# Recent Developments in Instrumentation of Functional Near-Infrared Spectroscopy Systems
## 2 Near-Infrared Systems Instrumentation  
As for any optical system at visible or NIR light ranges, instrumentation of NIRS system consists of an (i) emitter device to illuminate a small area of tissue with light at two or more wavelengths, namely red and infrared range, and (ii) a detector device to measure the back-scattered light emerging from the tissue, and (iii) a diffraction grating to enable differentiation and recording the intensity of different wavelengths [24]. Practically, several sources and detectors (that are called optodes) are required and the collective effect is measured. In order to choose the optimal wavelengths for sources, the effects relevant to fNIRS such as absorption and scattering need to be carefully considered. At NIR wavelengths, the atoms or molecules absorb a part of the light energy. The absorption level is determined based on the molecular composition \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Technique** & **Main Discussed Topics** & **Year** & **Ref.** \\ \hline \multirow{8}{*}{CW-fNIRS} & fNIRS major events 1977–2011 & \multirow{8}{*}{2012} & \multirow{8}{*}{[1]} \\  & Simple point and multi-channel CW fNIRS & & \\ \cline{2-3}  & Main fields of fNIRS applications & & \\ \cline{2-3}  & Technological design aspects (Sources and detectors) & & \\ \cline{2-3}  & Analysis of fNIR signals & & \\ \cline{2-3}  & Characteristics of key fNIRS and Diffuse Optical Tomography & & \\ \cline{2-3}  & (DOT) technologies & & \\ \cline{2-3}  & fNIRS Most relevant references & & \\ \cline{2-3}  & fNIRS applications & & \\ \hline \multirow{8}{*}{FD-fNIRS} & Fundamental instrumentation design of FD-NIRS & 1998 & [26] \\ \cline{2-3}  & Solving the forward model for light propagation in tissue & & \\ \cline{2-3}  & Developments in reconstruction methods & & \\ \cline{2-3}  & A tutorial of the development of Diffuse optical imaging and & & \\ \cline{2-3}  & its applications. & & \\ \cline{2-3}  & Diffuse optical imaging instrumentation & & 2014 & [29] \\ \cline{2-3}  & DOT image reconstruction algorithms and its & & \\ \cline{2-3}  & clinical applications & & \\ \cline{2-3}  & Basics of FD-NIRS and its application to functional & & \\ \cline{2-3}  & brain studies & & \\ \cline{2-3}  & Recent advances in acquisition and processing speed for & & \\ \cline{2-3}  & several Diffuse Optical Imaging (DOI) modalities. & & \\ \hline \multirow{8}{*}{TD-fNIRS} & Basic features of the TD approach and diffuse optics & & \\ \cline{2-3}  & Theoretical background, instruments, advanced theories & & \\ \cline{1-1}  & and methods & & \\ \cline{1-1} \cline{2-3}  & Brain monitoring clinical applications & & \\ \cline{1-1} \cline{2-3}  & Time-gated detection modality & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Most relevant review papers about CW-, FD-, and TD-fNIRS systems. of tissue, the wavelength of the emitted light, and the thickness of the tissue. Within the NIR window, molecules such as water and lipids are minimal absorbers compared to the iron-containing hemoglobin present within the blood. In this wavelength widow, deoxyhemoglobin (Hb) and oxyhemoglobin (HbO\({}_{2}\)) absorb light strongly. Figure 2 illustrates these absorption properties of the deoxyhemoglobin (Hb) and oxyhemoglobin (HbO\({}_{2}\)) as well as the so-called \"diagnostic/therapeutic window\" where water absorption is at its minimum. Thus, light in this optical window can penetrate deeper in tissue [34; 35; 36]. Unlike the absorption, a scattering interaction occurs when light strikes a particle and changes direction. Numerous factors, such as wavelength, particle size, and refractive index of tissue, contribute to the prevalence of scattering. There are two general types of scattering: elastic and inelastic. With elastic scattering, no energy is lost; the light simply changes direction. With inelastic scattering, some energy is lost from the incident light during the interaction, which would mean altered frequency and wavelength. The scattering considered here is the former.