## 5 Applications  
### Characterization of Absorbers/Tissue Constituents  
Absorption spectra of various absorbers are essential for the quantitative analysis of highly scattering media. In tissue optics, the characterization of highly scattering tissue constituents like collagen, elastin, lipid, thyroglobulin using traditional spectrophotometers pose several challenges [119]. An integration sphere could uncouple the effects of scattering, but falls short to address uncoupling from other effects like fluorescence, finite boundary effects, physical structure of tissue constituents, and so on. TD-DOS can be used to characterize highly scattering absorbers [120; 121]. By careful design of the measurement protocol, TD-DOS can effectively address the above-mentioned challenges to extract the absolute absorption spectra of various absorbers [119]. Collagen is a structural protein and an abundant tissue constituent present in various soft and hard tissues. Recent works have shown the correlation of collagen density with breast cancer [122; 123], other studies aimed at the non-invasive characterization of collagen in bone [26; 52]. The first time domain characterization of collagen tissue was presented by [PERSON] et al. over the range of 600-1100 nm [124; 125], [PERSON] et al. extended this range to 500-1700 nm by addressing challenges related to fluorescence contamination and decrease in reduced scattering beyond 1100 nm [121]. Figure 9 shows the broadband collagen spectrum with its characteristic peaks around 910, 1030, 1200, and 1500 nm. This opens up the opportunity to explore collagen content in shortwave near infrared (SWIR) range, which has great potential for penetration depth due to low tissue scattering. Elastin is a protein responsible for the elasticity in the tissues such as skin arteries, ligaments, and lungs. The ratio of elastn to collagen content is a cursor for dermal skin aging [126] and reduced elasticity of the tissue was correlated to breast cancer [127; 128]. Figure 10 shows the broadband elastn spectrum measured using elastn extracted from bovine neck ligament [129]. The key features of the elastn spectrum are peaks at 910, 1025, 1185, and 1275 nm. Elastin has similar spectral features to collagen, thus making it difficult to uncouple the two constituents in human tissue, where they often coexist. However, the peak at 1275 nm (and the relative weight of other peaks) could be used to distinguish elastn from collagen, as shown by a close comparison of the two absorption spectra [129]. Figure 10: Absorption (blue) and reduced scattering (red) spectra of elastn with standard deviation error bars. The visual appearance of elastn powder is also shown on the bottom right [129]. Figure 9: Absorption (black solid squares) and reduced scattering (red triangles) spectra of Type I collagen (bovine Achilles tendon) [122]. Lipids play a crucial role in the onset and progression of various fat-related diseases like obesity, metabolic disorder, lifestyle-related morbidity [130; 131]. Hence, the optical characterization of lipids could enable their non-invasive quantification for in vivo studies with various diagnostic aims. The time domain broadband (600-1100 nm) diffuse optical characterization of lipid was initially performed on purified pork fat, as reported in Figure 10a [125]. The extension of the characterization to the SWIR window is shown in Figure 11b, where the spectrum measured using TD-DOS [132] is overlaid on the one obtained with CW DOS performed at a small source-detector distance on beef fat. Combining the results in the two spectral ranges, one could obtain a broadband spectrum across the 600-1700 nm range. In the NIR range, an intense and sharp 930 nm peak is unique to lipid. Features of lipids in the SWIR range slightly overlap with the collagen spectrum. However, on careful notice, the lipid peak at 1400 nm is less intense and red-shifted as compared to the collagen peak. Recent studies on the near infrared (NIR) characterization of human thyroid in vivo revealed some unique features [53; 68; 133].