The principle of lock-in thermography is based on the application of a periodic input energy wave (i.e. thermal emitter, ultrasound, microwave, eddy current, flash lamp) to the surface of the object being examined and analyzing the resulting local temperatures on the surface of the object.

When the input energy wave penetrates the object's surface, is it absorbed and phase shifted. When the input wave reaches areas within the object where the thermophysical properties are not homogeneous in relation to the surrounding material, (ie. at delaminations or inclusions), the input wave is partially reflected.

The reflected portion of the wave interferes with the incoming input wave at the surface of the object, causing an interference pattern in the local surface temperature, which oscillates at the same frequency as the thermal wave.

The internal structure of the object being examined can then be derived by evaluating the phase shift of the local surface temperatures in relation to the input energy wave. The ability to derive internal thermophysical inconsistencies within the object, however, requires that the input energy source be used at an optimal frequency, which depends on both the thermophysical characteristics of the object as well as its thickness.