Lock-in Thermography 

LTLock-in Thermography is a technique of active thermography in which the heat is applied to the component in a sinusoidal manner. A thermal wave propagates near the surface of the component. This wave has the same frequency as the heating source, but is delayed in time and hence has a different phase. Once the component has settled down into a steady state, the applied and observed sinusoidal waves can be compared. Images of both the phase difference and the amplitude of the thermal wave can be generated. These images provide different information about the component.

If the thermal wave hits a defect or a disbond it is reflected and interacts with the incident wave, causing an interference pattern in the local surface temperature. Some depth information regarding the defect can be obtained. Lock-in refers to the synchronisation of the input wave and the measured wave to derive the phase difference.

A strong point of LT is the phase image, which is relatively independent of local optical and thermal surface features.

For more information on LT and its application see:

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