# Optical Characterization of Homogeneous and Heterogeneous Intralipid-Based Samples
## 1 Introduction  
Many physical processes, such as absorption, transmission, fluorescence, and scattering, occur when photons propagate inside turbid media, i.e., media in which the light scattering by medium constituents or generated irregular (randomly distributed) optical nonuniformities are of significant intensity. In particular, when photons are scattered inside these kinds of media, they can undergo different scattering events. If the energy of the scattered photon is the same as the incident photon, the photon-particle interaction is denoted as elastic scattering; if the energy of the scattered photon is lower or higher than the incident photon, the interaction is denoted as quasi-elastic or inelastic scattering. These processes have been widely exploited for the development, over the years, of many experimental techniques that provide a lot of useful information on scattering and structural parameters of the investigated samples, such as size, molecular weight, diffusion coefficient, and hydrodynamic radius [1]. For example, the elastic scattering processes can be analyzed using static light scattering(SLS) apparatus [2; 3]. Dynamic light scattering (DLS) [2] and diffuse wave spectroscopy (DWS) [4; 5] are the experimental techniques related to quasi-elastic scattering processes that have been employed to investigate important theoretical topics and to develop new characterization methods. In particular, DLS can be used in the single scattering regime when photons are scattered only once in the sample. DWS instead works in the multiple scattering regime when photons may be scattered hundreds of times. SLS, DLS, and DWS are usually indicated as light scattering techniques. The inelastic scattering processes are exploited in Raman spectroscopy, which nowadays is one of the most powerful tools in the macro and microscopic biochemical investigations. SLS, DLS, and DWS have been applied in a very large number of research fields spanning from biophysics [6] to microrheology [7; 8], microbiology [9], pharmaceutics [10], food characterization [11; 12], nanotechnology [13; 14; 15] and biomedicine [16; 17], just to name a few. Scattering properties of turbid media can also be investigated by using the time-resolved transmittance (TRT) technique which uses an ultrashort laser source with a pulse duration of few picoseconds or hundreds of femtosecond and a fast detector such as a streak camera or a photon counting system [18; 19; 20] to investigate the time behaviour of the light transmitted by the scattering sample. This technique has been largely applied in biomedical research (see [18] and reference therein), [21] but has also offered a certain number of applications in food characterization [22; 23; 24; 25] and in technology [26; 27]. The above-mentioned techniques represent very powerful tools that are selectively adopted by researchers for studying various samples with different scattering properties in very different experimental conditions as illustrated by the representative cited references and by the enormous number of related papers present in the literature. SLS and DLS require that the samples are highly diluted in order to be in the \"single scattering\" regime [28; 29], while TRT operates in a highly turbid medium or \"multiple scattering\" regime, and it describes the photon path inside the samples as a diffusive process [30]. In a previous paper [31], we reported a preliminary study on the use of SLS, DLS, and TRT as complementary techniques for the characterization of turbid media from an optical and structural point of view. In the present paper, we intend to extend the joint use of SLS, DLS, and TRT on a large number of samples that are representative of the different scattering regimes above-mentioned. These samples were prepared using one of the most employed scattering elements used for various applications: Intralipid 20%.