Research of imaging spectrometer is of great significance to improve the applicability and accuracy of the alternative calibration of remote sensor in orbit. According to the requirements of the imaging spectrometer, the structure of the imaging spectrometer is presented by combining the off-axis three-trans front optical system and the beam splitting system based on the convex grating. The design of the optical system of the imaging spectrometer with compact structure, small volume, high spectral resolution and good imaging quality is realized by the combination of the two, which meets the design requirements of the imaging spectrometer optical system. The operating band of the optical system covers 400 ~ 1000nm, the F-number is 3.3, the field of view is 11°. In order to verify whether the imaging spectrometer meets the design specifications, it is necessary to carry out laboratory assembly and testing of the instrument. Due to the influence of processing technology, there will be some processing errors in the processing of mechanical structures and optical components. In order to make the installation of each component as close to the ideal position as possible, reduce the influence of optical path offset on the optical system, and enable the detector to accurately detect the spectral information of the target, it is necessary to install and debug the spectrometer system to ensure the measurement accuracy of the instrument. According to the test results, the installation state of the optical slit and off-axis triple reaction system is continuously optimized until the best state is achieved. This research provides the technical support for the development of imaging spectrometer, and lays a foundation for the on-orbit radiometric calibration of remote sensor based on imaging spectrometer.
Spectral calibration accuracy of solar irradiance spectrometer plays an important role in the measurement of the absolute solar spectral irradiance. The solar irradiance spectrometer uses fèry prism to disperse composite light and linear CCD to control the scan of spectrum. The solar irradiance spectrometer has a spectral range of 380 nm ~ 2500 nm. In order to ensure the spectral response efficiency of different bands, three detectors are used to achieve spectral detection of different bands respectively. Single wavelength lasers distributed in different bands are used as the light source in the spectral calibration experiment, and the relationship between wavelength and CCD pixel is obtained through spectral scanning. The actual equivalent angle of fèry prism and spectral calibration equation are calculated. In this paper, two methods of calculating spectral calibration equation are studied. In the experiment, method one uses two characteristic wavelength points of visible near-infrared band to calculate the actual equivalent angle, which is taken as the common prism angle of the whole band. Method two uses the same two characteristic wavelength points as method one in the visible near-infrared band, and uses another characteristic wavelength point in the visible and short-wave infrared bands, respectively. Experimental analysis and calculation show that the pixel deviation of method two is 7.44 pixels and 23.78 pixels less than that of method one in visible and short-wave infrared bands, respectively. Therefore, calculating the actual equivalent angle of the prism in different wavebands can improve the accuracy of wavelength calibration.
The principle of self-developed hyperspectral reflectometer is introduced. The instrument has long-term automatic observation capability, and its spectral range covers the visible-short-wave infrared band. The functions of instrument include ground spectral reflectance observation, sky diffuse irradiance, total irradiance, diffuse total ratio measurement, atmospheric optical thickness measurement and long-term automatic observation. Among these the observation of the spectral reflectance of the ground is the core function of the instrument. According to the principle of ratiometric radiation measurement, the uncertainty of the reflectance measurement of the instrument depends on the uncertainty of irradiance calibration and radiance calibration. In order to verify the measurement accuracy of the instrument, the uncertainty of the instrument irradiance calibration and radiance calibration is quantitatively analyzed, and the uncertainty of the hyperspectral field reflectometer is calculated to be less than 1.74%, which meets the technical index requirements of less than 2%.
With regard to the visible and near-infrared hyperspectral irradiance-meter developed by ourselves, the theories and methods of spectral calibration are discussed.The visible and near-infrared hyperspectral irradiance-meter is composed of three optical modules: the visible spectral module(400-1000nm)(VIS), the near-infrared spectral module(900- 1700nm)(NIR), the short-wave-infrared spectral module(1600-2500nm)(SWIR).The detection units use flat-field concave grating to diffract and focus different wavelengths, use the linear array detector to detect signals.For the NIR spectral module(900-1700nm), two spectral calibration methods are adopted: argon lamp calibration and wavelength standard panel calibration.The theories of the two spectral calibration methods are expounded, and the corresponding calibration steps are designed.When the argon lamp or the wavelength standard panel is used in the laboratory to calibrate the NIR spectral module(900-1700nm), the correspondence between the response and the pixel of linear array detector is obtained through diffracting and focusing different wavelengths by the flat-field concave grating. Depending on the characteristic spectrum of the argon lamp or the wavelength standard panel, the spectral calibration equation in the range of 900-1700nm (the NIR spectral module) is derived through data polynomial fitting. The calibration results show that the fitting errors of the two methods are less than 0.45nm and 1 nm respectively, and the spectral calibration uncertainties are better than 0.5nm and 1.2 nm respectively.The two calibration methods verify that the design of the instrument’s spectral module is rational, and provide a meaningful reference for various spectral calibration methods of the near-infrared spectral module.
USB 3.0 specification was published in 2008. With the development of technology, USB 3.0 is becoming popular. LVDS(Low Voltage Differential Signaling) to USB 3.0 Adapter connects the communication port of spectrometer device and the USB 3.0 port of a computer, and converts the output of an LVDS spectrometer device data to USB. In order to adapt to the changing and developing of technology, LVDS to USB3.0 Adapter was designed and developed based on LVDS to USB2.0 Adapter. The CYUSB3014, a new generation of USB bus interface chip produced by Cypress and conforming to USB3.0 communication protocol, utilizes GPIF-II (GPIF, general programmable interface) to connect the FPGA and increases effective communication speed to 2Gbps. Therefore, the adapter, based on USB3.0 technology, is able to connect more spectrometers to single computer and provides technical basis for the development of the higher speed industrial camera. This article describes the design and development process of the LVDS to USB3.0 adapter.
In order to realize unmanned vicarious calibration, Automated Test-site Radiometer (ATR) was developed for surface reflectance measurements. ATR samples the spectrum from 400nm-1600 nm with 8 interference filters coupled with silicon and InGaAs detectors. The field of view each channel is 10 ° with parallel optical axis. One SWIR channel lies in the center and the other seven VNIR channels are on the circle of 4.8cm diameters which guarantee each channel to view nearly the same section of ground. The optical head as a whole is temperature controlled utilizing a TE cooler for greater stability and lower noise. ATR is powered by a solar panel and transmit its data through a BDS (China’s BeiDou Navigation Satellite System) terminator for long-term measurements without personnel in site. ATR deployed in Dunhuang test site with ground field about 30-cm-diameter area for multi-spectral reflectance measurements. Other instruments at the site include a Cimel sunphotometer and a diffuser-to-globe irradiance meter for atmosphere observations. The methodology for band-averaged reflectance retrieval and hyperspectral reflectance fitting process are described. Then the hyperspectral reflectance and atmospheric parameters are put into 6s code to predict TOA radiance which compare with MODIS radiance.
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