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光谱分光系统采用反向设计思路,由探测器性能和光栅线对数计算会聚模块和准直模块设计参数。会聚模块的初始结构在专利结构中选择,较小的系统F数,可以保证得到的图像不失真也能保证接收到较多的光谱信息[7],会聚模块设计参数如表1所示。
Parameter Value Wavelength range/nm 400 -1 000 Pixel/pixel 2 480×2 048 Pixel size/μm 3.45 Focus of convergence module/mm 30 Grating notch density/lp·mm-1 254 F number 4 Resolving power/nm 5 Field angle/(°) ±7.5 Table 1. Convergence module system parameters
由于系统包含光栅元件,使得视场非对称,所以设计时设置全视场进行优化。为达到轻小型化的目的需控制镜片数量及大小,系统中存在较大的球差和色差,矫正难度高。因此系统使用两组胶合镜对色差及球差进行矫正,两片单透镜使用消色差材料。设计完成的会聚模块系统如图2所示。由此可以看出使用较少的镜片会聚模块仍达到良好像质且系统体积较小,满足轻小型化的目的[8]。
为保证与空间维视场一致,需要放大倍率为1,因此选用与会聚模块对称结构,系统参数如表2所示。同时为与光栅及会聚模块更好的拼接,准直模块设计要考虑到光瞳匹配以及装调等问题[9],准直模块光学系统采用像方远心系统。考虑到系统中光谱范围较大,会存在较大的色差,因此准直模块还需要重点校正色差。上述几个问题叠加在一起均增加了系统总体设计难度。设计完成的准直模块如图3所示。图3(a)为准直模块结构图;图3(b)显示在截至频率处MTF接近衍射极限;从图3(c)可以看出掩膜板处点列图光斑最大RMS为3.1 μm小于掩膜板狭缝宽度,满足对掩膜版空间分辨的的要求,同时系统像质良好,MTF接近衍射极限。
Parameter Value Wavelength range/nm 400 -1000 Focus of collimation module/mm 30 F number 4 Field angle/(°) ±7.5 Mask slit width/μm 6.9 Table 2. Collimation module system parameters
Light and small mask spectral optical system
doi: 10.3788/IRLA202049.0414001
- Received Date: 2019-12-02
- Rev Recd Date: 2020-01-09
- Available Online: 2020-04-14
- Publish Date: 2020-04-24
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Key words:
- mask type /
- spectrometer /
- optical design /
- grating /
- miniaturization
Abstract: The mask spectrometer uses the beam splitter to separate the incident scene into two dimensions: spatial dimension and spectral dimension, then information fusion is used to achieve high spectral resolution of dynamic video signal acquisition. It has wide application value in the field of dynamic hyperspectral imaging. In order to solve the problem of miniaturization of mask spectrometer, for the part of the spectral optical system simplified design of the number of lenses, and the conventional prism was replaced by the grating, the linear dispersion was realized while the structure was further compacted. The problem of stray light generated by the unwanted stages of the grating was analyzed, and the feasibility of the system design was demonstrated. Finally, in the range of 400-1 000 nm, the spectral resolution was less than 4 nm, the average modulation transfer function (MTF) at the full field of view Nyquist frequency was greater than 0.4, and the image illumination uniformity was higher than 0.9, achieved good image quality. Meanwhile, the SNR generated by stray light was 0.06, which did not affect the collection of spectral information.