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.

在任何成像模式中，图像平面中物体的表观大小和形状都受相机参数和实验条件的影响。为了从图像中提取物体尺寸的准确测量值，了解这些参数对表观尺寸和形状的影响很重要。锁定热成像是一种先进的成像无损评估技术，广泛用于定量缺陷评估和材料表征。锁相热成像中的重要实验参数是激发频率，它极大地影响了所得相位和幅度图像的表观形状（图像清晰度）和尺寸。因此，在本研究中，尝试通过实验研究了解频率对锁定热成像中的图像定义（表观形状）和缺陷尺寸的影响。实验考虑了具有各种尺寸和深度的平底孔的不锈钢板。相位和幅度图像都被考虑用于分析。Canny 边缘检测技术用于通过提取平底孔的边缘来研究频率对图像清晰度的影响。边缘检测后得到的二值图像用于大小计算。在振幅和相位图像中，明显的形状和大小受激发频率和深度的影响很大。