## 4 Discussion
SCL are a great tool for teams aiming at instrumental and methodological developments. In the reviewed papers, 33% were aimed at the initial system developments, and 18% and 44% were focused on optical tissue estimations or methodological developments, respectively. Only 5% of the reported studies were focused on pure applications, like tissue oxygenation estimation or cancer monitoring. Indeed, the availability of these SCL sources in teams focused on methodological and instrumental developments have pushed their used to test new possibilities and enhance the reliability of the system by promoting standardization. It is worth mentioning that having a SCL is not mandatory in a lot of settings, and that these sources were used as they were available in most of the groups working on instrumental and methodological developments. Indeed, in the case of a CW system, we have seen that SCL systems were used to provide a high number of wavelengths. In that case, other broadband sources are available, like Tungsten Halogen Light Sources (THLS) or like the Laser-Driven Light Sources (LDLS). The main disadvantage of the THLS is the low power density/wavelength, compared to coherent sources, which can limit their possibilities in biomedical diffuse optics. However, we can still report that THLS were successfully used to developed simple CW instruments [144; 145]. On the contrary, the novel LDLS are of interest as they can produce a stable, high power broadband light. This technology is still relatively new and, to our knowledge, has not been used in biomedical diffuse optics yet. However, its attractive characteristics could be of interest for developing CW biomedical diffuse optics systems. The interested reader can refer to review [146] for more details. Finally, we can note that designing a multi-wavelength CW system by adding up several single wavelengths light sources, like LEDs, is possible, but it is limited to a low number of wavelengths as the bulkiness and cost of the systems would rapidly increase with the number of wavelengths. In the case of TD systems, the choice of the light source is also largely dictated by the number of wavelengths needed, as can be seen in Table 3, that summarizes the relative advantages and disadvantages of single wavelength laser diode sources versus SCL in two cases: (1) low and (2) high number of wavelengths for TD systems. The use of single wavelength laser diode sources is indeed well adapted when a low number of wavelengths is required, as these sources are compact and relatively not expensive regarding the SCL ones. However, if one wants to design a truly spectroscopic system, with a high number of wavelengths required, these advantages became less evident as, similarly to the CW case, the bulkiness and cost of the system increase as function as the number of wavelengths. This is even more pronounced for the TD cases as the electronics are more complex. Finally, we can mention that if one wants to focus on methodological development, when specific wavelengths are still not defined, the flexibility of SCL is then evident as this choice does not have to be made from the start of the designing process, and not all the wavelengths are available commercially with laser diodes. Therefore, even though other light sources with characteristics compatible with diffuse optics are available, the high power, broadband spectrum, and fast switching capabilities of SCL makes them a source of choice, sufficiently versatile to be able to explore various avenues without source limitations. This spectral flexibility offered by SCL has indeed been a true advantage regarding the standardization efforts, giving the ability to optically characterize over a large bandwidth material and tissue. Phantom materials, like Intralipid or Indian ink, have been able to be precisely characterized using SCL, enabling to produce reproducible recipes for liquid phantoms, or to produces reliable solid phantoms.