Optical reflectance apparatus for moisture content determination in porous media

Monitoring surface moisture content (MC) is a required activity and a topic of current interest in conservation studies. The present work examines the possibility of using the spectral reflectance factor at 970 nm (Rλ970) for the noninvasive quantitative determination of surfaceMC in building materials. Indeed, the quantification of surface liquid water by optical measurements is poorly exploited on porous media despite it is well documented in many remote sensing technologies such as in agriculture, soil science and some industrial application. The measurements apparatus is tested in a laboratory experiment on different specimens of building materials. The custom-made detector is a Si-core Avalanche Photo Diode (APD) operating in Geiger mode which satisfy the requirements of non-invasiveness and ease of use with the additional opportunity to developing economic, automated and reliable solutions. The results obtained for six representative cultural heritage building materials clearly show the dependence between Rλ970 and surface MC: Rλ970 increases as surface MC decrease following the drying process of an unsaturated porous matrix i.e. evaporative flux supported by capillary pumping. This work reports the possibility to use the reflectance value at a specific wavelength for an alternative and non-invasive measurements of MC.