## 4 UV-VIS-IR Radiation Detectors
### Infrared Detectors for Fourier-Transform Infrared (FT-IR) Spectroscopy
Infrared detectors can be classified into two classes: thermal detectors and photonic (quantum) detectors [229]. The former uses the infrared energy as heat and their photosensitivity is independent from wavelength; they do not require cooling but have the disadvantage that response time is slow, and the detection capability is low. On the contrary, quantum detectors offer higher detection performance and a faster response speed (ms) although their photosensitivity is dependent on wavelength. In general, quantum detectors must be nitrogen cooled for accurate measurement, except for detectors used in the NIR range [230]. A wide variety of detector materials have been adopted for the fabrication of photon- and thermal detectors. Traditional materials for cooled detectors are InSb for MIR and HgCdTe for MIR and NIR. Extrinsic doped and deeply cooled semiconductors, such as Si and Ge, are operated in the far-infrared spectral region. The most significant figure of merit used to compare detectors is the detectivity (D*), a figure of merit used to characterize performance, equal to the reciprocal of noise-equivalent power (NEP), normalized per square root of sensor area and frequency bandwidth (reciprocal of twice the integration time)  
\[\text{D*}=[\text{cm}\,\times\,\text{Hz}^{1/2}/\text{W}]\]  
A is the area of the photosensitive region of the detector, f is the bandwidth and Noise Equivalent Power (NEP) is the quantity of incident light equal to the intrinsic noise level of a detector. Pyroelectric detectors such as Deuterated Triglycine Sulfate (DTGS) are widely used in analyzing CH materials, despite their low D*, due to several advantages such as the capability to work at room temperature, without the need for cryogenic cooling. This can overcome difficulties related to operator safety and enables installation on portable instruments for in situ measurements. In addition, the possibility to cover a wide spectral range (10000-100 cm\({}^{-1}\)) in one shot enables the identification of some inorganic compounds that are not active in the MIR range (i.e., oxides or sulfides) [231, 232, 233]. Some issues related to the high signal/noise ratio can be avoided by increasing acquisition time and opening the field aperture, in order to increase the light flux to the detector element. When dealing with micrometric-size samples, higher D* detectors are required. HgCdTe (MCT) is a photoconductive detector, the spectral response of which changes with detector element temperatures: as the temperature increases, the D* decreases and the spectral response shifts to the shorter wavelength region. The D* of MCT is inversely proportional to the amplitude of the band and it is therefore possible to compare detectors of the same type, but with different areas, from their D* values [234], as shown in Table 2:A wide-band MCT detector offers approximately four times higher sensitivity than a DTGS, while the narrow-band MCT detectors has the highest mid-IR sensitivity and is ideal for low throughput analysis when high sampling rates are required [10]. Using a MCT, the spatial resolution cannot be smaller than the theoretical diffraction limit of approximately 10 \(\upmu\)m, leading to a low signal/noise ratio for beam aperture dimensions of 10 \(\times\) 10 \(\upmu\)m\({}^{2}\)[235]. In recent years, the development of scanning systems or imaging devices has enabled obtaining chemical maps at a micrometric scale. Single-element detectors are used to obtain point by point maps by using a motorized stage (X,Y,Z) and apertures to select the spatial resolution; alternatively, 1-D or 2-D imaging systems are much faster thanks to the use of multiple element detectors arranged in an array, with a linear or a rectangular/square format (Linear Array (LA) and Focal Plane Array (FPA), respectively) (Figure 13). Chemical imaging enables a detailed and spatially resolved chemical analysis, based on the sample spectroscopic properties, where each pixel of the image detects an entire FT-IR spectrum.