13 MTF average 0.347 187 12 98% lens MTF value >0.336 334 57 80% lens MTF value >0.343 335 32 50% lens MTF value >0.348 132 40 20% lens MTF value >0.351 250 35 10% lens MTF value >0.352 350 71 2% lens MTF value >0.353 558 39 -
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