Spectral, directional emittance of stainless steel at elevated temperatures

Stainless steel material samples with different surface roughness values were investigated to determine their spectral directional emissivity at elevated temperatures up to 1200 K in the visible to near-infrared wavelength range and in the infrared around 4 μm. The emissivity measurements were accomplished by measuring the radiation from an appropriately designed test specimen in a furnace heating facility with optical access. Blackbody radiation, needed to determine the emissivity, was generated through a cavity in the specimen itself. During the time of measurement, the sample was shielded from the furnace radiation through a retractable cold radiation shield. In the visible to near-infrared wavelength range, the specimen was imaged on the entrance slit of a 500 mm focal length spectrometer, enabling simultaneous measurement of a sample normal to the viewing direction, the blackbody cavity, and a tilted sample. In the infrared, a FLIR camera was used to image filtered radiation around 4 μm. Surface roughness increased emissivity significantly in comparison to polished samples. The actual roughness values only had minor effects. Preoxidation of the samples caused a significant increase in emissivity for polished surfaces but had only minor effects for rough samples. The measured emissivities show only a weak variation with temperature.