Comparison of image processing techniques for the on-site evaluation of damaged frescoes

Infrared thermography is a valuable tool for non-destructive evaluation of antique artworks. Active thermographic techniques can be applied on-site thanks to their contactless and non-invasive nature. On-site monitoring is a challenging task. The observed objects are often hard to reach and of unknown thermal and physical properties. Moreover there are usually hard constraints on the availability of the site, in terms of space and time. For these reasons the acquired data are typically inhomogeneous and need to be reorganized and post-processed, with dedicated algorithms, to enhance the analysis. The frescoes of the San Gottardo Church, located in Asolo, in the North-East of Italy, are showing multiple detachments due to the ageing process. More than 60 frescoed surfaces have been selected for evaluation via an active thermography procedure. Each area has been heated with handheld air heaters and a sequence of infrared images of the cooling process has been recorded. Several techniques are available for the post-processing of thermographic sequences. In this work standard algorithms, such as correlated contrast and principal component thermography, are compared with new ones. We propose two new algorithms, the first is based on sum and filtering, the second is an adaptation of the partial least squares method to thermography. The obtained results allow to identify and locate the most important detachments on the surfaces. © 2014 Copyright SPIE.