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滤光片轮高光谱相机的结构如图2所示,它是以滤光片轮为分光元件,通过转动滤光片轮获得不同波段的光谱图像,从而完成复色光到单色光的分光。滤光片轮通常是将一组具有不同波长透过率的窄带滤光片固定在轮式结构上,每曝光一次采用一个滤光片。控制滤光片轮的旋转速度,使其转动频率与传感器采样频率同步,从而保证每个滤光片对应的谱段都能在传感器上成像[4]。
滤光片轮高光谱相机的关键器件是滤光片轮,可以根据观测波段的不同替换相应谱段范围的滤光片轮,光路结构简单,谱段更换灵活。但是由于光谱通道之间的切换需要依靠轮式结构的转动来完成,旋转结构带来的振动对成像质量影响较为明显,成像所需曝光时间较长;且单次曝光只能获得指定光谱范围的图像,光谱响应曲线是离散的,无法获取连续谱段的图像,存在实时性的问题;同时滤光片轮上各个滤光片的共面情况以及厚度均匀性也会带来成像模糊等问题。除此之外,随着光谱成像技术的发展,探测波段数目越来越多,滤光片轮已无法满足宽谱段高分辨率的观测,因此越来越多地被用于多光谱探测中。
1994年,美国成功发射了对月探测卫星Clementine,该卫星的有效载荷:UV/VIS相机、NIR相机和HIRES相机都用到了滤光轮,覆盖波段及滤光轮的相关参数如表1所示。
Pay-
loadSpectral range/nm Filters Center wavelengths and bandpass widths/nm A B C D E F UV/
VIS415-
10006 415/
40750/
10900/
20950/
301000/30 — NIR 1070-2840 6 1100/60 1250/60 1500/60 2000/60 2600/60 2780/120 HIRES 395-
8006 395-
435535-
585625-
675725-
775450-
800— Table 1. Relevant parameters of the filter wheel used on Clementine
美国航空航天局研制的JWST,其上搭载的MIRI中波红外相机-光谱仪和NIRSpec近红外多目标光谱仪都用到了滤光轮。NIRSpec将滤光轮与光栅轮进行组合使用,其中滤光轮的主要作用是将光波分解为不同组分,再结合光栅轮进行更为精细的光谱分析。图3为NIRSpec所用滤光轮的示意图,该滤光轮覆盖光谱范围为0.6~5 μm,主要由四个边缘滤光片、两个不同谱段的条带滤光片、一个用于捕获目标的透明滤光片以及一个用于在轨校准的反射镜组成。
Figure 3. (a) Schematic diagram of filter wheel used in NIRSpec (b) Auxiliary structure diagram of filter wheel
MIRI也是JWST的主要载荷之一,MIRI主要由成像仪和两个光谱仪SPO、SMO组成,负责在5~28 μm的中红外波段内进行成像及中低分辨率的光谱分析。滤光轮在MIRI中主要起连通成像、光谱的作用。如图4所示,该滤光轮主要分为18个通道,包括十个成像滤光片、四个日冕滤光片、一个中密度滤光片、一个双棱镜、一个透镜、一个与棱镜配重的明暗位置。
Euclid是欧洲航天局目前在研的卫星之一,预计发射至第二个拉格朗日点,该卫星的主要任务是在五年之内完成对整个河外星系暗弱目标的探测,有效载荷主要为一个成像仪器和一个光谱仪器,其中光谱仪器采用由四个滤光片构成的滤光轮进行分光,主要负责近红外波段的探测,其中每个滤光片有8.5°的倾斜,防止在探测器上形成鬼像,图5为该光谱仪中滤光轮的早期设计模型[5]。
Development status and trend of filter hyperspectral camera (Invited)
doi: 10.3788/IRLA20210981
- Received Date: 2021-12-17
- Rev Recd Date: 2022-01-09
- Available Online: 2022-02-10
- Publish Date: 2022-01-31
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Key words:
- remote sensing /
- remote sensing payload /
- hyperspectral camera /
- filter splitting /
- high resolution
Abstract: While imaging the spatial characteristics of the target, the hyperspectral camera, which combine imaging technology with spectral detection technology, can form multiple narrow bands for each spatial pixel to carry out continuous spectral coverage. Spectral information can fully reflect the differences in physical structure and chemical composition of the ground features. Compared with traditional spatial two-dimensional imaging, it can obtain spatial and spectral information of the target at the same time. Under a certain spatial resolution, a wide continuous spectrum can be obtained. The unique continuous characteristic spectrum has outstanding advantages in accurate identification and detection of ground objects. It has become an important cutting-edge technology for remote sensing of the ground. And it is widely used in the investigation of agriculture, forestry, water, soil, minerals resources and environment monitoring. With the rapid development of filter coating technology, it has greatly promoted the development of hyperspectral cameras based on the principle of filter splitting. At present, this type of hyperspectral cameras become an important part of the hyperspectral remote sensing payload and have been widely used in the networking of nano-satellite hyperspectral constellations because of the wide range, high spatial resolution, and high spectrum. Hyperspectral cameras based on the principle of filter splitting were reviewed, and typical hyperspectral imaging payloads at home and abroad and other systems being studied were introduced. The technical solutions, performance indicators and application prospect of these systems were analyzed. The technical characteristics, advantages and disadvantages of hyperspectral cameras based on the principle of filter were explained. Finally, the development trend of filter hyperspectral was prospected.