A new transient temperature emissometer

Abstract

An emissometer is described which measures the temperature dependent total hemispherical emittance of flat samples coated with a selective surface. The construction of the instrument is such that heat losses occur only through radiation and then only from the surface under investigation. The emittances of several selective surfaces are measured and compared with other results on the same, or similar surfaces. These are measured in both other calorimeters and in instruments using reflectance methods. It is found that different calorimetric methods give close agreement, as do different reflectance methods. However, reflectance results tend to be about one third lower than those obtained through calorimetry. This seems to indicate that normal emittance and hemispherical emittance should not be equated for selective surfaces. There also appears to be a marked difference in the temperature dependence of emittance, depending on whether the measurements are made in a calorimeter or calculated from theoretical black body shifts applied to spectral reflectance curves.