Optical characterization of growing thin films at high temperatures by analysis of near infrared emissivity variations using CCD thermal imaging

Abstract

Due to interference effects which occur during the growth of surface thin films, the resulting spectral emissivity varies continuously. It is shown in this paper, that these temporal variations of emissivity allow the determination of the complex index of refraction (i.e. optical constants n, k) of a thin absorbing film, without explicit knowledge of the optical constants of the substrate layer. In particular, this work presents results obtained by the analysis of the thermal radiation of a thin thermally grown iron-oxide film on a heated steel specimen (at approx. 400°C) measured by a standard CCD camera with a near infrared band-pass filter (~1μm).

Experimental Determination of the Emissivity of Powder Layers and Bulk Material in Laser Powder Bed Fusion Using Infrared Thermography and Thermocouples

“…According to Zauner et al [31], the change of emissivity of steel surfaces due to the growth of an oxide layer reached a first maximum above ε > 0.8 for an oxide film thickness of about 100 nm.…”

“…These results were based on theoretical calculations. The fluctuations stem from interference phenomena of reflectance at oxide films at metallic surfaces which depended on oxide film thickness [31].…”