Active Thermography

Abstract

In active thermography, heat is directed into a test object in order to detect inhomogeneities and cavities. When a test object is heated or cooled, temperature differences at the surface are caused due to local variations in the thermal conductivities and heat capacities of the test specimen. These are recorded and evaluated using a thermography system. The most important methods used in this process are pulse thermography, pulsed phase thermography, and lock-in thermography.

This paper first describes the basic measurement set-ups and the functional principles. Then possibilities of how to heat the test specimens are explained. External energy sources, such as radiation, convection, and heat conduction, heat the surface. With internal energy sources such as ultrasonic excitation, microwaves and inductive stimulation, in contrast, the heating takes place in the volume of the test specimen. If the specimen is excited with a short pulse, then this is referred to as pulse thermography. A special evaluation of the pulse response is pulsed phase thermography. If the excitation ensues in a sinusoidal form, then this is known as lock-in thermography.