Low-e stands for low emissivity. Microscopically thin, nearly invisible, metal or metallic oxide layers are coated to the outside of the interior pane (thus located in the interstitial cavity) of a multi-pane Insulating Glass Unit (IGU) to reduce the heat conductance by minimising radiative heat flow. A low-e coating is usually transparent to the solar spectrum (visible light and short-wave infra-red radiation) and reflective of long-wave infra-red radiation. In triple glazed IGUs there sometimes are two low-e layers.