The vacuum and thermal environment of airless planetary surfaces, particularly those dominated by a particulate regolith such as the Moon and asteroids, produces intense near-surface thermal gradients that can substantially alter their thermal emissivity spectra when compared with spectra collected at ambient terrestrial conditions. Therefore, spectroscopic measurements acquired under conditions designed to simulate the radiation environment in which remote measurements of airless bodies are made should be used as the basis for interpreting those data. As a foundation for this goal, we report the radiometric calibration of thermal infrared emission data collected with a Fourier transform infrared spectrometer integrated with the custom Asteroid and Lunar Environment Chamber (ALEC) at Brown University. This chamber is designed to simulate the environment of airless planetary bodies by evacuating the atmospheric gasses to vacuum (<10-4 mbar), cooling the chamber with a flow of liquid nitrogen, heating the base and sides of samples with temperature-controlled sample cups, and heating the top of samples with an external light source. We present a new method for deriving sample emissivity based on the absolute radiometry properties of our system, focusing on the 400-2000 cm-1 wavenumber range. This method produces calibrated radiance spectra from calibration targets, and particulate samples and those spectra are used to derive emissivity spectra. We demonstrate that the ALEC system and data reduction methods successfully replicate independently determined spectral properties of particulate samples under both ambient and cold, vacuum conditions. The ALEC system is shown to be capable of supporting ongoing and future planetary exploration of airless surfaces by facilitating careful investigation of meteorites, lunar samples, and planetary materials at an array of environmental conditions.