Field Programmable Gate Array (FPGA) based Tapped Delay Line (TDL)- Time-to-Digital Converters (TDC) are widely used in photoelectric measurement due to flexible design and low cost. These types of TDCs require calibration to quantify the inconsistent bin width of the delay line for better accuracy. This paper proposes a code density method based on Geiger-Mode Avalanche Photodiode (GM-APD) avalanche signals for calibration. The paper analyzes the Poisson distribution of GM-APD’s avalanche signals. Then, using an experiment to verify that the proposed method is convenient and effective for TDL-TDC’s calibration, reduces the Differential Nonlinearity (DNL) and Integral Nonlinearity (INL) in measurement.
The space radiometric benchmark enables in-orbit traceable to SI system, calibrates in-orbit loads, improves data consistency across different space radiometric remote sensing missions, and facilitates long-term high-precision monitoring of global climate and environment. The transferring detector becomes an absolute power detector by quantum optical radiation measurement in the self-calibration mode, and is used to measure solar radiation in the observation mode. Si photodiode is used as transferring detector in the visible band, which requires high-precision radiometric detection at the photon level. Therefore, Si photodiode and its amplifying circuit need to have low noise. To expand the detection dynamic range of Si photodiode under low radiation condition, the noise sources of Si photodiode and its amplifier circuit are analyzed, and cooling of Si photodiode is proposed to reduce the noise. Vacuum cryogenic experiment was conducted to study the temperature characteristics of Si photodiode at 223K to 263K, and the temperature control design requirements were obtained. A sealed package Si photodiode based on two-stages TEC was developed, and a high-precision temperature control circuit with -40 °C ± 0.01 °C was designed. The shunt resistance of the cooled Si photodiode at -40°C was tested to be 8.5 TΩ, and the dark current was 1.2 fA. A wide dynamic range low-light-level irradiance source with adjustable radiation intensity is used to test the cooled Si photodiode with a low-noise transimpedance amplifier, the measurement signal-to-noise ratio was 1158 when the photocurrent was about 1pA.
To solve the problem that target recognition in infrared detection blindness, a DoFP infrared polarimetric imagery is designed. The top of the infrared detector pixel, based on a domestically made uncooled long-wave infrared focal plane array detector, integrates a micro-polarizer array with different polarization directions. Then, according to the infrared intensity image, infrared DoLP image, and infrared AoP image of the infrared blind spot scene, which have different physical properties and imaging characteristics, a multi-polarization parameter infrared polarization fusion method is integrated at the information processing end of the detector, and the fusion criterion of polarization color mapping is adopted. The test results show that the system can output the infrared intensity, DoLP, AoP and infrared polarization fusion enhanced image of the target scene in real time, and can effectively detect the target in the infrared blind area. The study has great potential for camouflage recognition and polarization remote sensing.
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.
Test site vicarious calibration provides an absolute radiometric calibration for sensors. Surface reflectance is a critical parameter to be measured during a vicarious calibration field campaign. In order to realize long-term high precision observations of surface spectral reflectance in solar reflective bands, Automated Self-Calibration Spectra-Radiometer (ASCSR) was developed. ASCSR measures the global irradiance and the ground reflected radiance respectively with high spectral resolution from 400nm-2400 nm, the ratio of the two measurements is the surface reflectance. The degradation influences of instrument sensors and optical elements are removed by ratio-measurements and self-calibration. In the past two years ASCSR deployed in Dunhuang test site for continuous spectral reflectance measurements over 4 weeks. The measurements result of ASCSR is compared with traditional measurements which used SVC spectra-radiometer.
A kind of novel calibration source with dual output modes, namely, narrow-band and broadband, was designed. The optical system of the source is refractive, in spectrometer-like optical configurations using a prism as the dispersion device. The Digital Micromirror Device (DMD) is used as the spatial light modulator, which locates at the focal plane of the dispersion unit. The dispersive wavelengths are located at the active area of DMD, every column of the DMD corresponds to a different wavelength and the rows of each DMD column correspond to the intensity of that wavelength. With the modulation of the DMD, it can produce narrow-band/monochromatic output like a monochromator by switching the corresponding columns on, and broadband output by switching several different columns on. The source’s operating band spans 450~2250nm, consisting of two independent parts which span 450~1000nm and 1000~2250nm, respectively. The narrow-band bandwidths spans 5~28nm for VIS-NIR and 20~40nm for SWIR subsystems. Several broadband target spectra, including sea water, plants and sun, were simulated by this source through spectral simulation algorithm. The source’s radiometric metrics are suitable to be traced to the absolute cryogenic radiometer (ACR), the most accurate optical power standard at present, which is helpful to improve the calibration accuracy for remote sensors at the beginning. The capability of simulating target spectra will reduce the calibration uncertainties caused by the spectral mismatch between calibration sources and targets viewed by the remote sensors. Based on the considerations above, the source is very appropriate and applicable for remote sensor’s calibration.
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