Scattering‐Enhanced Light Extraction for Radiative Thermal Load Mitigation in Fluorescent Films

To mitigate solar heating in colored objects, fluorescent coloration has been proposed as an alternative to traditional absorptive pigments. However, the Stokes-shifted photons generated by fluorophores predominantly remain trapped by total internal reflection (TIR), increasing the parasitic solar absorption and the radiative thermal load. This work introduces a scattering-enhanced light extraction strategy that overcomes the TIR limit in fluorescent films. A sequential quadratic programming-driven optimization model establishes the theoretical minimum radiative thermal load for both traditional and fluorescent-colored surfaces. Results reveal that while traditional absorption-based color achieves only 19.3% sub-ambient cooling chromaticity in the CIE 1931 color space, light extraction technology expands the range from 26.7% to 64.9% for fluorescent color. TiO2 nanoparticles enhance light extraction through multiple Mie-scattering, with Monte Carlo ray-tracing simulation identifying an optimal 0.5 wt% TiO2 nanoparticle concentration yielding 85.9% light extraction efficiency, significantly outperforming the TiO2-free fluorescent film (25.3%) and a higher 15 wt.% concentration (66.6%). Outdoor experiments confirm the optimal 0.5 wt% sample exhibits a 4.1 °C temperature decrease compared to the control (0 wt.%) sample. This approach offers cost-effective scalability advantages over microtexture-based light extraction methods.