The time-domain (TD) approach, based on the use of picosecond pulsed lasers and time-resolved single-photon detection, offers the possibility to discriminate photons on the basis of their arrival time and to retrieve absolute values of the optical properties. These parameters can be assessed using near infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS), respectively. Thanks to the key advantages described above, the most widespread application of DO is on biological tissues, where the aim is the non-invasive and real time assessment of the concentration of the main chromophores-e.g., oxy-(O 2Hb) and deoxy-hemoglobin (HHB)-and the microvascular blood flow.
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Due to its intrinsic non-invasive nature, DO has a strong potential in several applications both in laboratories and professional environments (industries and clinics). The main advantages of DO techniques are: (i) non-invasiveness and non-destructiveness: in-vivo measurements on human and industrial applications are possible without damaging the tissue/sample (ii) functional measurements: possibility to follow the temporal variations of the optical properties with a real-time feedback (iii) possibility to derive other parameters from the optical properties, such as-for biological media-the main tissue components (e.g., lipid, collagen and water) or metabolic and oxidative state indicators (e.g., oxygen saturations, hemoglobin and flux). Examples of diffusive media are fruits, vegetables, wood, plastic or resins, pharmaceutical tablets, fog and clouds, and biological tissues such as brain, muscles, breast, bones, thyroid and others. The retrieval of the optical properties, i.e., absorption ( μ a) and reduced scattering ( μ s’) coefficients, of a diffusive medium is the main purpose of diffuse optical spectroscopy (DOS). In diffusive media, the ballistic photons are also deviated in different directions (scattered) because of the structural heterogeneities of the medium. When light interacts with matter, it is typically attenuated because of the selective absorption of the various chromophores of the material investigated at different wavelengths.
Diffuse optics (DO) is a branch of optics which aims to study the optical properties of a diffusive medium using light in the near-infrared spectral window, that is the wavelength range where the water absorption is negligible.
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