Accurate spectral irradiance measurement is very important in the field of Earth observation. In the ideal condition, the measurement of spectral irradiance is expected to follow the cosine law of angular response, but the cosine diffuser does not exhibit the ideal cosine response in the actual measurement, the cosine error will affect the measurement result of the instrument. In order to understand the effect of cosine diffuser on the performance of the irradiance senor, a spectral irradiance experimental measurement system was established. The cosine errors of three spectrometers were measured. Two different methods, the traditional and offsetting angle response normalization method, were used to analyze the experimental results, respectively. The experimental results showed that cosine response characteristic was affected by many factors such as diffuser material and size, the adaptability of different angle response normalization method was different. Three different sensors were involved in this experiment: HOC spectral irradiance sensor, EVO spectrometer, and Fieldspec Spectroradiometer. The results indicated that the traditional normalization method is more suitable for HOC and EVO spectral irradiance sensors, and the former presents good cosine characteristics, but the latter is obviously wavelength dependent by the influence of element reflection and diffraction. Compared with conventional normalization method, the Fieldspec Spectroradiometer showed good cosine response characteristics using the offset method. When θ is in the range of 0~60°, the cosine error of thissensor is reduced from 11% to 6%, and it is almost independent of wavelength change.
Ultraviolet radiant exposure meter, also known as UV energy meter, is widely used in a multitude of fields, such as sterilization, climate change, solar photovoltaic, material aging, medical health, UV curing, lithography and so on. Ultraviolet exposure radiation meter is a commonly used instrument to measure ultraviolet radiation. Due to the particularity of the structure and the complexity of the influencing factors, the measurement error of commercial instruments is very high. Commonly used ultraviolet light sources include mercury lamps, LED light sources, metal halogen lamps and so on. This paper will study the calibration method of the ultraviolet exposure radiation meter, and evaluate the measurement uncertainty.
UV radiometers are used in many areas. There are many kinds of UV light sources with different peak wavelength and different wavelength range. The broadband UV radiometers are wildly used due to easy to use and low cost. However, there are some obvious disadvantages for the broadband radiometers. They cannot distinguish the spectral characteristics of UV sources. That will cause the spectral mismatch measurement error for the UV broadband radiometers calibration. Recently, the fiber spectroradiometer plays a more and more important role in this area. The fiber spectroradiometer is more portable and low cost compared to the double grating spectroradiometer. We can obtain the spectral characteristics and any UV irradiance using the fiber spectroradiometer. However, for most fiber spectroradiometers, we cannot use them to replace the UV broadband radiometers for the absolute irradiance measurement. There are four key effects for that. The first one is the stray light. Stray light effect is obvious for the fiber spectroradiometer, especially in the UV wavelength range. The second one is the temperature effect. The third one is the non-linearity effect. The fourth one is the bandwidth effect. This effect will cause the measurement error for the spectral distribution of the UV source. In this paper, we research the four factors that reduce the measurement accuracy of the fiber spectroradiometer in UV wavelength range.
In order to achieve the goal of spectral radiance (SR) and spectral irradiance (SI) calibration with an uncertainty of less than 1.0%, NIM set up a MC-C large-area high-temperature fixed point blackbody(HTFP BB) as the new generation of SR and SI reference source, which is composed of large WC-C fixed point cell and BB3500MP. It can be used directly for the SR and SI realization, thereby further reducing measurement uncertainty. The HTFP BB with WC-C fixed point has excellent stability, reproducibility and repeatability in the high temperature range of more than 3020K, which greatly improves the detection capability in the ultraviolet range (UV). Accurate measurement of melting temperature of HTFP BB is an important source of error, which is determined as the point of inflection (POI) of melting plateau. At the same time, POI is also an important reference point used in comparison experiments. Different calculation methods introduce different degrees of errors, which are critical. This paper studied the current three POI numerical calculation methods, namely “differential+second-order fitting” (DSF), “third-order direct fitting” (TDF), “histogram+Gaussian fitting” (HGF). The numerical calculation of the POI was performed on the data of WC-C14 and Re-C measured at NIM and WC-C14 and WC-C10 measured at VNIIOFI by the above three methods. Combining the fitting results and correlation coefficients to explore the characteristics of each method. Based on the experimental results, a more reasonable calculation method is proposed to reduce the calculation error of the POI of the previous data to less than 5 mK.
With the highly accurate calibration requirements of ocean remote sensing, it is crucial to provide long-term in-situ measurement and validation for on-orbit remote sensors by using sea-based validation sites. However, when the laboratory-calibrated spectroradiometer is transmitted to in-situ validation sites, the measurement results are greatly deviated due to the difference between the field and the calibration environment, which directly affects the accuracy of on-orbit remote sensors synchronous calibration. In this paper, both of above water measurement and undersea profile measurement are considered. The temperature effects and stray light characteristics of common spectrometers were studied. In addition, the characteristics of the classic calculation model and optimization calculation model of immersion factor are compared and analyzed.
In photometry and radiometry, photodetectors such as silicon detector and PMT detector are widely used. In precision metrology, the uncertainty of the nonlinearity should be considered. Superposition method is used to analyse the linearity. The silicon trap detector is measured using both nonmochromator light and monochromator light. First, integrating sphere with broadband light is used to test the linearity. The result shows that the nonlinearity is (1-3)×10-4 from 1uA to 1mA. The monchromator light result shows that the nonlinearity is below 3×10-4 from 1uA to 1mA, which is consistent with the integrating sphere method. For the PMT detector, the linearity is measured only using monochromator light. Experiment shows that the nonlinearity is less than 1×10-3 through three orders of magnitude.
Stray light due to the array spectroradiometer characteristic can’t be ignored in the ultraviolet region. In order to obtain a true spectral power distribution, stray light correction must be considered. Array spectraradiometer covering 200nm- 460nm is investigated using lasers and filters. First, several lasers are measured using the array spectroradiometer. Due to the fact that the wavelengths of the lasers are beyond the capabilities of the spectroradiometer, the response in the UV region is originated from stray light. Results show that the stray light contribution is at the level around 2×10-5. In order to correct the stray light, filters with different bandpass wavelength are used to correct the stray light from different wavelength region. Results show stray light consistency using lasers and filters.
CCD based array spectrometers are widely applied in remote sensing, earth observation, and other industries. However, the signals of ultraviolet region are very weak. Thus, the stray light is one of the most important factors on accurate measurements. In this work, the in-range stray light of commercial UV/VIS CCD array spectrometer and VIS/NIR CCD spectrometer were corrected by mathematical correction method. The measured stray light value at any pixel is of the order of 10−3 ~10−5 of the true in-range. A reduction of the stray light effect by 1-2 orders of magnitude can be achieved using a correction matrix based on line-spread functions (LSFs), which can be determined with the help of spectrally tunable lasers. On the other hand, the bandwidth of the commercial CCD array spectrometer was corrected due to the increasing needs for high accurate calibration and measurement of spectral radiometry. The correction outcome is in good agreement with the measured results by monochromator spectroradiometer.
CCD array based spectrometers are widely used in radiometric measurements. Ambient temperature and nonlinearity effects are significant factors for high accuracy measurement in the field. Here, a temperature correction method for the CCD array spectrometers was developed, which calculated the spectrometer response at each pixel. The deviation between measured and calculated spectrometer responses at a randomly selected temperature is less than 1%. In addition, the radiant power nonlinearity effects were investigated by supplementary-light methods. The gain settings nonlinearity effect was evaluated using FEL-type transfer standard lamps. The nonlinearity correction coefficients were calculated and analyzed based on the experiment, respectively.
Deuterium lamp is used as the transfer standard of air-UV spectral irradiance (200nm to 400nm). The CCPRK1. b comparison of spectral irradiance 200nm to 350nm took deuterium lamp as transfer standard lamp. Spectral irradiance is measured by a spectroradiometer with finite bandwidth. The bandwidth can cause measurement error. In order to correct the measurement error, we apply SS and DO bandwidth correction methods to the spectrum of Deuterium lamp. We obtain the correction effect preliminarily.
In 2011, new primary standard apparatus of spectral radiance was setup at Changping campus of NIM based on high temperature blackbody BB3500M and double-grating monochromator of M207D. The temperature of the BB3500M was measured by a LP4 thermometer with uncertainty of 0.64 K at the temperature of 2980 K, which was calibrated by the Pt-C and Re-C fixed point blackbodies, and checked by a WC-C fixed point blackbody. The consistency of the temperature at 3021 K was better than 70 mK between NIM and VNIIOFI. The image of the measuring source was focused on the entrance slit of the monochromator with magnification 1:1. A mask was put in front of the entrance slit to limit the target spot size of the tungsten strip and the water-cooled aperture was 0.6 mm wide by 0.8 mm tall rectangle. The solid angle of spectral radiance measurement was approximately 0.008 sr. Uncertainty of spectral radiance scale realization was analyzed in this paper. The source of the uncertainty scale includes repeatability of the signal ratio of the blackbody and the transfer lamp, lamp alignment, temperature measurement of HTBB, non-uniformity of HTBB source, instability of HTBB source, correction of different size of source (BB and lamp), nonlinearity of the measurement system, current passed through the transfer lamp, wavelength error, polarization effects, bandwidth etc. The measurement uncertainty (k=2) of spectral radiance was 1.8 % at 250 nm, 0.90 % at 400 nm, 0.64 % at 800 nm, and 1.3 % at 2500 nm respectively.
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