Emissivity is a value on a scale of zero to one given to materials based on the ratio of heat emitted compared to a perfect black body. A black body will have an emissivity value of 1 and a perfect reflector will have an emissivity value of 0.
Soda-lime float glass used in windows by nature is highly thermal emissive. To improve its solar insulation properties, a thin filmed silver coating (also known as low-e coating) is applied to the surface of the glass. There are 2 methods of applying this coating; pyrolytic chemical vapor deposition or magnetron sputtering deposition. Depending on region, pyrolytic coats are also referred to as hard coats or on-line coating, whereas sputter coats are also referred to as soft coats or off-line coating. Low-E coatings can also impart colour (tint) to the glass or impart a reflective tint to the glass.
Low-E coats are used in conjunction with monolithic, laminated or insulated glass units to improve on the products solar control performance.
