2PT	two-photon tomography
3PT	three photon tomography
AF	autofluorescence
APIs	active pharmaceutical ingredients
ATR-FTIR	attenuated total reflectance Fourier-transform infrared
BMDP-D	deuterated betamethasone dipropionate
CARS	coherent anti-Stokes Raman scattering
CLSM	confocal laser scanning microscopy
CRM	confocal Raman micro-spectroscopy
DMSO	dimethyl sulfoxide
EPR	electron paramagnetic resonance
FCIs	functional cosmetic ingredients
FCLSM	fluorescence confocal laser scanning microscopy
FITC	fluorescein 5-isothiocyanate
FLIM	fluorescence lifetime imaging
FCLSM	fluorescence confocal laser scanning microscopy
FP	fingerprint (400–2200 cm−1 spectral region)
FTIR-PAS	Fourier-transform infrared photoacoustic spectroscopy
HWN	high wavenumber (2500–4000 cm−1 spectral region)
LDA	linear discriminant analysis
MPT	multi-photon tomography
NAD(P)H	nicotinamide adenine dinucleotide phosphate
NMF	non-restricted multiple least squares fit
NMF molecules	natural moisturizing factor molecules
NNMF	non-negative matrix factorization
OCT	optical coherence tomography
PCA	principal component analysis
PCA	3-(Carboxy)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy
PLSR	partial least squares regression
RCLSM	reflectance confocal laser scanning microscopy
RCM	reflectance confocal microscopy
RCLSM	reflectance confocal laser scanning microscopy
RS	Raman spectroscopy
SC	stratum corneum
SERS	surface-enhanced Raman scattering
SHG	second-harmonic generation
Skin-PAMPA	skin parallel artificial membrane permeability assay
SRS	stimulated Raman scattering
TEMPO	2,2,6,6-Tetramethylpiperidin-1-yl)oxyl
TED-FTIR	thermal emission decay–Fourier-transform infrared
TERS	tip-enhanced Raman spectroscopy
TEWL	transepidermal water loss
THG	third-harmonic generation
tMCR-ALS	tailored multivariate curve resolution–alternating least squares
UV/VIS	ultraviolet/visible (light)
ω1PEF	frequency of one-photon-excited fluorescence
ω2PEF	frequency of two-photon-excited fluorescence
ω3PEF	frequency of three-photon-excited fluorescence
ωaS	frequency of anti-Stokes scattering light
ωp	frequency of probe laser beam
ωR	frequency of Rayleigh scattering light
ωS	frequency of Stokes scattering light