Skin spectroscopy and imaging for cosmetics and dermatology

Skin is one of the most significant parts of the human body. It connects us with the environment and has a vast number of functions, among which defensive function is of a high importance. Skin structure and its layers may vary with a number of factors such as sight, age, sex, race and the overall health state of the individuals. The latter affects skin water to lipids ratio and their depth profile in the skin. Smaller changes in the water to lipids ratio may result in skin type variations. In both cases, skin appearance will change along with variations of skin conditions. Given the great importance of the state of the skin, a number of methods and devices for measuring water and lipids content were developed over the years. The research presented in this thesis proposes methods to achieve simultaneous measurements of water and lipids content of the skin and their ratio. We also analysed the impact of these measurements on determining the skin condition. Skin appearance is also addressed through measurement of the skin gloss, using several methods such as the ratio of specular to diffuse component of the image, the slope of the gradient intensity of the image from specular to the diffuse component, and an approached based on number of weighted pixels. The method proposed for simultaneous water and lipids content measurement is described in the Chapter 2, and is based on light measurements, comprising 3 wavelengths that are sensitive to primarily lipids, primarily water and equally sensitive to both, these wavelengths are: 1720 nm, 1770 nm, and 1750 nm, respectively. We benchmarked our measurement with those obtained with a corneomenter and sebumeter – benchmark devices, on induced skin conditions corresponding to combinations of high, low and neutral levels of water and lipids content in the skin. The study showed good agreement. The state of the protective function of stratum corneum (SC) and distribution as function of depth of skin lipids and water are addressed by means of short wave infrared spectroscopy. The method does not give information as a function of depth. This obstacle was overcome by tape stripping of one SC layer at a time. Comparative measurement was performed with Raman confocal microscopy and is described in the Chapter 3. Our proposed method showed similar pattern of the depth profile for water as obtained with the corneometer and with Raman confocal microscopy, while trans epidermal water loss measurement indicated the point of the barrier breaking point. Lipids measurements obtained with our method also showed similar trends as Raman confocal microscopy. As expected, water concentration increased and lipids concentration decreased with increasing depth into the stratum corneum. Additionally, a low-cost method for quantifying skin appearance by measuring skin gloss is proposed in Chapter 4. The method has proven to be reliable for skin gloss measurements via comparison with benchmark devices, and it also shows a great potential for other gloss measurements in a wide range, i.e., from an almost absolutely matte surface to a mirror like one. The proposed method comprises surface imaging by hand-held low-cost camera with ring-illumination along with image post processing based on weighting specular and diffuse components of the image. A gloss value is assigned as the result of the processing. Looking ahead, we discuss in Chapter 5 how the methods developed in this thesis could potentially be combined in one hand-held device. There will be several challenges such as the presence of other chromophores in the skin along with the low absorption coefficient of water and lipids in the spectral region suitable for the camera. The abovementioned obstacles can be solved by measuring absorption and scattering coefficients separately by means of illumination with spatial frequency modulation. The presence of several chromophores will as well require separating their impact on the absorption coefficient, potentially using more extensive data processing algorithms than those used in this research.