A Transient Method for Total Emissivity Determination

A transient method for determining the hemispherical total emissivity of solids is investigated using an emissometer recently developed at the NPL. The emissivity is calculated from measurement of the sample surface temperature coupled with a knowledge of its bulk thermal properties. This was conducted as part of the current work to validate the new NPL apparatus for high temperature emissivity measurements. A theoretical study shows that when a thermally thick sample is allowed to radiate instantaneously into a cold environment, then the resulting transient surface temperature depends solely on its hemispherical total emissivity and effusivity. This approach is used to obtain a hemispherical total emissivity value for Fecralloy steel, and it is then compared with the normal total emissivity value obtained by iIntegration of normal spectral emissivity measurements in the wavelength range 2 to 9 mm.