In current thermographic NDE practice, the detection limits of various methods are typically undefined, beyond citing the deepest flat bottom hole that has been successfully detected in a particular material. Little distinction is made between the ability of a thermographic system to detect the presence of a subsurface defect, to measure its physical properties, or to resolve adjacent defects. Although many practitioners rely on a `rule of thumb', which states that the aspect ratio (diameter/depth) of a detectable defect must be greater than 1, it has been shown to be unreliable as a basis for determining the feasibility of a particular inspection. It is possible to characterize the performance of a thermographic system using a simple procedure, so that the ability of the system to detect, resolve, or measure defects in a particular material or sample type can be successfully modeled. The intrinsic detection limits derived from this approach effectively defines a thermal point spread function that defines the minimum detectable defect size at a particular depth. This information can be used to remove blurring due to lateral heat flow, and extend the depth at which resolution is possible.
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