Objective  With the development of infrared technology, infrared band detectors are widely used in military, industrial, agricultural, aerospace and other fields.The optical aperture of infrared payloads is increasing to meet the needs of higher resolution applications, and ultra large aperture infrared radiation sources can simulate the targets observed by infrared payloads in orbit.The infrared radiation energy of these targets is very weak. Under the normal temperature environment, the weak infrared target radiation will be submerged by stray radiation and the thermal radiation of the optical system itself, which cannot provide the true radiation for infrared payload detection targets.Therefor, an infrared radiometer is used to test ultra large aperture infrared radiation sources under vacuum and low-temperature environment, and to evaluate radiation/temperature uniformity.In order to accurately evaluate the uniformity of large-area black-body radiation source under vacuum and low temperature, a high-sensitivity mid-infrared radiometer is designed.

  Methods  The physical model of radiation calibration is given, and the signal-to-noise ratio of the target temperature of 200 K and 213 K is analyzed in detail. The signal-to-noise ratio of 200 K and 213 K target under vacuum and low temperature is 460 and 1 492 times respectively. The overall scheme of measuring 200 K target with high signal-to-noise ratio is designed, and the mid-infrared radiometer with high sensitivity under vacuum and low-temperature environment is developed. The transmission optical system and temperature coefficient matching stable high-rigid optical mechanical structure support is adopted to meet the vacuum and low-temperature environmental conditions. Calibration of temperature/radiation responsivity of mid-infrared radiometer with external blackbody, the modulator is innovatively used as the interral calibration radiation source. Using four-stage TE refrigeration mid-infrared detection unit, together with high-performance detection unit and 80 dB dynamic range synchronous integral phase-locked amplifier, the high-quality square wave signal with large duty cycle for radiation calculation is obtained.

  Results and Discussions   A high-precision measurement method for low-temperature targets of mid infrared radiometers in vacuum low-temperature environments is proposed to meet the performance evaluation requirements of large-scale mid infrared radiation sources under vacuum low-temperature conditions,a detailed analysis is conducted on the radiation model for measuring low-temperature targets, the system composition and working principle of the radiometer are introduced, and the optoelectromechanical and thermal control systems are designed in detail.Testing 213 K target blackbody source under the environment of 77 K and 10⁻⁵ Pa, the experimental results indicate that the signal instability within one hour is 0.24%. The NETD value is 0.034 K. The measurement accuracy is better than 2%. Mid-infrared radiometer meets the requirements of high-precision measurement of weak target in vacuum and low-temperature environment.

  Conclusions  Aiming at the requirement of performance evaluation of large-area mid-infrared radiation source under vacuum and low-temperature conditions, a high-precision measurement method of low-temperature target under vacuum and low-temperature conditions is proposed. The radiation model of measuring low-temperature target is analyzed in detail. The signal-to-noise ratio of measuring 200 K low-temperature target is 430 times. The system composition and working principle of mid-infrared radiometer are introduced, and the opto-electro-mechanical and thermal control systems are designed in detail. Finally, the mid-infrared radiometer is tested and evaluated in detail under vacuum and low-temperature environment. The test results show that the mid-infrared radiometer can be used for high-precision and high signal-to-noise ratio test of cryogenic targets in vacuum and cryogenic environment