## 5 Advancements in CH Conservation: The Role of Terahertz Technology
### THz Imaging for Art and Archeology  
Advancements in THz technologies have led to the development of various 2D and 3D imaging techniques for analyzing CH objects and their materials. These techniques are classified into two categories: narrow-bandwidth (continuous wave or pulsed) THz sources operating in the frequency domain and wide-bandwidth (pulsed) sources in the time domain, as indicated in Figure 17[168; 150; 169]. Figure 16: Virgin with Child and a Saint, a 14 th-century icon (32 \(\times\) 39 \(\times\) 0.8 cm) in the Public Library, Taormina, Sicily. This damaged icon underwent diagnostic assessment before conservation, using THz and imaging techniques like VIS (Visible Imaging Spectroscopy) and IR-RTI (Infrared Reflectography and Transmitted Imaging). Inserts show THz applications: (A) locating gilding edges; (B) visualizing obscured gilding; (C) assessing structural condition at the wood-preparation interface. Image courtesy of the Danish Technical University [163; 164]. Their effectiveness depends on specific measurement objectives [169]. Continuous-wave (CW) systems provide higher peak power, better image quality, and faster acquisition times and can be more cost-effective than pulsed systems. Pulsed THz imaging is ideal for obtaining detailed multivariate information or when time-of-flight measurements pose fewer challenges than CW methods. Numerous methods exist for reconstructing two-dimensional (2D) images from measured data [170], and the most suitable approach for a given situation depends on the characteristics of the object. One effective technique for identifying different pigments in a painting involves creating a false-color RGB (red, green, and blue) THz image [171]. This method is similar to the multi-spectral false-color techniques currently employed by conservators. It works by segmenting the spectrum into three distinct frequency regions, each corresponding to a specific color in the RGB model. The combination of pigments in the painting generates a diverse array of colors in the resulting RGB false-color image [171]. An image can be reconstructed by assigning false colors to each pixel based on amplitude at a specific frequency [172] or by integrating amplitude over a frequency range [169]. In the time domain, images can be generated by analyzing pulse peak amplitude, integrating a pulse or time window, or measuring pulse time delay [168; 170; 169]. Several methods exist for obtaining three-dimensional (3D) THz images that reveal the internal structure of objects. In reflection mode, the time-domain waveforms capture signatures corresponding to reflections from various layers within the object. By assigning a false color or grayscale value to the amplitude at each point in the time domain and plotting it against position, we can create a cross-sectional representation of the internal structure of a layered object [171; 172], known as a B-scan. This representation is analogous to images produced by ultrasound scans. Alternatively, in transmission mode, X-ray computed tomography techniques can be employed using both time-domain and CW sources. Terahertz 2D Imaging for Art and Archaeology  
_Narrowband Frequency-Domain Techniques._ These methods utilize THz sources with a narrow instantaneous bandwidth for continuous wave and pulsed outputs. For pulsed sources, the pulse duration is typically long enough (approximately in the microsecond range) that the output can be regarded as quasi-monochromatic. This concentration of  
Figure 17: THz imaging and spectroscopy methods used in Cultural Heritage Science [168]. output energy within a narrow frequency band allows narrowband systems to achieve high spectral resolution and substantial output power. Obtaining spectral information requires frequency sweeping, retuning, or simultaneous operation at multiple frequencies [165]. Even without frequency tuning, narrow-band systems remain valuable for single-frequency imaging. [PERSON] et al.