Used on most new windows and doors, low-emissivity glass reflects up to 90% of long-wave radiation, which is heat, but lets in short-wave radiation, which is light. Typically applied to the protected inner surface of double or triple-pane glass, the low-e coating is a microscopic, transparent, metallic substance with heat-reflecting ability. Low-e glass helps to limit interior heat gain during hot weather, and limit interior heat loss during the heating season.