Heat flow thermography

With heat flow thermography, you can see into objects to a limited extent. In this way, z. B. Determine layer thicknesses or detect imperfections under the surface. In contrast to thermography, heat flow thermography does not evaluate static thermal images, but rather analyzes the dynamics of the heat distribution after a pulse-shaped excitation. The suggestion can e.g. B. consist of an infrared flash.

If the lightning strikes a surface, depending on its optical properties, the material is initially only heated near the surface. The introduced heat is then distributed as a heat 'wave' in the depth, whereby the surface cools down again. The speed depends on the specific heat capacity and the thermal conductivity of the material. If there are defects under the surface, this process is locally disturbed and can be perceived as a temperature difference with a fast, sensitive infrared camera. The depth of the defect can be determined via the time interval between the heat pulse and the detection of the temperature difference. The possible depth depends on the lateral extent of the defect.

A classic signal processing technique with periodically modulated excitation is the lock-in technique. It can be used to detect temperature fluctuations down to the µK range.

literature

• Michael Sackewitz (Ed.): Guide to Heat Flow Thermography (Volume 12). Fraunhofer-Verlag Stuttgart, 2011, ISBN 978-3-8396-0234-8