## 4 UV-VIS-IR Radiation Detectors
### Infrared Detectors for Fourier-Transform Infrared (FT-IR) Spectroscopy  
Fourier-transform infrared spectroscopy (FT-IR) is a molecular technique that exploits the interaction of IR radiation with matter by absorption, emission, or reflection. It is used to study and identify in a solid, liquid, or gas. Absorption of IR radiation results in small energy differences in the vibrational and rotational states of molecules. Typical frequency units used in IR spectra are reciprocal centimeters (wave numbers), with the symbol cm\({}^{-1}\). IR wavelength units are commonly given in micrometers, symbol \(\upmu\)m, which are related to wave numbers in a reciprocal way. Molecular bonds with an electric dipole moment that can change due to atomic displacement are IR active. These vibrational modes are quantitatively measurable by IR spectroscopy, providing a unique, label-free tool for studying molecular composition and dynamics without perturbing the sample. The IR region of the electromagnetic spectrum is conventionally divided into three regions: near infrared (NIR: 14000-4000 cm\({}^{-1}\)), to study the harmonic and combined modes of molecular vibrations; mid-Infrared (MIR: 4000-400 cm\({}^{-1}\)), the region most used to detect fundamental vibrations; finally, the far infrared (FIR: 400-10 cm\({}^{-1}\)), to study low-frequency vibrations. The infrared sources used for FT-IR analysis are generally thermal sources, electrically heated (between 1500-2200 K) inert solid filaments. For example, Globar sources (silicon carbide filament, SiC) have a spectrum of approximately 4000 cm\({}^{-1}\) in the mid-IR, while mercury arc lamps emit in the far-IR region. In addition to the abovementioned conventional sources, synchrotron radiation (SR) sources are used to analyze samples \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Measurement** & **Detectors** & **Instruments** & **Photometric and Colorimetric Quantities Measured** \\ \hline Light characterization & Silicon photocell & Illuminometer & Illuminance (lx) \\ (photometric techniques) & Silicon photodiode array & Spectroradiometer & Light intensity (cd) \\  & & & Luminance (cd/m\({}^{2}\)) \\ Color specification & RGB sensors & Colorimeter & Color coordinates \\ (colorimetric techniques) & Silicon array photodiode & Spectrophotometer & SRF (\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Type of measurements, detectors, instruments and measured photometric and colorimetric quantities. offering huge advantages in terms of brilliance, spectral quality (signal/noise ratio, lateral resolution) and extended spectral range from NIR to FIR [219, 220, 221, 222, 223]. The INFN Da\(\Phi\)ne-Light synchrotron radiation IR source [224] offers beam time to users from different disciplines, including cultural heritage material analyses. FT-IR spectroscopy is usually used in the field of CH for the characterization of the main constituents of works of art, such as pigments, binders, degradation products and synthetic materials due to restoration treatments. [225, 226]. The IR region of the electromagnetic spectrum is conventionally divided into near infrared (NIR: 14000-4000 cm\({}^{-1}\)) to to study the harmonic and combined modes of molecular vibrations; mid-infrared (MIR: 4000-400 cm\({}^{-1}\)), the region most used in the field of CH to detect fundamental vibrations; finally, the far infrared (FIR: 400-10 cm\({}^{-1}\)), to study low-frequency vibrations. FT-IR spectroscopy can be performed in different modalities, depending on the shape and size of the sample: macro-FT-IR, single point micro-FT-IR and mapping or imaging. Different detectors are used depending on the acquisition mode. Therefore, each configuration of the FT-IR experiment, and consequently the choice of the detector, is strictly dependent on several factors: the characteristics of the analyzed sample (shape, size, etc. ), the spectral range of the analyses, the spectral resolution, and the optical throughput (percentage of the beam reaching the detector) [227, 228].