Image Definition and Defect Sizing in Lock-in Thermography: An Experimental Investigation

In any imaging modality the apparent size and shape of the object in the image plane is influenced by the camera parameters and experimental conditions. To extract an accurate measurement of object size from the image, the understanding of the influence of these parameters on the apparent size and shape is important. Lock-in Thermography is one of the advanced imaging Non Destructive Evaluation techniques which is widely used for quantitative defect evaluation and material characterization. The important experimental parameter in Lock-in Thermography is the excitation frequency, which greatly affects both the apparent shape (image definition) and size in resultant phase and amplitude images. Hence, in this study, an attempt is made towards understanding the effect of frequency on the image definition (apparent shape) and defect sizing in Lock-in Thermography through experimental study. Stainless Steel plate with flat bottom holes of various sizes and depths is considered for the experiment. Both phase and amplitude images are considered for analysis. The Canny edge detection technique is implemented for studying the effect of frequency on image definition by extracting the edges of the flat bottom hole. The binary image obtained after edge detection is used for sizing calculation. The apparent shape and size were greatly affected by excitation frequency and depth in both amplitude and phase images. A relation between excitation frequency and depth was established which can be used to predict the excitation frequency where the sizing measurement is accurate.