###### Abstract  
While radiation thermometry is well-developed for laboratory calibrations using high-emissivity sources, the effect of spectral emissivity in real-world conditions, where emissivity ranges from 0 to 1, is usually not considered. Spectral parameters that influence non-contact temperature measurements are often neglected even in laboratory conditions. These parameters become more important with decreasing emissivity and at lower temperatures, leading to increased uncertainty contributions to the measurement result. In this manuscript, we analyze the impact of various influential spectral parameters using the constructed spectral Monte Carlo simulation of radiation thermometry. The investigation covers the influence of spectral and related parameters, namely spectral emissivity, reflection temperature, spectral sensitivity and atmospheric parameters of temperature, relative humidity and distance of the path in the atmosphere. Simulation results are compared to experimental results, overestimating sensitivity to humidity by 23-27% and sensitivity to emissivity and reflected temperature within 10% at given conditions. Multiple cases of radiation thermometer (RT) use are simulated for measurement uncertainty: high temperature RT use as the reference in calibration by comparison, the use of a flat plate calibrator for RT calibration, measurements with a RT using emissivity input data from literature with relatively high uncertainty and temperature measurements with a RT using emissivity data, obtained with FTIR spectroscopy with relatively low uncertainty. Findings suggest that spectral uncertainty contributions are often unjustifiably underestimated and neglected, nearing extended uncertainty contribution of 1.94 \({}^{\circ}\)C in calibration practices using flat plate calibrators with emissivity within 0.93 and 0.97 and 1.72 \({}^{\circ}\)C when radiation thermometers with spectral ranges, susceptible to atmospheric humidity, are used on black bodies.  
28 April 202526  
reflective uncertainties, the increasing number of flat-plate calibration devices with decreased emissivity presents a challenge for calibration laboratories, because it requires an evaluation of the uncertainty of emissivity. Meanwhile, based on our experience, the calculation of the measurement uncertainty in real conditions is rare. In our laboratory, which serves as a national laboratory for thermodynamic temperature and humidity, we maintain a reference value and perform calibrations of radiation thermometers for various industries and institutions. While calibrations are performed on laboratory equipment and the calibration certificate only states the best-case calibration uncertainty for laboratory conditions, this uncertainty does not account for actual real-world conditions, where influence parameters are non-ideal and often poorly estimated or known by the end users. As the uncertainty calculation is complex and users are often not familiar with radiation thermometry, the calibration uncertainty from a calibration certificate is often presumed, even in a real-world use, thus often underestimating the uncertainty of measurement by an order of magnitude.  
Having identified the problem in the perception of radiation thermometers, the authors of this paper strive to investigate the actual uncertainty contributions in a real-world setting. This research focuses on uncertainty contributions, caused by unavoidable influential spectral parameters and immediately related parameters in the process of radiation thermometry. Specifically, the spectral parameters are emissivity and reflectivity, transmissivity of the atmospheric transfer path and spectral sensitivity of the instrument. These parameters are physically further influenced by reflected temperature (or rather hemispherical combination), as well as the distance in atmospheric transfer path, its relative humidity, temperature and atmospheric pressure.