Active Thermography

Active thermography is the induction of a heat flow by energetically exciting a test object. This can be done either by applying energy in a pulsed form (pulsed thermography) or in a harmonic modulated way (lock-in thermography). The propagation of the heat flow inside the object directly affects the temporal behaviour of the surface temperature. By recording the surface temperature with an infrared camera and by applying an appropriate mathematical analysis, an image is calculated, which shows the internal structure of the part with the defects. Active thermography offers different inspection methods as well as a variety of measuring techniques. So the measurement procedure can be optimally adapted to different materials and parts with different structural properties. The inspection methods mainly differ in the type of excitation source used, in the way the thermal energy is applied and in the mathematical analysis.

With the pulse thermography method, a short energy pulse is applied to the measuring object. High-energy flash systems are most commonly used. Typical applications are the inspection of thin-walled components or the characterization of thin layers. Main advantages of this technique are the short inspection times (Less Than 1 second, up to a few seconds), and the ability to determine the depth of defects.

With lock-in thermography the surface of the test object will be excited with periodical harmonic-modulated energy. Typical excitation sources are, for example, normal halogen lamps or hot-air blowers. The depth range can be set through the modulation frequency of the excitation source, in order to obtain the best possible result. Compared with the transient method, measuring times are considerably longer.