## 5 Applications
### Standardization and Performances Assessment  
In the biophotonics field, the need is recognized for shared procedures for performance assessment of newly developed instruments to ensure the reproducibility of results and reliable comparisons between different instruments [170]. Diffuse optics is pursuing this approach [49; 50; 51] as it is undergoing great technology progresses, like miniaturization of pulsed laser sources [171], an increase of detector light-harvesting capability [172], high throughput timing electronics [173; 174; 175], and innovative measurement technique [176; 177; 178]. In the medical imaging field, in particular, it is crucial to enable a solid validation of new or upgraded instruments in laboratory settings, performing measurements on tissue-mimicking phantoms instead of on humans and anticipating possible technical issues that could occur during expensive clinical trials. Consequently, important human and financial investments are being spread, leading to the use of many different materials for the fabrication of phantoms. Phantoms must rely on well-defined recipes, devised thanks to a proper knowledge of the spectral characteristics of the different phantom constituents, and on precise final characterizations to verify their targeted optical properties. Therefore, the role of TD-DOS facilities is crucial. Our purpose here is not to provide an exhaustive review of all the different materials involved in this process, but to make examples where broadband TD-DOS allowed one to solve specific issues. Ref. [179] presents a phantom kit composed of a homogeneous epoxy resin bulk phantom with a hole, in which different epoxy resin rods can move along one direction (Figure 24a). Each rod contains a different absorption perturbation [180], thus allowing to move the inhomogeneity with respect to the position of the source-detector pair. The kit has been fabricated using titanium dioxide particles to obtain the targeted \(\mu_{\mathbf{s}}^{*}\) and black toner powder to obtain the desired \(\mu_{\mathbf{a}}\). The optical properties of the bulk phantom are shown in Figure 24b. The absorption coefficient is quite flat up to 1100 nm, with a maximum difference in the range of about 0.05 cm\({}^{-1}\) due to faint absorption peaks proper of epoxy resin. At longer wavelengths, larger absorption peaks of the epoxy resin matrix are clearly visible. In some cases, even if perishable within a few weeks, the high flexibility of liquid phantoms is required to finely change the optical properties during measurement or to freely move various perturbations. For this reason, phantoms based on water dilutions of Intralipid-20% and India ink have been widely employed. In a multi-center study, the optical properties of these phantoms were characterized in a broad wavelength range (between 633 and 916 nm) [181]. Reference values were identified for the \(\mu_{\mathbf{a}}\) of India ink and the \(\mu_{\mathbf{s}}^{\prime}\) of Intralipid-20%, with uncertainty \(<\)2%, providing a solid base for the preparation of liquid phantoms. Multimodal imaging is emerging as a novel approach to improve the diagnostic performance of a single medical examination by combining the strengths of different measurement techniques. In the field of diffuse optics, there is quite a strong effort in combining, for instance, ultrasound imaging with  
Figure 24: (**a**) Layout of the switchable phantom with dimensions in millimeters. (**b**) Absorption and reduced scattering coefficients over a broadband wavelength range [179]. diffuse optics [182; 183] or different diffuse optics techniques [184]. An example is the combination of DOS and diffuse correlation spectroscopy (DCS) [1]. DCS exploits the autocorrelation function of light speckles intensity, which depends on the speed of moving scattering particles (like red blood cells) to measure tissue microvascular blood flow. Hence, it can provide a complimentary information to DOS.