Low size-of-source effect (SSE) infrared optical system design and experimental validation are critically involved. SSE is commonly explored in infrared radiation measurements. The main causes of SSE are the diffraction of the field aperture, the reflection of optical components and objective aberrations. The optical path design and the internal components scattering have an important influence on SSE. Reflective optical system is commonly used in infrared radiation measurements with high temperature region and wide wavelength range, which can eliminate chromatic aberration and reduce coma. A reflective infrared optical system is designed and built based on the high-temperature Fourier transform infrared (FTIR) spectrometer infrared radiation measurement facility at NIM. The ambient scattered radiation and the thermal effect of optical components are controlled via the water-cooled scattered radiation shielding bin and limitation apertures. Experimental validation of the SSE characteristics of the FTIR infrared optical system is carried out via the uniform blackbody radiation source at 500 °C and various sized apertures using the direct measurement method. The corresponding calculation model will be described in the paper. SSE on 3.9 μm is measured via the direct measurement method by using a standard reference blackbody with good temperature uniformity as the radiation source. The effect of reflection is reduced via the high emissivity coating on the apertures. The results show that the effect of the SSE on the FTIR measurement facility at the wavelength of 3.9 μm is less than 2×10-4. Details and results of the infrared optical system SSE measurement will be reported in the paper. All measurements can be traceable to the National Standards of P. R. China.
KEYWORDS: Black bodies, Monte Carlo methods, Infrared radiation, Temperature metrology, Ray tracing, Reflectometry, Integrating spheres, Metrology, Infrared materials, Environmental sensing
Integrated blackbody is a blackbody radiation source with the emissivity approximate to 1, which is coupled of materials with different infrared emissivity as the cavity wall and the cavity bottom. The integrated blackbody theory is the theoretical basis of materials infrared spectral emissivity measurement device at ultra-high temperature developed by National Institute of Metrology, China (NIM). Infrared emissivity is the most important factor for evaluating the infrared radiation characteristics of integrated blackbody. Based on Monte Carlo simulation, infrared emissivity of the integrated blackbody coupled of different high emissivity materials and infrared emissivity of the integrated blackbody coupled of low emissivity material and high emissivity material are calculated theoretically in the near infrared wavelength band at room temperature and high temperature environment. The simulation results show that, the infrared emissivity of the integrated blackbody coupled of different high emissivity materials can reach 0.998 and the infrared emissivity of the integrated blackbody coupled of low emissivity material and high emissivity material can reach 0.988. The above simulation results of integrated blackbody infrared emissivity have reached the requirements of industrial blackbody radiation source. The simulation results are verified based on the method of blackbody infrared emissivity measurement by integrating sphere reflectometer at room temperature environment at wavelength of 0.633 μm. Measurement results on multiple wavelengths show that the deviation between the integrated blackbody infrared emissivity simulation results and the measurement results is less than 0.3 %.
The spectral responsivity of Fourier Transform Infrared Spectrometer (FTIR) measurement system of high temperature blackbody infrared radiation characteristics is calibrated via ThermoGage HT9500 high temperature reference blackbody furnace from National Institute of Metrology, China (NIM). A calculation model of the spectral responsivity calibration of FTIR measurement system is established. The infrared spectrum of the blackbody radiation source is measured in the temperature range from 1273 K and 1973 K on the wavelength range from 1 μm to 14 μm. Calibration is carried out within the temperature range from 1373 K to 1873 K on the wavelength range between 1 μm and 13 μm. The infrared spectral radiation characteristics of ThermoGage HT9500 high temperature reference blackbody furnace are represented. The results indicated that the method of piecewise linear calibration was practicable. The measured infrared spectrum in the temperature range from 1373 K to 1873 K on the wavelength range between 1 μm and 13 μm was compared with the calculation which showed the signal divergence less than 1%. And the calculated temperature obtained by inverse calculation in this temperature region was compared with the actual temperature which showed the temperature divergence less than 0.45%.
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