A New Standard for Calibration of High Temperature Emissivity: Laboratory Intercalibration at PEL of DLR and ALEC of Brown University

Abstract

Emissivity or emittance is a non di-rectly measurable characteristic of each material. There are several methods to derive it, using a direct or an indirect formula for retrieval. Whichever method is used, a reference body (often called a “blackbody” in the thermal infrared range) needs to be measured. Many good, black paints are available on the market, most have peak emissivity values around 0.99 with little variation across the infrared spectral range (1 to 100 μm) we usually observe. Unfortunately, these paints are chemical products that can not withstand temperatures around 600 K for more than few minutes. At the Planetary Emissivity Laboratory (PEL) of the German Aerospace Center (DLR) in Berlin, we have spent the last few years investigating materials that could act as a good “blackbody” under challeging conditions. We need a reference body that: (1) has a very high emissivity, (2) has an emissivity spectrum nearly flat across the 1 to 100 μm spectral region, (3) does not outgas and its emissivity spectrum does not change in shape when heated in vacuum above 700K. Such high temperatures are needed to simulate the Mercury surface conditions and to get a good signal close to 1 μm, where Venus’ atmosphere is transparent to the planet’s emitted radiation. The only material fulfilling these criteria is blast furnace slag, a residue from metal production. We present in this paper a joint calibration/evaluation campaign between the PEL and RELAB laboratories to define the spectral characteris-tics of 2 blast furnace slags.