Processing and analysis of THz time-domain spectroscopy imaging applied to cultural heritage

This contribution discusses the improvement approaches of Terahertz Time-Domain Spectroscopy (THz-TDS) to cultural heritage objects. The technique is gathering attention in the conservation field for its ability to retrieve information on material composition and surface topography in the far-infrared region (3-300 cm−1 ), but it can pose major challenges when applied on works of art. THz-TDS generates hyperspectral images that are corrupted by frequency-dependent blur and noise and are distorted by surface warping. These degradation effects can make the analysis of heterogeneous materials rather complicated because they substantially limit the frequency range where THz images have good spatial resolution and hinder the identification of materials. This work tackles these inherent limitations of the THz-TDS imaging and shows how to improve the analysis by computationally restoring the hyperspectral data. The experiments were undertaken using a bench-top THz-TDS instrument equipped with an advanced laser design and a precise mechanical delay stage that can achieve a peak dynamic range of 100 dB and a bandwidth of more than 6 THz. The exploitable region of frequencies is limited, however, between 0.2 and 3.5 THz because of the size of the THz beam waist and noise present at high frequencies. Such limitations were addressed by adopting a twofold computational strategy that involves (i) the removal of the surface warping and (ii) the application a fast deblurring-denoising algorithm for image restoration tailored to THz images. Experiments undertaken on an archaeological coin demonstrate how the approach effectively reduces degradation effects and how unwanted surface warping (whether it is a simple tilted plane or a more complex distortion) can be removed. The computational restoration of the images significantly improves their resolution, enabling the analysis above 4 THz. The application of such computational strategies makes possible the characterisation of artistic and archaeological materials up to 250 cm−1 . These results concretely open new possibilities in the use of THz spectroscopy imaging on objects with complex geometries and push the boundaries of THz data and image processing.