Radiative Cooling Emitter Designer

Emitting Heat to the Cold Sky

A radiative cooling emitter is a thermal-infrared emitter on a reflective silver mirror. Silicon-dioxide and silicon-nitride layers radiate strongly in the 8–13 µm atmospheric transparency window, dumping heat to the cold sky, while the silver mirror reflects sunlight so the surface can cool below ambient even in daylight. This tutorial requires mid-infrared optical constants (Kischkat 2012).

Kirchhoff's Law and the Phonon Band

By Kirchhoff's law a surface's spectral emissivity equals its absorptivity, so a layer that absorbs in the 8–13 µm window also emits there. SiO₂ and Si₃N₄ have strong phonon resonances in exactly this band, giving high emissivity, while the opaque silver back-mirror reflects the solar spectrum to minimize daytime heating.

On an opaque mirror transmission is zero, so emissivity is just 1 − R. The phonon resonances near 9 and 12 µm drive the absorptance up inside the sky window. More or thicker layers broaden and strengthen the emission band.

Interactive design tool

This page includes a live transfer-matrix calculator for this design. Adjust the parameters to recompute spectra, layer stack, and performance figures in the browser, then open the design in the full calculator.