Fig. 1

TPEF and SHG generation mode[10]（W is the frequency of absorbed photons, Wf is the frequency of emitted fluorescence）"

Fig. 2

TPM images of the subject’s facial skin before and after using the cosmetics with anti-wrinkle effect"

Fig. 3

Fluorescence intensity map after 740 nm excitation and corresponding fast （τ1） lifetime color maps obtained upon excitation wavelength at 800 nm[29] （The color of the image represents the fluorescence lifetime of the region, the weaker the fluorescence lifetime, the bluer the color, and the stronger the fluorescence lifetime, the greener the color. The red boxes indicate the regions of being studied in which the normalized fluorescence intensities are calculated and show that fluorescence intensities decrease linearly with increasing skin color lightness）"

Fig. 4

2D SHG and TPEF imaging of in vivo absorption of recombinant human collagen （R-hc） labeled with infrared fluorescence probes[33] （Green represents the SHG signal emitted by endogenous collagen in histological structures of the skin, while red represents the TPEF signal emitted by R-hc labeled with infrared fluorescence probes, and white arrow represents reticular structure. Merge image reveals intrinsic collagen and exogenous R-hc are mutually linked to reticular structures in the dermis）"

Fig. 5

Reconstruction of 3D surface visual Imaris rendering images of recombinant human collagen （R-hc） labeled with external fluorescent probes during in vivo absorption process: （A） SHG imaging of endogenous collagen in dermis （Red）, arrow represents cavities in the hair follicle structure; （B） Exogenous R-hc TPEF imaging （Blue）; （C, D）Aggregation sites of endogenous collagen and exogenous R-hc imaging overlay （C: Cross-section, D: Sagittal plane）; （E, F） Partial enlarged view（Exogenous R-hc is concentrated in the hair follicle and a small amount spreads from the hair follicle to the dermis, with a depth of up to 150 μm）, arrow represents R-hc diffuses from hair follicles to the dermis [34]"

Tab. 1

Application of multi-photon imaging technology in cosmetic evaluation"