Classification and application of spectral imaging technology
Spectral imaging technology is a new type of multi-dimensional information acquisition technology that combines imaging technology and spectral technology. It can detect and obtain a data cube composed of two-dimensional spatial information and one-dimensional spectral information of the measured target. After data processing, different ground objects can be obtained. spectral curve.
Spectral Imaging Technology Classification
Spectral imaging technology originated in the 1980s, and its predecessor was multispectral remote sensing imaging technology. Because spectral imaging has good information acquisition capabilities, spectral imaging technology has developed rapidly. A variety of spectral imaging technologies have been developed, and imaging spectrometer products are constantly updated.
There are various classification standards for spectral imaging technology. According to different light splitting methods, it can be divided into dispersive and interference type spectral imaging technologies. Both dispersive spectral imaging technology and interference spectral imaging technology obtain the two-dimensional spatial information and one-dimensional spectral information of the target through push sweep or swing sweep, which require high stability of the platform and are obtained in the same exposure. Spectral information for all spectral bands. For spectral imaging solutions that use filters, whether multiple filters are used to obtain image information at multiple wavelengths in parallel, or filters are switched sequentially, the appropriate exposure time needs to be set according to the spectral response of the system. thereby obtaining the maximum signal-to-noise ratio.
1) Dispersive imaging spectrometer
·Dispersive prism spectroscopy technology
The dispersive prism is the most commonly used and simplest spectroscopic element in spectral imaging. The picture above is a typical application of a dispersive prism in a spectral imager. As shown in the figure, the incident slit is located on the front focal plane of the collimating system. After the incident light passes through the collimating system, the slit is imaged on the focal plane detector by the imaging system through the prism according to the wavelength.
·Diffraction grating spectroscopy technology
The application method of diffraction grating is the same as that of dispersion prism. The incident slit is located on the front focal plane of the collimation system. After the incident light passes through the collimation system, the slit is imaged on the focal plane detector according to the wavelength through the grating.
Another use of diffraction grating is to place it in a divergent beam. The light incident from the slit is directly incident on the grating without a collimation system. After diffraction by the grating, a spectral virtual image of the target slit can be obtained. The imaging system will Slit-by-wavelength imaging is performed at different positions of the area array detector. This imaging technology has been applied to the conceptual design of the tactical remote sensor of the OrbView-4 satellite.
At present, the relatively mature dispersive spectrometers used in airborne and aerospace applications in the world are all based on diffraction gratings, such as AVIRIS of the Jet Propulsion Laboratory in the United States, CASI of Canada, AISA of Finland, and instruments and equipment such as the spectroradiometer MODIS. .
·Binary spectroscopic element spectroscopy technology
The binary optical element is both a dispersion element and an imaging element. A monochromatic area array detector is used to scan the selected band imaging range along the optical axis. Each position corresponds to the imaging area of the corresponding wavelength. Binary optical elements converge the incident light just like ordinary lenses, but they are based on the principle of diffraction. The effective focal length of the chromatic aberration caused by diffraction is inversely proportional to the wavelength.
Unlike prisms or grating elements that disperse along the direction perpendicular to the optical axis, binary optical elements dispersion along the axis. The spectral resolution of an imaging spectrometer using binary optical elements is determined by the size of the detector. This structural imaging spectrometer has a compact structure and high diffraction efficiency.
·Acousto-optic tunable filter spectroscopy technology
Acousto-optic tunable filter (AOTF) is a new type of dispersion element, which consists of three parts: acousto-optic medium, transducer array and acoustic terminal. According to the principle of acousto-optic diffraction, when complex-colored light is incident on an acousto-optic medium at a specific angle, due to the interaction between acousto-optic and acousto-optic, the incident light that meets the momentum matching conditions is diffracted by ultrasonic waves into two orthogonal monochromatic lights, each located at the zero order. Light both sides. Changing the frequency of the radio frequency signal changes the wavelength of the diffracted light accordingly. Continuously and rapidly changing the frequency of the radio frequency signal can achieve rapid spectral scanning within the wavelength range of diffracted light.
2) Interference imaging spectrometer
Since the spectral resolution of the dispersive imaging spectrometer is inversely proportional to the width of the incident slit, to obtain higher spectral resolution, the width of the slit must be continuously reduced, so that the luminous flux of the system is reduced, resulting in low detection sensitivity. As the technical requirements for imaging spectrometers increase, especially in terms of spatial resolution, spectral resolution, and weak signal detection capabilities, dispersive imaging spectrometers are gradually unable to meet the requirements. Interference imaging spectrometers have the advantages of high spectral resolution and high energy utilization in principle, and can meet the ever-increasing application requirements, and have gradually become a research hotspot in the field of imaging spectroscopy technology.
The main spectroscopic techniques of interference imaging spectrometers include Michelson interference, triangular common optical path interference, birefringence interference, etc. In recent years, technology has been developed that uses liquid crystal tunable filters to obtain polarized light and then interfere. In addition to the above two-beam interference technology, there is also spectroscopy technology based on multi-beam interference.
3) Filter type imaging spectrometer
Filter-type imaging spectrometers add filters as spectroscopic elements in the optical path, and obtain different spectral channels by replacing the filters. Filter-type spectral imaging changes the center wavelength through electrical tuning. The wavelength is adjusted once and the camera is exposed once. The system records the two-dimensional image information of this band and then sets the next transmission wavelength. Loop in sequence until the image collection tasks of all wavelengths are completed and the final spectral data cube is obtained.
Applications of Spectral Imaging Technology
Spectral imaging technology is a perfect combination of spectral analysis technology and image analysis technology. It has both spectral resolution and image resolution capabilities. It can conduct qualitative, quantitative and positioning analysis of the measured object. Using the spectral differences of the surface components of the object, the target can be achieved The precise identification and positioning has wide applications in the fields of material identification, remote sensing detection, medical diagnosis and other fields.
The development of spectral imaging technology has gone through three stages: multispectral, hyperspectral, and hyperspectral imaging. Precisely because the imaging spectrometer can obtain multi-band image data with a very narrow band width, it is mostly used for spectral analysis and identification of ground objects. As the spectral resolution continues to improve, the target spectral information obtained becomes more refined, and it is increasingly used in fields such as military, agriculture, medicine, resource exploration, and geological surveys.
In terms of military use, because the imaging spectrometer has the ability to distinguish types of ground objects in the spectrum, it shows strong advantages in fine classification of ground objects, target detection and change detection, and is called an important battlefield reconnaissance method. Spectral images can distinguish real targets and camouflaged targets on a natural grass background, and quickly detect small tactical targets on a desert background.
In terms of civilian use, spectral imaging originated from the identification research of geological mineral resources, especially the detection of special minerals such as the detection of mineralized altered rocks, and gradually expanded to vegetation ecology, ocean and coastal water color surveys, water body detection, ice and snow, soil and Atmospheric research. Spectral images with fine resolution have the characteristics of integrating maps and spectra, and can accurately detect the spectral information of vegetation growth characteristics. They can even identify and estimate the chlorophyll or suspended matter content in waters, and detect chemical pollution of water quality. Fine spectral imaging has become a hot research topic at home and abroad. Scholars use fine spectral imaging technology to quantitatively conduct detection and research on material mechanisms on a more microscopic scale.
Review Editor: Huang Fei
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