## 4 Discussion
This now gives the possibility to accurately and quantitatively test and compare newly developed instruments and algorithms. We can note the effort undertaken by the BitMap project that have started comparing instruments, from different institutions all over Europe, using the same sets of solid phantoms [124; 125]. SCL can also be used as companions to develop new electronical components. Indeed, one of the big instrumental development in the recent year have been the refinement of the detection scheme. Especially, a lot of improvement have been seen in novel solid state detectors [147] which enable to boost light harvesting detectors electronics. The development of gated detector [148] also enables techniques like null source distance to be practically implemented, which can improve the resolution and sensitivity of the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{4}{c}{**Single Wavelength Sources**} & \multicolumn{2}{c}{**Supercontinuum Laser**} \\ \hline \multirow{2}{*}{Few Wavelengths} & High number of & \multirow{2}{*}{Few Wavelengths} & High Number of \\  & & Wavelength & & Wavelength \\ \hline Cost & ++ & \(--\) & \(--\) & \(+\) \\ Stability & + & + & \(--\) & \(-\) \\ Compactness & ++ & \(--\) & \(--\) & \(+\) \\ Wavelength Choice & Limited & Limited & Unlimited & Unlimited \\ Overall & ++ & \(--\) & \(--\) & \(++\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main relative advantages and drawbacks of single wavelength laser diode sources and SCL as function as the number of wavelengths used for TD systems. (\(+\) advantages \(/-\) disadvantages). FW: Few Wavelength, HNW: High number of Wavelengths. technique [77]. In these works, the availability of SCL sources enabled to perform a precise characterisation of these detectors in situ, both in terms of temporal resolution, thanks to the short pulse width of the SCL, and in terms of spectral responsivity over a large bandwidth. The combination of the SCL capabilities with these new detectors can notably boost the capabilities of TD-NIR instrument, overcoming some current limitations like, for example, the penetration depth [22; 149]. The inherent ability of SCL sources to construct systems gathering multidimensional information, i.e., spatial/spectral/temporal, also makes them a perfect tool to test novel algorithms able to exploit this large amount of data. The recent works by [PERSON] and colleagues [117; 118; 119] show the advantages of using multi-dimensional datasets, by enhancing the accuracy of the optical properties estimation of tissues by using simultaneously the temporal and spatial dimensions. [PERSON] and colleagues [121] also showed this utility, here combining the spectral and the spatial dimensions. It worth noting that this work is based on broadband CW-NIRS, but that the results of this algorithm has been tested against a TD-NIRS instrument based on SCL as a source. Combining all the dimensions available is certainly a really exiting avenue for the field of diffuse optics, to make the system even more robust and precise. Finally, we can mention that in the recent years, progress has also been made in individual pulsed laser source, which now achieve several 10 th of mW and a pulse width shorter than 100 ps. A good example of the uses of these sources can be found in [150], where an 8 wavelengths TD-NIRS system, compatible for breast imaging for example, has been developed. This paves the way on the use of these sources to enable miniaturisation of TD-NIRS instruments, making them truly compatible with the clinic. These sources could then replace the SCL in clinical settings. However, depending on the application and the chromophores targeted, the wavelength selection is a process that requires some investigations in order to balance the system complexity, and the possible crosstalk effects between the chosen wavelengths. Therefore, systems based on SCL able to easily explore various combinations of wavelengths are once more extremely useful.