Optical coherence tomography and non-linear microscopy for paintings – a study of the complementary capabilities and laser degradation effects

Abstract

This paper examines for the first time the potential complementary imaging capabilities of Optical coherence tomography (OCT) and non-linear microscopy (NLM) for multi-modal 3D examination of paintings following the successful application of OCT to the in situ, non-invasive examination of varnish and paint stratigraphy of historic paintings and the promising initial studies of NLM of varnish samples. OCT provides image contrast through the optical scattering and absorption properties of materials, while NLM provides molecular information through multi-photon fluorescence and higher harmonics generation (second and third harmonic generation). OCT is well-established in the in situ non-invasive imaging of the stratigraphy of varnish and paint layers. While NLM examination of transparent samples such as fresh varnish and some transparent paints showed promising results, the ultimate use of NLM on paintings is limited owing to the laser degradation effects caused by the high peak intensity of the laser source necessary for the generation of nonlinear phenomena. The high intensity normally employed in NLM is found to be damaging to all non-transparent painting materials from slightly scattering degraded varnish to slightly absorbing paint at the wavelength of the laser excitation source. The results of this paper are potentially applicable to a wide range of materials given the diversity of the materials encountered in paintings (e.g. minerals, plants, insects, oil, egg, synthetic and natural varnish).

Non-Linear Microscopy: A Well-Established Technique for Biological Applications towards Serving as a Diagnostic Tool for in situ Cultural Heritage Studies

“…Specifically, NLM and OCT techniques have been combined for the study of model paintings [77]. It has been proved that non-linear imaging techniques can act synergistically with optical coherence tomography measurements providing complementary diagnostic capabilities for the precise in-depth characterization of various model painted artworks.…”

“…It has been demonstrated that OCT is capable of providing information from deeper layers of a model painted artwork while it cannot distinguish between two types of equally transparent varnishes. On the other hand, the MPEF modality presents limited penetration depths, while it is capable of discriminating different transparent varnishes based on the detected fluorescence intensity variation of the samples [77].…”

Laser cleaning of paintings: in situ optimization of operative parameters through non-invasive assessment by optical coherence tomography (OCT), reflection FT-IR spectroscopy and laser induced fluorescence spectroscopy (LIF)

“…It is a Fourier domain OCT instrument utilising near infrared (770-970 nm) radiation and a spectrographic detection. With the power of a probing beam at the object below 0.8 mW and an exposition of a given spot of the object to the radiation for 45 μs only, and thus fluence of about 30 mJ/cm 2 , it is considered safe, even for sensitive materials [26].…”

An assessment of multimodal imaging of subsurface text in mummy cartonnage using surrogate papyrus phantoms

“…It compares near infrared light that has scattered back from different depths in a sample to a reference signal in an interferometric approach which can provide micrometre resolution to a depth of approximately 1-2 mm in tissue. It is becoming increasingly common in heritage applications [21], particularly when looking for underdrawings [22,23].…”