# Infrared Spectroscopy-Quo Vadis?
## 2 Infrared Spectroscopy: The Past
From a technological point of view, long-lived radiation sources such as silicon carbide-based blackbody radiators along with high wavelength stability ensured via narrow-band laser sources such as helium-neon (HeNe) lasers have led to a customer-friendly mode of operation. Further technological developments have enabled the implementation of highly sensitive room temperature operated pyroelectric deuterated (L-Alanine) tri-glycine sulphate (DTGS, DLaTGS) or thermoelectrically cooled photoelectric or photovoltaic mixed-semiconductor (e.g., mercury-cadmium-telluride; MCT) detectors facilitating the evolution of MIR spectroscopy not only for routine demands, but also as a high-end research tool. Further developments have led to exceedingly robust and miniaturized interferometers (e.g., Michelson interferometers) providing fast wavelength modulation with a compact device footprint. Hence, entire system footprints on the order of magnitude of a sheet of letter sized or A4 paper and even handheld devices have since entered the markets. However, device scaling remains associated with a trade-off between the highest spectral resolution (i.e., requiring a long travel pathlength of the moving interferometer mirror), fast scanning speed, and portability resulting in dedicated device technology-and performance figures-of-merit-adapted to specific application scenarios. Yet, potent computers enabling fast-Fourier transform algorithms basically in real-time, the introduction of attenuated total reflection (ATR) based sensing techniques, and the increased quality of optical components albeit still requiring specialized MIR transparent materials such as ZnSe, CsI, KBr, CaF\({}_{2}\), etc. have led to MIR spectroscopy catching up with NIR methods in almost any field of application [7]. Figure 4 displays schematically the possible applications of such IR-transparent materials for the use of FTIR spectroscopes with an interferometer in combination with various sample accessories for the detection of gas, liquid and solid applications. The introduction of ATR techniques has significantly facilitated the probing of aqueous or water-containing samples, in any way, shape or form, as water itself is a pronouncedly strong IR absorber and commonly gives rise to a largely opaque sample matrix in the MIR. Additionally, probing solid samples has been eased in terms of sample preparation, i.e., usually encasing into KBr pellets, which has become almost obsolete in most application scenarios placed by ATR techniques. Consequently, MIR spectroscopy has increasingly ventured into applications requiring analytical spectroscopy with inherent molecular discrimination exemplarily--but not limited to--environmental monitoring, as hyphenated analytical tool (e.g., GC-IR, LC-IR, etc. ), in food quality [8], for recycling of plastics [5], in wastewater monitoring [9], for surveillance, purposes in winery applications [10; 11; 12], for restauration [13], in industrial quality control (e.g., surface coatings quality) [14], in paint analysis, as well as for pharmaceuticals [15; 16] and in oil/fuel analysis [17]. Lastly, FIR or THz spectroscopy at the other end of the spectrum primarily uses antenna technologies to take advantage predominantly of antenna, and has developed into a tool mainly used in space observation, as various gases provide distinct features in the THz region [18]. Similarly, FIR is applied to the study of low-energy lattice vibrations, such as minerals, but is increasingly adopted for investigating large (bio)molecular constructs as proteins and DNA. These distinct features are detected by the excitation of low-energy modes that cause all atoms in the molecule to move collectively and therefore lead to these spectral signatures of many organic molecules in the terahertz range.