Revealing inscriptions obscured by time on an early-modern lead funerary cross using terahertz multispectral imaging

Abstract

The presence of a corrosion layer on lead art and archæological objects can severely impede the interpretation of inscriptions, thus hampering our overall understanding of the object and its context. While the oxidation of lead that dominates corrosion may be chemically reversible via reduction, potentially providing some access to inscriptions otherwise obscured by time, corrosion damage is overall neither entirely reversible nor is the reduction process in all cases easy or feasible to carry out. In this study, by taking advantage of the unique penetration ability of terahertz radiation and the abundant frequency bands covered by a single-cycle terahertz pulse, we perform nondestructive terahertz multispectral imaging to look under the corrosion on a sixteenth century lead funerary cross (croix d'absolution) from Remiremont in Lorraine, France. The multispectral images obtained from various terahertz frequency bands are fed into a judiciously designed post-processing chain for image restoration and enhancement, thus allowing us for the first time to read obscured inscriptions that might have otherwise been lost. Our approach, which brings together in a new way the THz properties of the constituent materials and advanced signal- and image-processing techniques, opens up new perspectives for multi-resolution analysis at terahertz frequencies as a technique in archæometry and will ultimately provide unprecedented information for digital acquisition and documentation, character extraction, classification, and recognition in archæological studies.

Figure 1

(a) Photograph of the Pb cross under investigation in this study. The dimensions of the cross are $\sim$11.5 cm by $\sim$11.5 cm with a thickness of $\sim$2 mm. The whitish appearance is due to corrosion. The presence of a text is suggested by incised features in the whitish areas with several characters readable where the corrosion is less well-formed. This is the condition in which the cross arrived in our laboratory. The horizontal arm of the cross is selected as the region of interest for THz imaging. (b) The THz reference signal in our system, which is recorded by setting a metal plate at the sample position. (c) The power spectrum of the reference THz pulse. The full spectrum of a broadband THz pulse is divided into six frequency bands with a bandwidth of $\sim$0.5 THz. THz multispectral images are obtained based on the frequency components in each frequency band.

Figure 2

(a) The THz reflected signal acquired at pixel (250, 190). The THz reflected signal mainly contains two echoes corresponding to the reflections from the air/corrosion and corrosion/Pb interfaces. (b) Deconvolved signal after performing the FWDD algorithm. The two positive peaks indicate the location of the interface, and the optical delay between the two peaks corresponds to the corrosion thickness. (c) The distribution of the corrosion thickness across the Pb cross.

Figure 3

THz multispectral images obtained based on the components in frequency bands within (a) 0.1 and 0.5 THz, (b) 0.5 THz and 1.0 THz, (c) 1.0 THz and 1.5 THz, (d) 1.5 THz and 2.0 THz, (e) 2.0 THz and 2.5 THz, (f) 2.5 THz and 3.0 THz. Features associated with the inscriptions start to appear in the frequency bands above 1 THz, and images from the low frequency bands mainly reflect the surface morphology of the cross. All the images are normalized to their maximum pixel values.

Figure 4

The output images from each step of the post-processing chain. (a) The input image $R_{\mathit{in}}(x,y)$. (b) The image after intensity correction. (c) The image after inpainting. (d) The image after blind deconvolution. (e) The image after contrast enhancement. All the images are normalized to their maximum pixel values.

Figure 5

Comparison between the optical and THz images regarding the visualization of inscriptions. (a) Photograph of the original Pb cross. (b) Final THz image of the Pb cross after post-processing. (c) Photograph of the Pb cross after chemically removal of the corrosion. Typical regions are highlighted, in which inscriptions are revealed in the THz image but are barely visible in the optical image before removing the corrosion. As can be read from the THz image (b) and confirmed after removal of corrosion (c), the inscriptions in the top right boxes are ‘tuum fiat voluntas tua’ and part of ‘quotidianum’; the ones in the bottom left boxes are a portion of ‘imittimus’ and ‘tentationem’, respectively.