Low emissivity

Low emissivity (low e or low thermal emissivity) refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect and emit radiant energy according to Planck's Law but here, the primary concern is a special wavelength interval of radiant energy, namely thermal radiation of materials. In common use, especially building application, the temperature range of approximately -40 to +80 degrees Celsius is the focus, but in aerospace and industrial process engineering, much broader ranges are of practical concern.

Emissivity is the value given to materials based on the ratio of heat emitted compared to a perfect blackbody, on a scale from zero to one. A blackbody would have an emissivity of 1 and a perfect reflector would have a value of 0.

Kirchhoff's law of thermal radiation states that absorption equals emissivity opaque for every specific wavelength/frequency (materials often have quite different emissivities at different wavelengths). Therefore, if asphalt has an emissivity value of 0.90 at a specific wavelength (say wavelength of 10 micrometer, or room temperature thermal radiation), its thermal absorptance value would also be 0.90. This means that it absorbs and emits 90 percent of radiant thermal energy. As it is an opaque material, the remaining 10 percent must be reflected. Conversely, a low-e material such as aluminum foil has a thermal emissivity/absorptance value of 0.03 and as an opaque material the thermal reflectance value must be 1.0 - 0.03 =0.97, meaning it reflects 97 percent of radiant thermal energy. Low-emissivity building materials include window glass manufactured with metal-oxide coatings as well as housewrap materials, reflective thermal insulations and other forms of radiant thermal barriers.

The thermal emissivity of various surfaces is listed in the following table.

Window glass is by nature highly thermally emissive, as indicated in the table above. To improve thermal control (insulation and solar optical properties) thin film coatings are applied to the raw soda-lime glass. There are two primary methods in use: pyrolytic CVD and magnetron sputtering. The first involves deposition of fluorinated tin oxide (SnO2:F see Tin dioxide uses) at high temperatures. Pyrolytic coatings are usually applied at the float glass plant when the glass is manufactured. The second involves depositing thin silver layers with antireflection layers. Magnetron sputtering uses large vacuum chambers with multiple deposition chambers depositing 5 to 10 or more layers in succession. Silver-based films are environmentally unstable and must be enclosed in insulated glazing or an Insulated Glass Unit (IGU) to maintain their properties over time. Specially designed coatings may be applied to one or more surfaces of insulated glass. One type of coating (low-e coatings) reduces the emission of radiant infrared energy, thus tending to keep heat on the side of the glass where it originated, while letting visible light pass. This results in glazing with better control of energy - heat originating from indoors in winter remains inside (the warm side), while heat during summer does not emit from the exterior, keeping it cooler inside.

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