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水下平行光管光学系统如图2所示,系统由光源、毛玻璃、鉴别率板、准直物镜组和平板窗口组成。
水下平行光管的前端为针对水体折射率设计的准直物镜组以及平板窗口,后端为鉴别率板和照明组件。光源发出的光经过毛玻璃匀光后,照明鉴别率板,再经过准直物镜组、平板窗口后在水中形成平行光。鉴别率板选用美标鉴别率板USAF1951,如图3所示。
美标鉴别率板USAF1951每1 mm的线对数如表1所示。
表 1 USAF1951线对数(单位:lp/mm)
Table 1. USAF1951 line pair count (Unit: lp/mm)
Element Group number 0 1 2 3 4 5 6 1 1.00 2.00 4.00 8.00 16.00 32.00 64.00 2 1.12 2.24 4.49 8.98 17.95 36.00 71.80 3 1.26 2.52 5.04 10.10 20.16 40.30 80.60 4 1.41 2.83 5.66 11.30 22.62 45.30 90.50 5 1.59 3.17 6.35 12.70 25.39 50.80 102.00 6 1.78 3.56 7.13 14.30 28.51 57.00 114.00 -
设定水下平行光管的波段为可见光(380~780 nm),全视场角4.8°,水中焦距300 mm,入瞳直径30 mm。选用无限远柯克物镜与平板窗口结合作为初始结构,针对水体光学参数,采用光路倒置进行水下平行光管的光学系统优化。优化后像差曲线如图4所示。
图4(a)、(b)分别为水下平行光管的调制传递函数(Modulation Transfer Function, MTF)曲线图、场曲畸变曲线图。全视场分辨率可达到120 lp/mm以上,准直光学系统最大畸变为0.0447%。球差、色差等各类像差得到很好校正,满足水下光学系统检测的需求。
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水下平行光管作为水下相机成像检测的标准仪器,需要确保其本身出射光的准直性。由于水下平行光管工作于水中,无法采用常用的平行光管装调方法[18-20]。
水下平行光管在水中和空气中的像差曲线如图5所示。首先分析水下平行光管在水中和空气中的像质差异。图5(a)、(b)分别为水下平行光管在水中和空气中的MTF曲线。将水下平行光管置于空气中时,成像质量会发生严重的衰减。这是由于色差的影响,不同波长的光经过准直物镜组会聚后焦点位于沿轴的不同位置。
进一步分析水下平行光管在水中可见光和空气中单色光的像质差异。如图6(a)~(d)分别为水下平行光管在水中可见光照明、空气中635 nm光源照明、空气中532 nm光源照明和空气中470 nm光源照明在同样焦面位置的MTF曲线。
对比发现,水下平行光管在空气中635 nm光源照明下的MTF曲线与水中可见光照明的MTF曲线基本一致。说明可以采用单波长的照明条件,在空气中进行水下平行光管的装调。选用波长为635 nm的红色激光光源,使用型号为威尔特T3A的经纬仪调节确定水下平行光管最佳焦面位置。
据此,在空气中采用标准传递法完成水下平行光管装调、检测[21]。在空气中635 nm光源照明条件下,使用经过严密校准的焦距1200 mm的平行光管对水下平行光管进行检测。水下平行光管可清晰辨认的鉴别率板单元对应线对数达到120 lp/mm,说明水下平行光管具备良好的成像分辨率检测能力。
Measurement of underwater camera imaging resolution using underwater collimator
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摘要: 水下光学成像是重要的水下探测方式。现有水下相机成像检测方法受到水体本身以及测量方法的影响,难以准确进行成像分辨率检测。提出了基于水下平行光管的水下相机成像分辨率检测技术,通过在水中产生平行光束,直接对水下相机成像分辨率进行检测。通过仿真得出:水下平行光管在水中可见光和空气中单波长的调制传递函数(Modulation Transfer Function, MTF)基本一致。利用这一结论,提出了水下平行光管空气中装调检测的方法。针对实验室所研制的一款水下相机开展实验测试,其在水中可见光与空气中635 m光源照明条件下的分辨率相同。实验结果表明,所提出的基于水下平行光管的水下相机成像分辨率检测方法可有效消除水体对分辨率测量的影响,实现水下相机成像分辨率的准确测量。Abstract: Underwater optical imaging is an important underwater measurement method. The existing underwater camera imaging resolution measurement method is affected by the water body and the measurement method, and it is difficult to accurately measure the imaging resolution. The underwater camera imaging resolution measurement technology based on an underwater collimator is proposed to directly measure the underwater camera imaging resolution by generating parallel light beams in the water. Through simulation, the modulation transfer function (MTF) of the underwater collimator in visible light in water and single wavelength in the air are basically the same. In conclusion, a method for adjusting the underwater collimator in the air is proposed. The experimental test is carried out for an underwater camera developed in the laboratory, and its resolution under the condition of visible light in water and a 635 nm light source in the air is the same. The experimental results show that the proposed underwater camera imaging resolution measurement method based on the underwater collimator can effectively eliminate the influence of water on the measurement and realize the accurate measurement of the underwater camera imaging resolution.
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表 1 USAF1951线对数(单位:lp/mm)
Table 1. USAF1951 line pair count (Unit: lp/mm)
Element Group number 0 1 2 3 4 5 6 1 1.00 2.00 4.00 8.00 16.00 32.00 64.00 2 1.12 2.24 4.49 8.98 17.95 36.00 71.80 3 1.26 2.52 5.04 10.10 20.16 40.30 80.60 4 1.41 2.83 5.66 11.30 22.62 45.30 90.50 5 1.59 3.17 6.35 12.70 25.39 50.80 102.00 6 1.78 3.56 7.13 14.30 28.51 57.00 114.00 -
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