Spatial resolution optimization of a cooling-down thermal imaging method to reveal hidden academic frescoes

Abstract

A safe cooling-down infrared thermography technique was considered to reveal paintings covered by a limewash layer for cultural heritage investigations. The transient infrared image sequence permits to reconstitute the subsurface pattern by processing the thermal contrast. Given the interest in detection methods, we propose a theoretical optimization study of the spatial resolution of the method. This analysis, achieved with thermal simulations, is based on both the thermal sensibility and a specific formal spatial resolution criterion. Considering a bar pattern painting covered by a 0.5 mm thick limewash, we have shown a thermal sensibility up to 1.5 K. This value is about 75 times bigger than the thermal noise threshold of the used camera. The proposed spatial resolution criterion allows to establish optimal temporal settings giving the sharpest thermal contrast image. An academic hidden painting sample was especially made to validate this theoretical approach. Results show the suitability of the method to perfectly reveal hidden patterns and confirm the spatial resolution estimations predicted using the proposed theoretical model. Simulated and experimental thermal contrast profiles are in good agreement.