Emissivity Measurement Under Vacuum from $$4\,\upmu \hbox {m}$$ 4 μ m to $$100\,\upmu \hbox {m}$$ 100 μ m and from $$-40\,^{\circ }\hbox {C}$$ - 40 ∘ C to $$450\,^{\circ }\hbox {C}$$ 450 ∘ C at PTB

Abstract

A new facility of the Physikalisch-Technische Bundesanstalt for emissivity measurements under vacuum was brought into operation, capable of measuring directional spectral emissivity in the wavelength range from $$4\,\upmu \hbox {m}$$ to $$100\,\upmu \hbox {m}$$ and temperature range from $$-40\,^{\circ }\hbox {C}$$ to $$450\,^{\circ }\hbox {C}$$ . The wide spectral and thermal ranges in combination with a target uncertainty of less than 0.01 meet the needs of the solar thermal energy industry and climate research. Emissivities determined in these ranges and at that uncertainty levels would allow significantly improving solar thermal absorber coatings and designing improved reference blackbodies for remote sensing. Here the measurement scheme which is based on the measurement of the spectral radiance of a sample with respect to the spectral radiance of two blackbodies at different temperatures is presented, and the subsequent evaluation of the emissivity is discussed. The first results of emissivity measurements under vacuum in the wavelength range from $$4\,\upmu \hbox {m}$$ to $$100\,\upmu \hbox {m}$$ and temperature range from $$-40\,^{\circ }\hbox {C}$$ to $$450\,^{\circ }\hbox {C}$$ are presented and compared with results obtained in air, and it is shown that samples with very low emissivity can be measured with the required low target uncertainty of less than 0.01.