The physical structure, physiological characteristics, and health status of skin tissues are closely related to the quality of daily life and safety of the human body. Measuring human facial skin structures and characteristics has attracted increasing attention recently, because it can reveal the health conditions of the body and the dense distribution and complex structures of skin tissues in different regions. Polarization imaging is a non-contact label-free method that can provide abundant optical and structural information of tissues. Thus, it has been applied on many biomedical studies including facial skin tissue detection. However, there is still a lack of systematic and quantitative studies on the skin tissue characterization of different facial regions using polarization imaging parameters, which hinders further dermatological applications of polarimetry. To deal with this problem, we proposed a non-contact and in vivo measurement method of facial skin structures and characteristics based on polarization imaging; this method can obtain the heterogeneous distribution of five skin parameters in different facial regions under three polarization modes. Our results suggested ways of improving the effectiveness of non-contact and quantitative measurements of appropriate facial skin parameters by polarization imaging in future dermatological applications.

For human facial skin imaging, we first developed a polarization-adjustable instrument that is mainly composed of a light source, the linear polarization modulators, and a camera. It can extract the information of the skin surface and deep skin layer by using three polarization imaging modes, namely, non-polarized imaging (NPI), parallel-polarized imaging (PPI), and cross-polarized imaging (CPI). We recruited 15 healthy male (age 30.4±7.7 years old) and 5 female (age 29.4±7.4 years old) volunteers, respectively, their front, left, and right facial regions were imaged under the three polarization modes introduced above. The L^*, b^*, a^*, and individual typological angle (ITA) parameters based on the CIE Lab color space, which are often used to assist dermatological studies, and the t parameter of the skin texture feature extracted by the Frangi filter were selected to further analyze the measurement results of distinct facial regions (cheek, upper eyelid, forehead, nose, and perioral area). The skin structures and characteristics of different facial anatomical regions of the volunteers were evaluated by dermatologists of the China Academy of Chinese Medical Sciences as the analysis criteria in this study.

The results indicated that there are significant differences in the measurement results of five parameters for the subjects’ whole facial skin under different polarization imaging modes (P<0.01) (Table 1). Different polarization imaging modes and facial anatomical regions have significant differences in the measurement results of five parameters (P<0.01). The results suggested that the polarization imaging mode should be carefully selected to obtain accurate facial structure and feature parameters in follow-up researches. Regarding the characteristic parameters of facial skin, both the L^* and ITA are related to skin pigmentation, which can hardly be evaluated by polarization imaging modes (Fig. 3 and Fig. 6). The parameters a^* and b^* focus on analyzing the degree of deposition information of chromophores in the deeper skin layers, which can be successfully evaluated by the CPI mode (Figs. 4 and 5). Specifically, among the selected facial regions, the cheek region has the lowest combined concentration of chromophores in the epidermis and dermis, showing the lightest skin tone. Similarly, the forehead region is the most hyperpigmented. For the parameters of skin texture, the t parameter is related to skin aging and hydration. Compared with the NPI mode, the PPI mode can better reflect the length, width, and depth of skin stratum corneum wrinkles and hair shaft information, while the t parameter in the CPI mode can provide hair shaft texture information (Fig. 7).

In this study, we proposed a non-contact and in vivo measurement method of human facial skin structures and characteristics based on polarization imaging. The influence of different polarization imaging modes on the measurement of five facial skin parameters is analyzed. Using this imaging system, we quantitatively explored the effect of three different polarization imaging modes, namely, NPI, PPI, and CPI, on the measurement of facial skin parameters of L^*, a^*, b^*, ITA, and t. The results showed that for certain facial skin parameters, the selection of an appropriate polarization imaging mode for different facial anatomical regions can improve the measurement accuracy, better characterize the facial skin tissue structure, composition, and metabolism of the region of interest, and facilitate the evaluation of facial skin health conditions and detection of pathologies. Simultaneously, the results suggested that for the quantitative detection of other body skin areas, we still need to establish a clear relationship between the corresponding skin structure, composition, metabolism, and polarization imaging parameters.