Confocal Imaging at 0.3 THz With Depth Resolution of a Painted Wood Artwork for the Identification of Buried Thin Metal Foils

Abstract

A compact confocal terahertz microscope working at 0.30 THz based on all-solid state components is used to locate buried thin metal foils in a painted wood artwork. Metal foils are used for decoration and their precise localization under the pictorial layer is relevant information for conservation scientists and restorers, which cannot be obtained neither by X-ray radiography nor by spectroscopic imaging in the infrared, as we directly show here. The confocal microscopy principle based on the spatial pinhole concept is here implemented by positioning the first focus of an ellipsoidal reflector at the phase center of horn antennas coupled to Schottky diode detector and emitter mounted in rectangular waveguide blocks, together with an optical beamsplitter. The second focus of the reflector is mechanically scanned inside the sample in three dimensions. The predictions of diffraction theory for a confocal microscope at an imaging wavelength of 1.00 mm with numerical aperture of 0.53 are verified experimentally (1.2 mm and 2.8 mm for the lateral and the axial resolution respectively). These values of resolution allow a precise determination of the position of buried metal foils in ancient piece of art hence making restoration interventions possible.

New Frontiers in the Digital Restoration of Hidden Texts in Manuscripts: A Review of the Technical Approaches

“…THz confocal microscopy has been recently applied to works of art [79] and has been able to reveal different inks used for music notes and pentagrams of a parchment manuscript [68].…”

“…THz cameras have already provided great advantages for imaging purposes in terms of discriminating between different materials, and they allow the real-time monitoring of the conservation state of artworks in a short amount of time [80,81]. THz confocal microscopy has been recently applied to works of art [79] and has been able to reveal different inks used for music notes and pentagrams of a parchment manuscript [68].…”

Terahertz confocal microscopy based on Fibonacci super-oscillatory lens

“…In 2018, Ciano et al built a THz reflective confocal microscopy imaging system using a 0.3 THz source (λ= 1 mm) which was applied to the protection of cultural relics. Experiments show that the lateral resolution and longitudinal resolution of the system are 1.2 mm (1.2 λ) and 2.8 mm (2.8 λ), respectively [10]. In 2019, considering the influence of the point spread function of the system [11], Ning et al adopted the deconvolution method to process the data, which improved the resolution of the image obtained by the 0.3 THz (λ = 1 mm) confocal microscope, and obtained the lateral resolution of ~ 0.8 mm (0.8 λ) [12].…”

Guided mode resonance flat-top bandpass filter for terahertz telecom applications

“…A WR2.8 diagonal horn antenna after the last multiplier acts as a point source emitting a Gaussian beam in free space with 6°divergence (half width at half-maximum), linearly polarized along the vertical direction with respect to the optical table. In fact, the horn antenna transforms the rectangular waveguide mode TE10 into a pure transverse electromagnetic (TEM00) mode [21], which can be manipulated optically [22], in this case to obtain a perfectly collimated beam. To do so, we precisely positioned a plano-convex polymethylpentene (TPX) lens (D 50 mm, f 75 mm) with its focus coinciding with the virtual focus of the WR2.8 horn antenna [23], indicated as L1 in Fig.…”

“…As shown in Fig. 4(a), a Schottky diode zero-bias rectifier positioned in a focus of L4 and connected to a lock-in amplifier was finally employed to measure the transmitted power for each generated frequency (on-off amplitude modulation at 1 kHz was applied to the AMC circuit) [22]. The Keithley voltage source and the lock-in amplifier were connected to a computer with custom software for automated acquisition of identical spectra for each sample and reference.…”