Last update: 10.06.2019
Thermal insulation
4.

Emissivity

According to EN 12898, emissivity is the ratio of the energy emitted by a given surface at a given temperature to that of a perfect emitter (i.e. a black body with normal and corrected emissivity of 1,0) at the same temperature.

According to EN 12898, emissivity is the ratio of the energy emitted by a given surface at a given temperature to that of a perfect emitter (i.e. a black body with normal and corrected emissivity of 1,0) at the same temperature. 
In practice, the corrected emissivity is used for describing the radiation exchange between glass surfaces facing each other in multiple glazing and glass surfaces facing a room.

The normal emissivity εn is determined by measuring the normal spectral reflectance Rn with photospectrometers in 30 steps within the range of long wave infra-red (IR) radiation of λ = 5 and 50 µm according to the following equation:

Emissivity


 

 

The normal emissivity is given by:  εn = 1 - Rn

In order to determine the corrected emissivity ε for float glass and coated float glass – used for calculating the Ug value of glazing – the normal emissivity εn is multiplied with a ratio given in the EN 12898 standard. 


The corrected emissivity of float glass or float glass with a coating that has no effect on the emissivity is 0.837 (EN 673).