A new experimental device for infrared spectral directional emissivity measurements in a controlled atmosphere is presented. The sample holder, which permits to measure spectral directional emissivity up to 1050K, is placed inside a stainless steel sample chamber that can be evacuated or filled with different gases. The signal detection is carried out by means of a Fourier transform infrared spectrometer. The experimental results focus on the capability of the device to perform emissivity measurements as a function of temperature, emission angle, and in situ surface state evolution. A careful study of the sample temperature homogeneity and the measurement method has been done, including the background radiation, the apparatus response function, and temperature differences between the sample and the blackbody radiator. As a consequence, a compact expression for the sample emissivity that generalizes those previously obtained for the direct radiometric measurement method is found. The error assessment shows that the main contribution to the emissivity uncertainty is related to the sample temperature. The overall uncertainty at intermediate temperature is estimated to be around 3% at short wavelengths. Emissivity measurements of Armco iron were used in order to check the accuracy of the experimental device. The experimental results show an excellent fit with direct emissivity data accessible in the literature, as well as with the theoretical emissivity obtained from the Hagen-Rubens relation.

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Directional spectral emissivity data in different environments are needed in a great number of scientific and technological applications. In this work, the normal spectral emissivity of Armco iron is studied as a function of temperature under a controlled atmosphere. Emissivity values are calculated by the direct radiometric method. The evolution with thermal cycling, the dependence on temperature, and the effect of surface roughness are considered. Additionally, the electrical resistivity is calculated by using the Hagen-Rubens emissivity relation. This work makes progress in the use of Armco iron as an emissivity reference.

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