Opaque, transparent, and colored low-emissivity materials for mid-infrared thermal management

Mid-infrared (MIR) low-emissivity (εMIR) materials are central to radiative thermal management, enabling non-contact heat regulation in aerospace systems, infrared camouflage, and energy-efficient building envelopes. Although significant advances have been made in reducing emissivity through compositional optimization and structural engineering, most studies remain confined to conventional material classifications and focus narrowly on emissivity metrics. Crucially, the optical properties in the visible range, especially coloration which governs camouflage and functional aesthetics, as well as transparency, have been largely overlooked despite their relevance to multifunctionality. This review addresses gap by establishing a structure–property–optics framework classifying low εMIR materials into three categories based on visible-light behavior: opaque, transparent, and colored. Each category is analyzed in terms of microstructural design, spectral regulation strategies, and application requirements. Emphasis is placed on interplay between electromagnetic responses and structural features such as surface topology, photonic resonance, and interfacial coupling. Advanced approaches for MIR thermal regulation are discussed, including radiative cooling, selective heating, and tunable emissivity, with attention to mechanisms such as bandgap modulation, phase transition, and localized mode control. The review concludes by outlining challenges in broadband control, structural durability, scalable fabrication, and proposing future directions toward multifunctional, optically adaptive, thermally efficient low εMIR systems.