Effective emissivity ratio in the long-wave infrared region for evaluating selective band radiative cooling performance

This study introduces the Effective Emissivity Ratio (EER) as an innovative metric for evaluating and predicting radiative cooling (RC) performance by analyzing long-wave infrared emissivity profiles, particularly within the atmospheric window (8–13 μm). Combining theoretical modelling and empirical validation, the research investigates how selective band emissivity patterns of RC materials affect cooling capabilities under different atmospheric conditions. By analyzing historical meteorological data from Ningbo, China, the minimum emissivity requirements for achieving sub-ambient cooling were established. Results show that achieving net cooling requires materials with high reflectivity (above 80%) and optimized emissivity distributions both inside and outside the atmospheric window. Experimental validation across different seasons demonstrated sub-ambient temperature drops of up to 2°C in summer, 6°C during transitional periods, and 8°C in winter, closely aligning with theoretical predictions. A strong linear correlation was observed between theoretical EER values and measured temperature reductions, confirming the reliability of the proposed metric. This work advances radiative cooling technology by providing a region-specific approach to material emissivity optimization, supporting the development of more targeted and climate-adaptive RC materials. Abbreviations: EER: Effective Emissivity Ratio; RC: Radiative cooling; SRI: Solar Reflectance Index.