Improvement the temperature signal filtering in lock-in thermography

At present, lock-in thermography is widely used non-destructive method for defects detection. The informative images in lock-in thermography (e.g. phasegram) are obtained after temperature signal filtering of the raw data. The postprocessing in lock-in thermography is more complex than in other active thermography methods and very important for defects detectability. In some cases the standard postprocessing can significantly decrease the quality of the temperature signal filtering, respectively decreasing defects detectability, although the parameters of lock-in thermography measurement are selected correctly. The aim of the paper is to study the quality of temperature signal filtering in lock-in thermography depending on used offline postprocessing. For this reason, a methodology based on modelling and measurements of temperature signals from infrared thermography for determination of cases, in which the temperature signal filtering quality is decreased significantly, is used and corresponded methods for correction are proposed. The results from modelling and from real lock-in thermography measurements shows that by using of the proposed methods can be avoided decreasing of temperature signal filtering quality due to improper postprocessing. In addition, the proposed methods allow same defect detectability at lower energy, induced in tested sample, which is very useful for materials that are not sufficiently heat-resistant.