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Passive daytime radiative cooling (PDRC) is an emerging, electricity-free cooling approach that can achieve sub-ambient temperatures by emitting thermal radiation through the atmospheric transparency window (8–13 μm). It therefore offers a promising route to mitigate the growing demand for cooling across a broad range of applications. As the number of reported PDRC materials and demonstrations increases rapidly, robust and broadly comparable figures of merit (FoMs) and testing protocols are increasingly needed to assess performance and to translate laboratory results to diverse outdoor conditions. However, commonly used FoMs and experimental methods are often reported inadequately or without key experimental information, which hampers reproducible benchmarking and cross-study comparison. This perspective critically reviews the main classes of spectral and thermally derived FoMs. In doing so, it discusses numerical and experimental testing approaches by highlighting recurring limitations that drive discrepancies between studies. Building on this analysis, we propose a metrological framework for the classification and comparison of PDRC materials and provide a minimum reporting checklist to enable efficient inter-laboratory comparison and more reliable performance assessment.
Werle et al. (Tue,) studied this question.