Table 2.

Summary of simple ratiometric values used in the work, their molecular sources and depth behavior.

Ratiometric indicator	Molecular source	Depth behavior	Related values
Fl-FP (Raman-normalized fluorescence)	Melanin and protein oxidation products in the epidermis and dermal fluorophores	Stepwise increase near the basal layer	–
$\frac{I_{1244}}{I_{1298}}$	Collagen in the papillary dermis	Stepwise increase near the dermal–epidermal junction	Non-negative matrix factorization component 3 (Fig. 4C)
$\frac{I_{1412}}{I_{1620}}$	Natural Moisturizing Factor in the stratum corneum	Localization and monotonous decrease in the stratum corneum	Non-negative matrix factorization component 4 (Fig. 4D)
$\frac{I_{1540}}{I_{1655}}$	Melanin in the epidermis	Local maximum near the basal layer	Spectral decomposition amplitudes and (Fig. 2) and non-negative matrix factorization component 2 (Fig. 4B)