Principle of optically excited lockin-thermography

The optical lockin thermography (LIT) method is based on heat flow analysis. It is often referred to as heat flow thermography.

The surface of the object under investigation is heated with halogen lamps. The resulting heat flow into the material is altered by thermal obstacles such as impacts, delaminations or material thickness variations of the object. Thermal signatures form on the surface, which are captured by imaging with an infrared camera. The sensitivity of the method is very high due to the use of the lockin technique. The intensity of the halogen lamps is modulated sinusoidally and the surface temperature is recorded at the same time. A phase and amplitude image is calculated from the measured image sequence by means of a Fourier transformation. In the phase image, interference effects resulting from different emission coefficients or inhomogeneous excitation are significantly reduced. The statement about the interface state and/or the material thickness or the coating thickness is now possible.