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选用一个具体设计实例对上述方法进行说明讨论。设计实例选用0.48~0.68 μm和0.7~0.9 μm增强型CMOS面阵探测器,像元数为1920×1080,像元尺寸为2.8 μm×2.8 μm,对角线长度为6.2 mm。具体设计指标如表1所示。
Parameter Value Work wavelength/μm 0.48-0.68 and 0.7-0.9 Field of view (FOV)/(°) 38.1×22.0-2.5×1.4 Effective focal length/mm 8-120 F-number 5 Overall length/mm ≤90 Back focal length/mm ≥5.5 Work temperature/℃ −40-60 Table 1. Design requirements
光学系统前固定镜组(A-part)承担系统主要光焦度,采用四组五片式结构,其中首片正透镜采用HZF88材料,尾片负透镜采用HZF3来平衡色差和热差;变倍镜组(Z-part)行程较大,承担整个光学系统变倍作用,采用色散能力较强的HZF88做正透镜,进行色差补偿,保证整个变倍组具有较小的残余色差;中间固定组(M-part)为典型的单透镜+双胶合结构,具有正光焦度;补偿组(C-part)具有正光焦度,主要用作补偿因变倍组移动引起的像面偏移,其中的双胶合元件具有负的合成色散系数,可保证补偿组色差的独立校正;后固定镜组(B-part)靠近像面,构成场镜结构,主要用于校正系统场曲和畸变,另外较长的后截距也给系统的滤光片切换机构预留了足够的空间[8-12]。
采用CodeV软件对上述光学系统进行优化分析,图2为可见-近红外无热化连续变焦光学系统优化结果示意图。所述系统采用HZF88、HZF3、HFK95、HZPK1、HZLAF78等七种普通光学玻璃材料,共12组16片透镜,光机选用铝合金材料,光学系统靠近物面一侧表面至焦面,总长90 mm,单透镜最大口径小于45 mm,双胶合透镜最大口径小于40 mm。
对上述光学系统不同温度、不同视场的MTF评价如表2和表3所示。可以看出,所设计的光学系统在−40~60 ℃范围内,在长焦、中焦和短焦不同位置,空间频率为180 lp/mm处均具有较好的传函分布,表明该系统具有较好的成像质量,也证明了上述设计方法的可行性。利用编写的宏程序对上述宽波段连续变焦光学系统进行凸轮曲线优化设计,图3为优化完成的变焦系统凸轮曲线示意图,可知整个变焦系统变倍补偿过程平滑无拐点,可满足工程应用要求[13-15]。
−40 ℃ 20 ℃ 60 ℃ EFL=8 mm EFL=70 mm EFL=120 mm Table 2. MTF of the zoom optical system in 0.48-0.68 μm at −40 ℃, 20 ℃ and 60 ℃
−40 ℃ 20 ℃ 60 ℃ EFL=8 mm EFL=70 mm EFL=120 mm Table 3. MTF of the zoom optical system in 0.7-0.9 μm at −40 ℃, 20 ℃ and 60 ℃
取前固定镜组和焦面的轴向位置作为装调补偿,各镜组和镜组间的公差要求如下:各镜组内材料折射率公差为±0.001,阿贝数公差为±0.002;面型公差为N=3,ΔN=0.3;厚度公差为±0.03 mm;偏心公差为±0.01 mm;倾斜公差为±1′。镜组间用镜筒整合,整合公差:偏心公差为±0.03 mm,倾斜公差为±1′,均具有优良的可实现特性。对应公差条件下可见谱段的MTF概率累积分布如图4所示。
Design of visible-near infrared athermal continuous zoom optical system
doi: 10.3788/IRLA20210090
- Received Date: 2021-02-04
- Rev Recd Date: 2021-04-11
- Publish Date: 2021-09-23
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Key words:
- optical design /
- continuous zoom /
- wide band /
- athermal /
- optical dehazing
Abstract: The primary aberration of each lens group will be changed with their movement, and the change of environment temperature will also lead to defocus, both of which will cause a lot of difficulties in the zoom lens design process. To solve this problem, aberration functions do depend and not depend on group movement were introduced based on optical aberration theory, and lens power distribution and material selection method were discussed with the achromatic and athermal design model futher. A visible-near infrared (Vis-Nir) optical system under the requirements of F/5, focal length of 8-120 mm, focal plane diameter of 6.2 mm, work waveband of 0.48-0.68 μm and 0.7-0.9 μm was designed with mechanically compensated method and optical passive athermal technology. The proposed zoom lens system, which used 7 kinds of common optical glass, consists of 12 groups 16 lenses, total length of only 90 mm, has good image quality and tolerance character among the zoom range within −40~60 ℃.