1.Anhui Institute of Optics and Fine Mechanics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China 2.University of Science and Technology of China, Hefei 230026, China
Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2019YFF0303400), the National Natural Science Foundation of China (Grant No. 41941011), the Key Research Program of Frontier Sciences of Chinese Academy of Sciences (Grant No. QYZDY-SSW-DQC016)
Received Date:29 January 2021
Accepted Date:29 March 2021
Available Online:07 June 2021
Published Online:20 September 2021
Abstract:Under different ambient temperatures, the thermal aberration certainly affects the imaging quality of infrared multi-spectral camera. Therefore, an athermalized model of infrared multi-spectral cameras is established, and in this model the ambient infrared multispectral camera is equivalent to a separated dual-lens optical system. In the case of the fixed focal length, the influence of the back focal length on the change of the focal power of the front lens and back lens is analyzed. Now, the variation range of the front and rear lens interval is assumed to be restricted. When the back focal length is smaller than the focal length, the ratio of the absolute value of the focal power of the front lens to the absolute value of the focal power of the back lens decreases with the back focal length increasing. The material of the front lens and the back lens have a longer interval on the thermogram. When the back focal length is greater than the focal length, the scenario becomes exactly opposite. Combined with the judgment method of the positive value and negative value of the focal power on the thermogram, the selection range of materials is constrained by the positive value, negative value, and absolute value of focal power, thus realizing the rapid selection of the optical materials. This method is used to design an athermalized infrared multispectral camera with a waveband of 8–14 μm, a focal length of 50 mm, and an F number of 1.4 in a range from –40 ℃ to +60 ℃. Through the simulation analysis, the value of the athermalized infrared multispectral camera, at the Nyquist frequency of 30 lp/mm reaches 0.39, which is close to the diffraction limit; the root mean square radius of the diffuse spot is smaller than the Airy spot radius of 19.17 μm; the axial aberration is less than 0.02 mm, and the design results show that this method can make the long back-focus infrared optical system maintain stable imaging quality in a large temperature range. The SF6 gas is detected experimentally, and the experimental results demonstrate the excellent optical performance of the system. Keywords:infrared multispectral camera/ long back focal length/ athermal design/ hybrid refractive-diffractive system
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2.1.无热化原理
对于一个具有K块薄透镜的光学系统, 分割出K-1个空间, 如图1所示, ${L_i}$为第i块透镜, ${M_i}$为第i块透镜的材料, hi为近轴光线在第i块透镜上的高度. 图 1 具有K块薄透镜的光学系统 Figure1. An optical system containing K thin lenses.
以${f'} = 50$ mm为例, 结合(12)式与(13)式绘出不同后焦距下前组透镜归一化光焦度的变化情况, 如图3所示. 图 3 不同后焦距下前组透镜的归一化光焦度变化情况 Figure3. Change of normalization power of front lens under different back focal length.
从图3可以看出, 通过增大或减小$ \left| d \right| $可以抑制前组透镜归一化光焦度的变化趋势, 但是需要对$ \left| d \right| $进行大幅度的增减, 当$\Delta {f_{\rm{b}}}$取值较大时, $ \left| d \right| $的取值不再合理. 所以, 当后焦距小于焦距时, $\left| {\phi _{{\text{front}}}'} \right|$随着后焦距的增大呈减小趋势, 且后焦距越大趋势越明显, 如图3中的A, C区域, 此时$\left| {\dfrac{{\omega _{{\text{front}}}'}}{{\omega _{{\text{back}}}'}}} \right|$变大, 应当选择无热图中相隔较远的两种材料, 如图4(a)所示. 当后焦距大于焦距时, $\left| {\phi _{{\text{front}}}'} \right|$变化情况完全相反, 如图3中的B, D区域, 此时$\left| {\dfrac{{\omega _{{\text{front}}}'}}{{\omega _{{\text{back}}}'}}} \right|$变小, 应当选择无热图中相隔较近的两种材料, 如图4(b)所示. 图 4 无热图中长后焦距系统材料的选择 (a)后焦距小于焦距; (b)后焦距大于焦距 Figure4. Material selection of long back focal focus system on the athermal chart: (a) Back focal length is smaller than focal length; (b) back focal length is larger than focal length.