Amplitude sensitive modulation-thermography to measure moisture in building materials: Fid-Bau Portal

Amplitude sensitive modulation-thermography to measure moisture in building materials

Wild, W. / Büscher, K.A. / Wiggenhauser, H.

There have been reports about moisture detection in building walls by reflective IR-thermography. Typically, only limited results could be obtained because of the emission coefficient variations, leaking radiation or inhomogeneous illumination of the object. In addition, the quantitative relation between remission spectra and the moisture has often been unclear. Reflectography uses constant excitation illumination which is recorded by the infrared camera. With the use of the 'Lock-In-Technology' a low frequency modulated signal of an infrared radiation source is coupled with the thermo camera and a frequency and phase sensitive signal from the thermal images can be derived. The advantage is, that emission coefficient dependencies are eliminated and that leaking radiation does not have any influence on the measured signal. The selective water measurement is possible, because there is an interference filter mounted in front of the radiator which has its transmission maximum at the wavelength of an absorption band of water. The area investigated is therefore illuminated under well defined circumstances and quantitative moisture measurement on the surface of building materials becomes a possibility. The illumation modulation is done with a sine wave to facilitate the calculation of the temporal intensity behaviour of the amplitude signal. Subsequently, the amplitude image is used to determine the distribution and the level of moisture quantitatively. Point measurements in the laboratory were carried out on several building materials with changing moisture levels. It could be shown that this method successfully eliminates disturbing contributions to the measured signal like surface effects or leaking radiation.