# Infrared Spectroscopy-Quo Vadis?
## 1 Introduction  
The infrared (IR) spectral region provides the analytical spectroscopist with a vast amount of molecular information. Infrared spectroscopic measurement techniques are in general versatile non-destructive methods based on the interaction of infrared light with matter by the excitation of vibrational transitions in molecules. They are used in a variety of scenarios for sensing gases, liquids and solids. Due to this inherent molecular selectivity, IR spectroscopy can be used quantitatively and for obtaining chemical, structural and compositional information on analytes which can be analyzed by absorption and transmission techniques [1]. Since this would otherwise go beyond the scope of this review, we refer here to [PERSON] and [PERSON] [1], where the basic principles and main scientific drivers of infrared spectroscopy are described in detail. Commonly, the mid-infrared (MIR) is defined as the regime of electromagnetic waves with wavelengths ranging from about 2 \(\upmu\)m to about 20 \(\upmu\)m. Whereas the near-infrared (NIR) and the far infrared (FIR) are extending the MIR to the shorter wavelength (about 800 nm to 2 \(\upmu\)m) and the longer wavelength (15 \(\upmu\)m to about 1 mm) regime, respectively. However, in the MIR, wavenumbers are commonly used, as wavenumbers represent inverse wavelength and thus provide a clear overview ofthe important fingerprint region. As this spectral region covers a rather wide wavelength regime, the MIR is split in more detailed regions depending on the field of application with respective conventions via DIN (Deutsches Institut fur Normung, German Institute for Standardization), CIE (International Commission on Illumination) or ISO (International Organization for Standardization). Hence, common classifications are short wave mid infrared (SWIR, IR-A, 1.4-3 \(\upmu\)m), mid wave infrared (MWIR, IR-B, 3-8 \(\upmu\)m) or the long wave infrared (LWIR, IR-C, 8-15 \(\upmu\)m). Additionally, the infrared can be classified into bands-similar to the radio frequency bands-which is mostly used in infrared astronomy, following atmospheric transparent windows. In the infrared, there are bands such as the J, H and K bands that cover the near-infrared wavelengths as well as the L, M, N, and Q bands, which refer to the mid-infrared spectral region [2]. A brief overview of the respective band positions in the IR is given in Figure 1.  
As evident by the manifold definitions of the IR, this spectral window is of growing interest (see Figure 2) and high importance to equally manifold fields of application. Given the recent evolution from an analytical method that was rather exclusively used by specialized laboratories, into a widely used routine method, the according development of IR-based tools is evident. To derive possible future applications of IR spectroscopy, such as medicine, the food industry, and the environment, as well as its potential as a non-invasive, on-line, fast, and in-situ analytical method, it is necessary to review and briefly discuss the historical development as well as the previous obstacles that have limited the broader application of IR spectroscopy. Furthermore, recent trends and adaptions of IR spectroscopy leading to future deployment scenarios will be elucidated. Finally, the potential role and positioning of IR spectroscopy in view of emerging challenges within the next decades is discussed in this perspective review.  
Figure 1: Electromagnetic wavelength regime defined as ‘infrared’ with associated classification of transitions.