## 1 Introduction  
The emissivity of a material is an important physical quantity that describes its thermal radiation ability. It reflects the ratio of the radiation ability of the material's surface per unit area to that of a blackbody under the same conditions and temperature [1]. With the growing demand for extensive measurements of material emissivity across various industries represented by military and defense technologies, particularly in fields such as satellite remote sensing and deep space exploration, there is an urgent need to obtain reliable emissivity data for materials under vacuum and low-temperature conditions. The research on the measurement of normal emissivity of materials under vacuum and low-temperature conditions has significant importance and application value [2, 3]. Based on different testing principles, the methods for measuring emissivity can be classified into calorimetric methods [4], reflective methods [5], energy methods [6], and multi-wavelength methods [7]. Currently, the energy comparison method is generally used for measurement [8]. This involves comparing the radiation power of the sample with that of a blackbody radiation source under the same measurement conditions and temperature [9]. In the research progress of measurement methods, from 2000 to 2004, the National Institute of Standards and Technology (NIST) developed a measurement device based on FTIR spectrometers, primarily aimed at measuring transparent materials [10, 11, 12]. In 2008, [PERSON] from Harbin Institute of Technology designed a measurement device for emissivity in a space environment simulation chamber, used to test space materials, achieving a maximum uncertainty of less than 2% [13]. In the same year, [PERSON] et al. proposed a method for directly measuring the optical properties of different materials at low temperatures ranging from 20 K to 200 K, achieving an emissivity error of approximately 2% at 60 K [14]. In 2011, [PERSON], [PERSON] et al. developed an experimental device for measuring the normal spectral emissivity at different temperatures. The temperatures of the samples and blackbodies were heated by the same heating device. Using this device, three steel samples having different percentages of constituents were measured, and the analysis showed that the measurement uncertainty was approximately 5% [15]. In 2015, [PERSON] et al. utilized a new vacuum emissivity measurement facility to achieve an uncertainty of 1% for samples with very low emissivity at PTB (Physikalisch-Technische Bundesanstalt), in Germany [16]. In 2016, [PERSON] et al. developed a measurement device for material emissivity under low-temperature conditions, utilizing a vacuum liquid nitrogen channel to provide a cooling environment. They measured the normal emissivity of copper oxide and high-emissivity ceramic samples as a function of temperature and wavelength, obtaining results with an uncertainty of less than 6% [17]. In the same year, 2016, [PERSON], [PERSON] et al. conducted an interlaboratory comparison of infrared spectral emissivity scales among National Measurement Institutes (NMIs) in the United States, Germany, France, Italy, and Japan. They found that 98% of the data points were consistent, with uncertainties of less than 5% [18]. In 2020, [PERSON] and his team developed a novel device for measuring the apparent emissivity under controlled atmosphere, conducting measurements within the temperature range of 973 to 1423 K and the spectral range of 0.85 to 1.1 \(\upmu\)m. The measurement results indicated that the combined standard uncertainty of the device was below 0.0590 (\(k=2\); \(k\) is the coverage factor) across the entire temperature range [19]. In 2021, [PERSON] et al. developed a measurement device based on a fiber optic spectrometer, evaluating the combined uncertainty of emissivity measurements to be less than 3% [20]. In 2022, [PERSON], [PERSON] et al. developed a device capable of achieving spectral directional emissivity measurements under controlled environmental conditions.