Low-E coatings are also called Low-Emissivity coatings. Emissivity is defined as the relative power of a surface to emit radiant heat. Low-E coatings are microscopic layers of a metallic oxide, which are used to coat the surface of a window's glass. These coatings allow the window glass to reflect heat energy back to its source. Low-E coatings have been around since the 1980s.