# Recent Developments in Instrumentation of Functional Near-Infrared Spectroscopy Systems
## 3 Continuous Wave fNIRS Instrumentation
The system uses Time-Division Multiplexing (TDM) of the fNIRS channels. Interestingly, they aimed to have such a system as an open-source instrument. The suggested module offers four dual-wavelength fNIRS channels using 750 and 850 nm with a quite broad emission of 30 to 35 nm. The incoherence and uncollimated characteristics of these sources allow for (i) a stronger strength used for tissue examination, (ii) the optodes to be in direct contact with the scalp as a result of almost no heating of the tissue, and it is (iii) no harm for the eyes. When using TDM, various factors have to be taken into accounts such as inter-channel crosstalk, heating, and battery consumption. More importantly, the SNR is restricted by the width of the used time frames. Figure 7 shows this open-source fNIRS system. Following that, the original system has been significantly improved and called the ninjaNIRS as shown in Figure 8 [60]. The new system has a very small footprint, scalable that supports up to 128 optodes. They are basically the core of this variable system. The optode itself digitizes the system, the signal and therefore the interface is a purely digital bus. It has a Field Programmable Gate Arrays (FPGA) onboard, at the side of the multi-wavelength led and photodetector. The long-term goal of this study has been to build a high-density fNIRS-EEG-Eye-tracking system that has a long interval to continuously monitor brain activity in real-time during movement, social interaction, and perception whilst being portable, miniaturized, lightweight and wearable. Figure 7: Complete system with (**a**) single 4 channel fNIRS module and (**b**) Bluetooth module. Reproduced from reference [59]. Another very interesting scheme has been proposed in 2017 by [PERSON] et al. to achieve a wearable and modular fNIRS system with four wavelengths [40] as shown in Figure 9. The scheme features three main characteristics: (i) the ability to measure short-separation (SS) and long-separation (LS) channels, (ii) four wavelengths can be utilized, and (iii) modular optode design that can be put on different brain regions. High modularity is obtained via a miniaturized hardware design of optode modules. It is worth mentioning here that sources and detectors can be individually connected to a central unit. For many fNIRS applications, in particular BCIs, the ability to measure the SS and LS channels can help to detect the desired signal and compensate for unwanted signals. Also, to achieve more robust estimates of concentration changes, four wavelengths are included. The following aspects have been carefully considered during the design process of this system and are summarized in Table 2. \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Required** & **Achieved** \\ \hline Signal quality (SNR) & 40–60 dB 60 dB & 64 dB \\ \hline \multirow{2}{*}{Sampling frequency} & \multirow{2}{*}{Above 6 Hz} & \multirow{2}{*}{\(\mathrm{NS}\) source; ND detector} \\  & & \(100\ \mathrm{Hz}/\left[\mathrm{NS}\times\left(1+0.06\times\mathrm{ND}\right)\right]\) \\ \hline \multirow{2}{*}{Safety} & Temperature should not exceed 42 \({}^{\circ}\)C & \(\leq\)41 \({}^{\circ}\)C \\  & Optical power below 10 mW & 1 mW \\ \hline \multirow{2}{*}{Usability} & \multirow{2}{*}{Small weight} \\  & & Graphical user interface \\ \hline Modularity & & The number of channels is scalable \\ \hline \end{tabular}
\end{table} 
Table 2: Main CW required and achieved parameters [40]. Figure 8: ninjaNIRS optodes and controller. Reproduced from reference [60]. Figure 9: A compact fNIRS instrument. (**a**) The PCB next to a 10-cent euro coin. (**b**) Picture of the fNIRS system with two optode modules. (**c**) Conceptual sketch illustrating the arrangement of the system. Reproduced from reference [40]. The third wearable CW-RINRS system has been suggested by [PERSON] et al. [61]. It is based on silicon photomultiplier detectors and lock-in amplification with fiber-less and multi-channels.