Terahertz imaging super-resolution for documental heritage diagnostics

Abstract

Electromagnetic waves are widely used in nondestructive testing of cultural heritage. The operating frequency region determines the waves’ role because the materials’ responses depend on the frequency. For example, noninvasive material analysis is possible using fibre optic reflectance spectroscopy (FORS) in the ultraviolet, visible, and near-infrared frequency ranges, while Fourier transform infrared spectroscopy (FTIR) is commonly used as a chemical analysis tool. Applying waves in the terahertz (THz) range, also called the far-infrared range, to diagnostics began about 15 years ago. THz spectroscopy and imaging are among the emerging technologies for cultural heritage science. THz time-domain spectroscopy (TDS) technology was developed in the 1990s. A THz-TDS system uses narrow pulses with pulse widths of the order of 1 ps. By applying a fast Fourier transform to the pulse in the time domain, spectra in the frequency range of about 0.2-4 THz are commonly obtained. Imaging techniques using THz pulses in the time domain can show the internal structures of opaque objects based on the time-of-flight technique. The THz waves can penetrate more deeply than infrared light and can obtain structural information from dielectric materials that are relatively transparent to X-rays. The present application of THz technology to artworks is focused on spectroscopic imaging, including the detection of hidden graphic signs and material identification. However, the spatial resolution of THz pulsed imaging is diffraction-limited to about the wavelength, which is of the order of 1 mm for THz waves. This project will focus on the development of a far-field super-resolution THz imaging system based on a freestanding knife edge and a transmission or reflective confocal configuration for the THz beamline, as employed in THz-TDS systems [7]. The super-resolution THz imaging system will be applied to the study of the text in ancient documents on paper or parchment substrates, where graphic signs with lateral sizes less than 1 mm are usually found [8]. Super-resolved THz images of inks, pigments, and dyes used in the past were acquired. The ancient materials and the mockups for system optimization have been obtained in collaboration with the National Central Library (Rome, Italy).