## 4 Discussion  
We have summarized the use of SCL sources in biomedical diffuse optics. Historically, these sources were allowed to develop TD-NIRS system as they could deliver a pulsed coherent light at several MHz with narrow pulse width in order to reduce the IRF [137], and high power, of several mW/Wavelength, which were characteristically difficult to achieve in the early 1990s. Moreover, the development of commercially available compact SCL in the 2000s also helped the development of the use of these sources, and we have reported that more companies (see Figure 1) are now able to provide such sources. The main advantage of these sources is that they make it easier to explore more wavelength, enabling a multispectral or even broadband or hyperspectral measurement. Several measurement strategies can be implemented in order to do so. The first one is to use the white light as a source and achieve the spectral unmixing at the detection stage. The first available option to do so was to use an imaging spectrometer coupled with a streak camera, which achieve very high temporal resolution (in terms of arrival time of photons). However, this solution was bulky, and the sensitivity of these camera was low compared to the newest detection technologies. The same method can be used by either coupling the spectrometer with an ICCD camera, or a 16-channel TCSPC. This approach of performing the spectral unmixing on the detection can achieve faster measurement since no spectral switching mechanism is employed. However, the resolution of these approaches, either spectrally or temporally, can be limited. For example, typical ICCD cameras have a gate width of 200 ps, which limits the temporal resolution of the DTOF. On the other end, the 16-channel TCSPC system used by [PERSON] and colleagues [64] allows spectral bands of 12.5 nm, limiting the spectral resolution. The other option to perform the spectral unmixing is to do a sequential acquisition, by filtering the white light on the source side. To do so, two main approach are used, (1) using an AOTF, with the ability of fast switching (few microseconds) but with a potential degradation of the spectral response (i.e., side lobs), and (2), using a dispersing prism which can be mounted on a rotating axis, which can achieve better spectral performances but at the cost of a slower switching time. Therefore, the choice of the instrumentation depends on the end application, and trade-off needs to be made between spectral quality and acquisition speed. Whatever the requirement, we have seen that SCL can be used and coupled with various type of instrumentation in order to accommodate to the requirements of the applications. From an application point of view, the ability to monitor a large number of several wavelengths has several advantages. First of all, it enables to provides a true spectroscopic analysis, both in terms of absorption and scattering. This is extremely appealing for cancer application, where this type of \"optical biopsy\", enables to distinguish between healthy and cancerous tissue. This type of application has been extensively used in breast cancer [138; 139], and more recently, thyroid applications [50; 140]. In this type of application, where the spectral quality is the most important criteria, the second instrumental approach, based on spectral unmixing with a dispersing prism, was used. Secondly, the first instrumental approach, based on spectral unmixing at the detection stage, or the second instrumental approach using ATOF, give the possibility to acquire several wavelengths, either parallelly, or with a fast switching, in a time scale compatible with functional imaging. With these approaches, several authors have explicitly noted that the adoption of SCL enabled them to design or update existing systems, in order to retrieve dynamic physiological information [67; 75].