Sky‐Directional Radiative Cooling Meta‐Films for Daylighting Windows

ABSTRACT Radiative cooling of vertical surfaces is constrained by the trade‐off between visible transparency and directional thermal emission. Existing asymmetric emitters suppress ground‐directed radiation but are opaque, preventing window integration. Here, we present a transparent asymmetric emitter (TAE) that resolves this limitation via a geometry‐mediated optical design. The TAE employs a microprism array featuring ground‐facing facets with partial silver coverage, enabling decoupled control of visible transmittance and mid‐infrared emissivity. Angle‐resolved measurements show high visible transmittance (75%) and strong anisotropic thermal response, with sky‐facing emissivity of 0.95 and near‐zero ground‐facing absorption. Controlled vacuum experiments demonstrate maximum temperature reductions of 26.4°C. Outdoor experiments using solar‐transmittance‐matched references confirm consistent cooling, with maximum temperature reductions of ∼10°C at ground temperatures of 31.5°C. A global energy model incorporating location‐specific, angle‐dependent optical properties and realistic ground temperatures is developed to capture radiative effects in vertical windows. The model predicts a latitude‐dependent trend, where energy savings are maximized in low‐latitude regions and gradually diminish toward higher latitudes. Notably, the TAE achieves energy performance comparable to the ideal limit of seasonal emissivity‐modulation technologies. As a scalable platform, the TAE establishes a viable route for integrating radiative cooling into windows without compromising daylighting.