Differential Evolution Fitting-Based Optical Step-Phase Thermography for Micrometer Thickness Measurement of Atmospheric Corrosion Layer

This article introduces differential evolution fitting-based optical step-phase thermography for fast, contactless, and high-accurate measurement of micrometer thickness, which is widely needed in industrial applications. In this process, frequency-domain characteristics of thermal wave propagation are extracted to estimate micrometer thickness. The quantitative relationships between the frequency-domain thermal characteristics and different thicknesses of micrometer structures are precisely discussed. The effect of sampling frequency is emphasized for measuring micrometer thickness and improving the sensitivity of the frequency-domain features as measurement tools of thickness. In particular, differential evolution is introduced to implement adaptive global optimization of transient thermal responses fitting and to eliminate the frequency aliasing in micrometer thickness measurement when the sampling frequency is not satisfactory. Finally, experimental studies have been conducted to compare the effectiveness of principle component analysis, independent component analysis, and the proposed method for quantitative evaluation of atmospheric corrosion. It can be seen that the frequency-domain features imaging is capable of quantitatively characterizing micrometer thickness in a visualized way, and the influences of abnormal surface states and nonuniform heating are completely eliminated.