Xiaobing Zheng

Variability of the downwelling diffuse attenuation coefficient with consideration of inelastic scattering

In situ time-series measurements of spectral diffuse downwelling irradiance from the Bermuda Testbed Mooring are presented. Averaged diffuse attenuation coefficients of downwelling irradiance, Kd,and their elastic and inelastic components are investigated at seven wavelengths. At shorter wavelengths (<510 nm), Kd is weakly dependent on the solar zenith angle owing to the prevailing scattering effect and therefore can be considered a quasi-inherent optical property. At longer wavelengths (>510 nm), Kd shows a strong dependence on the solar zenith angle. As depth increases, inelastic scattering plays a greater role for the underwater light field at red wavelengths.

Absolute spectral radiance responsivity calibration of sun photometers.

Sun photometers are designed to measure direct solar irradiance and diffused sky radiance for the purpose of atmospheric parameters characterization. A sun photometer is usually calibrated by using a lamp-illuminated integrating sphere source for its band-averaged radiance responsivity, which normally has an uncertainty of 3%-5% at present. Considering the calibration coefficients may also change with time, a regular high precision calibration is important to maintain data quality. In this paper, a tunable-laser-based facility for spectral radiance responsivity calibration has been developed at the Key Laboratory of Optical Calibration and Characterization, Chinese Academy of Sciences. A reference standard radiance radiometer, calibrated against cryogenic radiometer, is used to determine the radiance from a laser-illuminated integrating sphere source. Spectral radiance responsivity of CIMEL CE318-2 sun photometer is calibrated using this new calibration system with a combined standard uncertainty of about 0.8%. As a validation, the derived band-averaged radiance responsivity are compared to that from a Goddard Space Flight Center lamp-based sphere calibration and good agreements (difference <1.4%) are found from 675 to 1020 nm bands.

Method to intercalibrate sunphotometer constants using an integrating sphere as a light source in the laboratory

A calibration method is introduced to transfer calibration constants from the reference to secondary sunphotometers using a laboratory integrating sphere as a light source, instead of the traditional transferring approach performed at specific calibration sites based on sunlight. The viewing solid angle and spectral response effects of the photometer are taken into account in the transfer, and thus the method can be applied to different types of sunphotometers widely used in the field of atmospheric observation. A laboratory experiment is performed to illustrate this approach for four types of CIMEL CE318 sunphotometers belonging to the aerosol robotic network (AERONET). The laboratory calibration method shows an average difference of 1.4% from the AERONET operational calibration results, while a detailed error analysis suggests that the uncertainty agrees with the estimation and could be further improved. Using this laboratory calibration approach is expected to avoid weather influences and decrease data interruption due to operationally required periodic calibration operations. It also provides a basis for establishing a network including different sunphotometers for worldwide aerosol measurements, based on a single standard calibration reference.

Quantum efficiency calibration of opto-electronic detector by means of correlated photons method

A new calibration method of detectors can be realized by using correlated photons generated in spontaneous parametric down-conversion (SPDC) effect of nonlinear crystal. An absolute calibration system of detector quantum efficiency is performed. And its principle and experimental setup are introduced. A continuous-wave (CW) ultraviolet (351 nm), diode-pumped, frequency-doubled, and solid-state laser is used to pump BBO crystal. The quantum efficiencies of the photomultiplier at 633, 702, and 789 nm are measured respectively. The coincidence peaks are observed using coincidence circuit. Some measurement factors including the filter bandwidth of trigger channel, the detector position alignment and polarization of the pump light are analyzed. The uncertainties of this calibration method are also analyzed, and the relative uncertainties of total calibration are less than 5.8%. The accuracy of this method could be improved in the future.

Absolute spectral radiance responsivity calibration of the radiance transfer standard detector

Radiance transfer standard detector (RTSD) is a trap-based facility used as transfer standard detector for radiance measurements. In this paper, a RTSD with band-center wavelength of 550nm is calibrated and the result of absolute spectral radiance responsivity (ASRR) calibration is presented. The relative uncertainty in the absolute spectral radiance responsivity calibration of the RTSD is less than 8×10-3.

Experimental verification of self-calibration radiometer based on spontaneous parametric downconversion

Based on spontaneous parametric downconversion process, we propose a novel self-calibration radiometer scheme which can self-calibrate the degradation of its own response and ultimately monitor the fluctuation of a target radiation. Monitor results were independent of its degradation and not linked to the primary standard detector scale. The principle and feasibility of the proposed scheme were verified by observing bromine–tungsten lamp. A relative standard deviation of 0.39 % was obtained for stable bromine–tungsten lamp. Results show that the proposed scheme is advanced of its principle. The proposed scheme could make a significant breakthrough in the self-calibration issue on the space platform.

Measurement, Correction and Validation of Out-of-Band Response for Multi-spectral Remote Sensing Instruments

Out-of-band response (out-of-band light leaks) is a significant uncertainty factor in multi-spectral remote sensing instruments radiometric calibration. In this paper, a method using a spectrum-tunable reference light source (STIS) which possess new functions of continuous and discrete spectral coverage to measure, correct and validate the 443 nm band of a multi-spectral camera has been mentioned. Adjusting STIS output two discrete radiance spectrum with its spectral width of about 50 nm in the out-of-band spectral region to directly measure out-of-band radiance responsivity. Correcting out-of-band response via spectral radiance measured by reference spectral radiometer and out-of-band radiance responsivity with the help of out-of-band response correction model. Validating out-of-band response correction accuracy via contrasting the relative departure between band-weighted radiance measurement of 443 nm band with out-of-band response correction and reference spectral radiometer measurement, under the conditions of measuring the test continuous spectrum produced by STIS. The result shows that the relative departure between the band-weighted radiance and the reference is decreased from 8.8 to 3.1% before and after the out-of-band response correction. The results demonstrate that STIS is suitable for measuring, correcting and validating the out-of-band response of multi-spectral remote instruments.

Development of Self-Calibration Spectral Radiometer of Correlated Photons on Orbit

Optical sensor on orbit is usually calibrated by on-board calibrator or some astronomical objects such as moon, sun or other stars in the visible and near infrared band, primary standard and standard transfer train are the most important sources of uncertainty during the process of calibration . Currently many satellite’s calibrators are needed to be installed on orbit. Calibrator can be used to calibrate satellite sensor periodicity, it is helpful to improve the quality of science data products derived from sensor observations, but it’s difficult to evaluate itself change of calibrator, calibration accuracy on orbit is difficult to less than 3%.A new self-calibration spectral radiometer of correlated photons is developed based on spontaneous parametric down-conversion (SPDC) effect, including radiometric observation and self-calibration mode, radiometric observation can be used to measure radiance of optical sensor and transfer to other satellite sensor using cross calibration method. Self calibration mode can calibrate response change of optics and electronic on orbit and modify calibration coefficient of optical sensor periodicity, compared with other calibration method on orbit, correlated photons is absolute and not dependent on primary standard and standard transfer train, only by measuring the ratio of the number of coincidence events to the number of trigger detection, trigger channel optical transmittance, the quantum efficiency of trigger detector is not need to be konwn.Principle of the new spectral radiometer is introduced, optics system of eight channel radiometer is designed, electronic and signal processing is analysed. ICMOS low light sensor is used to observe of space and spectral distribution of correlated photons, some measurement results are introduced in detail, its uncertainty is assessed, combine uncertainty is less than 0.35%. It is hopeful that the self-calibration spectral radiometer will be used in the fields of on-orbit calibration, cross calibration of other satellite sensor, field observation instrument calibration and climate change observation in future, etc.

Error analysis and Stokes parameter measurement of rotating quarter-wave plate polarimeter

In this paper, we present a simple Stokes parameter measurement method for a rotating quarter-wave plate polarimeter. This method is used to construct a model to describe the principle of how the magnitudes of errors influence the deviation of the output light Stokes parameter, on the basis of accuracy analysis of the retardance error of the quarter-wave plate, the misalignment of the analyzing polarizer, and the phase shift of the measured signals, which will help us to determine the magnitudes of these errors and then to acquire the correct results of Stokes parameters. The method is validated by the experiments on left-handed circularly polarized and linear horizontal polarization beams. With the improved method, the maximum measurement deviations of Stokes parameters for these two different polarized states are reduced from 2.72% to 2.68%, and from 3.83% to 1.06% respectively. Our results demonstrate that the proposed method can be used as a promising approach to Stokes parameter measurement for a rotating quarter-wave plate polarimeter.

Development of automated self-calibration spectra-radiometer

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.