## 5 Applications
### Standardization and Performances Assessment
A joint phantom for multimodal optical measurements has to be liquid to allow the proper Brownian motion of the scattering particles. With respect to a standard liquid phantom, in this case, there is also the need to tune the Brownian diffusion coefficient \(\mathrm{D_{b}}\) independent of the optical properties. For this reason, the possibility was tested to tune the phantom viscosity by adding glycerol, which has high viscosity and negligible absorption [184]. Unfortunately, its refractive index is very close to that of fat droplets, reducing their scattering power [185]. A TD-DOS system was employed to characterize the \(\mu^{\prime}_{s}\) variation with the relative content of glycerol in water between 600 and 1100 nm (Figure 25b), making it possible to reliably compensate for the decreased scattering power with a calibrated increase of fat droplets concentration, and thus enabling the independent tuning of optical and dynamic properties (Figure 25a). To evaluate the performance of a system in retrieving the relative concentrations of different tissue constituents, it is possible to use water-based phantoms with the addition of other chromophores. Water and lipids are major tissue constituents, thus water-based phantoms are the best starting point for mimicking the absorption spectrum of tissues. In this case, lipid dilutions must be replaced with a more realistic fat, like the purified pork fat. Being water and lipid immiscible, different recipes have been developed, based on the simple emulsion of the two using a professional homogenizer (nature emulsion), or with the addition of a thickener, like Agar or Triton-X [186]. The optical properties of these phantoms were characterized on a broad spectral range (600-1100 nm), as shown in Figure 26 for phantoms containing Triton-X. This allowed one to identify the range of tissue optical properties that can be covered in the 3 cases. While nature emulsions are more limited due to the need to mix them in a similar ratio, Agar and Triton-X allowed one to simulate a much broader range of tissue compositions. Figure 25: (**a**) Absorption spectrum of phantoms with different glycerol content. The good overlapping of the measurement points proves the capability to independently tune optical and dynamic properties. (**b**) Reduced scattering coefficient of glycerol-based phantoms at 785 nm at different concentrations of scattering centers (\(x\) axis) and of glycerol content (symbols). The reduced scattering power upon increasing glycerol content is proved by the correspondingly reduced slope of the interpolating lines [184]. In this paper, we have discussed broadband TD-DOS exclusively. An alternative method to measure the same tissue properties is based on frequency domain (FD) recordings of the diffuse light propagating through turbid media such as e.g., human tissue. Frequency domain measurements can be limited to one modulation frequency (as compared to the equivalent of all possible modulation frequencies in TD-DOS), and can thus be conducted with few photons for the same signal-to-noise ratio. This benefit always comes to the expense of less information provided. For some applications, a smaller amount of information might suffice, and then FD measurements may be a good way to go. Frequency domain measurements may be of particular interest if a dynamic event is to be recorded in a homogeneous medium and thus the recording time is limited by the rate of changes in tissue. Another example could be the immediate response of tissue to rapid changes in tissue blood perfusion. However, in most other cases, TD-DOS may be the preferred technique for its much higher informative content. In particular, the depth information is retrieved in a straight forward way by time gating or tagging late photons in time of flight curves.