What is the principle of thermal imaging detection of defects?

The main advantage of thermal imaging methods is that they generate long-range maps of surface temperature distribution, whatever the cause. This principle has been successfully applied to detect subsurface defects that may occur due to any deterioration in the bonding quality of coatings and substrates.

Detection of disbonded areas by resistance to heat flow through the substrate-thin film system

As shown, if there are obstacles such as defects or peeled areas blocking the surface of the specimen, the heat flux from the inside of the specimen towards the surface can make a more or less significant difference in the uniform temperature of the surface. The shape of the temperature drop area is visible on the surface by an infrared sensor. This is a fairly accurate replica of the real defect, which would otherwise be invisible. As the distance between the defect location and the surface decreases, the agreement between the true shape of the disbonded area and its "image" on the surface becomes better. Optically, this situation is similar to shadows formed from distant objects.

As a rule of thumb, the linear dimension of the disbonded area should be at least twice the distance from the interface to the surface. Under these conditions, apparent defect reproduction on the surface can be expected.