Development of a Co-aligned Multiwavelength OCT System for the Analysis of Cultural Heritage Materials

Over the past two decades, Optical Coherence Tomography (OCT) have become an indispensable, non-invasive technique for examining cultural heritage materials (1). Still, commercial OCT systems have mostly been developed for biomedical applications and operate at central wavelengths from 850 nm to 1550nm, achieving micrometre-scale resolution and penetration depths of 1–2 mm in biological tissues (2). In highly scattering media such as paintings, longer wavelengths are preferred because they increase penetration depth; however, achieving similar resolution at these wavelengths requires much larger bandwidths. This compromise is fundamental to OCT system design. For instance, the optimal spectral window for imaging the subsurface structure of paintings, localising underdrawings, and characterising most historic artists’ pigments is approximately 2.2 μm (3). OCT systems operating at 1300-1500 nm provide a compromise between surface resolution for characterising varnishes and penetration depth (2,4). This project develops a multi-wavelength OCT system to image transparent and opaque materials at different depths and scales with high resolution, using two NIR wavelength bands. Mock-up samples have shown that this setup enables fast, non-invasive analysis of artistic materials while addressing challenges in analysing artefacts with mixed material compositions.