Dual mode imaging in mid infrared with thermal signal reconstruction for innovative diagnostics of the \u201cMonocromo\u201d by Leonardo da Vinci

Claudia Daffara,Dario Ambrosini,Paola Ilaria Mariotti,Simone Parisotto

doi:10.1038/s41598-021-01837-8

Abstract

Dual mode imaging in the mid infrared band, a joint use of thermography and quasi-thermal reflectography, was recently proposed as a full field diagnostic tool in cultural heritage. Here we discuss for the first time, to the best of our knowledge, a detailed application of such non destructive technique to the diagnostics of frescoes, with an emphasis on the location of detachments. We also investigate the use of a thermographic method based on TSR (thermal signal reconstruction), in a long pulse stimulus scheme, as well as the spatial registration of thermal images after post-processing analysis to their visible counterpart, so as to obtain a fine resolution diagnostic map. As an exemplar case study, we report about the application of dual mode imaging with a 500 {upmu }hbox {m} pixel size at object plane on the “Monocromo”, a fresco by Leonardo da Vinci located in the Sforza Castle (Milan, Italy). Our technique was used to guide the conservators during the restoration works, opening new perspectives in artwork diagnostics.

Highlights

Dual mode imaging in the mid infrared band, a joint use of thermography and quasi-thermal reflectography, was recently proposed as a full field diagnostic tool in cultural heritage
Thermography in the LWIR (Long Wavelength InfraRed) region from 8 μm to 12 μm is very effective in the detection of in-depth alteration of the wall support[7], but it is less suitable for some specific problems that require a sharp imaging; the most crucial one is the analysis of the detached areas of the pictorial paint and plaster layers[8]
A recognized critical issue is that a fine resolution map requires to perform the mosaicking and the spatial registration of the thermal image dataset to a reference visible image, which is difficult or in some cases not possible due to the lack of reference points in the thermal signatures, both in the LWIR and MWIR wavelengths

Summary

Introduction

Dual mode imaging in the mid infrared band, a joint use of thermography and quasi-thermal reflectography, was recently proposed as a full field diagnostic tool in cultural heritage. It has been shown that the use of the thermal MWIR region allows the design of a dual mode imaging approach[11], in which a two-step acquisition is performed in the reflective and the emissive domains and where the two datasets can be jointly analyzed in order to obtain information from both the surface and the subsurface of the painting. The proof of concept of frescoes diagnostics by MWIR dual mode imaging method, coupled to a variant of thermal signal reconstruction (TSR)[18], is shown on an exemplar case study, the Leonardo’s “Monocromo” in the “Sala delle Asse” of the Sforza Castle (Milan, Italy)

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