Infrared reflectance of metallic objects coated with a class of varnishes or absorbing pigments

摘要

The accurate and non-destructive estimation of the properties of coatings on metals via infrared spectroscopy is investigated in the present paper in order to inspect the traceability of the metals. First of all, the class of homogeneous coatings such as varnishes is examined, focusing on the extraction of the electrical properties and thickness through simple spectroscopic measurements. The thickness extraction originates from the multiple interferences that occur when an incident electromagnetic wave is reflected from a finite thickness layer via a procedure known as InfraRed Interference Method. The theoretical analysis is realized via the combination of fundamental electromagnetic and transmission line theories, while several measurements at, the infrared region are conducted on varnishes with different electrical properties and thicknesses atop metals, in order to facilitate the accuracy of the proposed method. Moreover, the analysis is extended towards inhomogeneous materials such as highly absorbable pigments that are utilized to decrease efficiently the radar cross section of aircrafts. The Kubelka-Munk theory is utilized to estimate the scattering and absorbing properties of such materials. Additionally, various measurements and simulations are conducted on samples of different thicknesses to determine the penetration depth of infrared light and estimate the absorbing efficiency.