## 3 The Use of Supercontinuum Lasers in Biomedical Diffuse Optics
### Methodology  
We have seen that the SCL has been used to develop several systems, mainly aiming at characterising the optical properties of tissues over a large bandwidth or to non-invasively follow the oxygenation and/or metabolic changes both in humans and in preclinical models. Both of these capacities are extremely appealing for clinical applications. However, the transition to the clinical application of system based on SCL and more generally TD-NIRS/DOT is still happening at a slow pace. One of the reasons of the slow adoption of optical techniques is the lack of standards in the community. Indeed, standardisation is key in order to test all the systems on the same ground, and reinforce the trustworthiness of the technique. In the recent years, standardisation have been a main goal of the NIRS community, and several protocols have been developed in order to compare and test the capabilities of NIRS systems. The main produced protocols were the MEDPHOT [92], BIP [93], and nEUROpt [94]. It is out of the scope of this paper to describe these protocols in detail, but the cornerstones of all of these are to produce standard phantoms, well characterized spectrally in terms of absorption and scattering coefficients, in order to be able to quantitatively compare different systems or methods. In this matter, SCL based systems were extensively used to characterize the material used to produce these phantoms and the actual final phantoms used as standard. These optical phantoms can either be solid (easier to store and stable over a long period of time) or liquid (easier to precisely tune the optical properties of the solution and to adjust it dynamically). For the liquid phantoms, the core principle is to use a fatty solution to control the scattering coefficient of the solution, and ink to control their absorption properties. Several recipes have been tested, like using Agar and Triton [95], but the most common recipe to date is to use a combination of water, intralipid and Indian ink. This choice has arisen after a careful characterization of both intralipid and Indian ink, principally with instrument based on SCL [96; 97; 98; 99; 100]. We can note that for the precise optical characterisation of liquid, techniques based on integrating sphere are often used. However, this methodology cannot be applied for large solid phantoms. In this matter, TD-NIRS has proved a good methodology to accurately estimates the optical properties. For example, in 2010, [PERSON] and colleagues used a TD-NIRS setup based on a SCL source filtered at 610 nm in order to estimate the optical properties of phantom with an absolute error estimates of 0.01 cm\({}^{-1}\) (11.3%) and 0.67 cm\({}^{-1}\) (6.8%) for the absorption coefficient and reduced scattering coefficient, respectively [101]. On top of designing phantom able to mimic the background optical properties of tissues, some methods have emerged to mimic a perturbation of the absorption coefficient within a homogeneous medium. To that end, one can either use a liquid phantom for the homogeneous medium, and a small black PVC target for the perturbation. A protocol has been designed and it is possible to precisely set the absorption perturbation value based on the size of the black PVC inclusion [94]. This protocol has been validated with Monte Carlo simulations and in-situ using a system based on SCL [102]. A similar phantom based on a completely solid phantom, with a movable solid rod, has also been designed and characterised [103]. Lastly, some phantoms have also been characterized in order to be a tool to evaluate quantitatively the responsivity of NIRS systems [104]. Finally, we can also mention new approaches based on digital phantoms. Basically, rather than physically simulating the process of light propagation in tissue, the digital phantom provides the detector with light signals mimicking the signals obtained in-vivo.