Recent progress on non-invasive skin evaluation technology with various optical technologies is reviewed.

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This paper gives a short review of Jones matrix optical coherence tomography (JM-OCT) and its application to skin imaging. JM-OCT is an extension of OCT which provides multiple types OCT three-dimensional OCT images including high sensitive OCT, local attenuation coefficient tomography, OCT angiography, birefringence tomography and polarization uniformity tomography. A basic concept of JMOCT and its implementation are explained. Examples of in vivo skin imaging of a fingertip and inner forearm are also demonstrated.

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The makeup foundation presented in this study has the effect of correcting the appearance of the skin by coating the spots and fine lines that develop on the surface of the aging skin. Furthermore, it helps the wearer look healthy and have a natural, bare-skin look. However, there is still no technique to measure the visual impression and the light penetration distribution in the depth direction, which are important aspects in the design and evaluation of foundations. In this study, we introduced a visible-light optical coherence tomography (OCT) system using an RGB LED light source. We presented a method of simultaneously measuring and imaging the surface state of the foundation and the distribution of light invasion in the depth direction.

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In vivo confocal laser scanning microscopy (CLMS) using 830 nm diode laser has been reported in Dermatology. CLMS is a device that can non-invasively and in real time obtain horizontal cross-sectional images up to around upper dermis (up to 300 μm). CLMS image has the characteristic that places with high refractive index and scattering coefficient are captured as high-brightness reflection images. Therefore, melanin is the best contrast source for CLMS image in human skin. In this time, not only facial benign pigmentation, such as solar lentigine, melasma, and acquired dermal melanocytosis, but also extramammary Paget’s disease are reported.

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We briefly described the absorption of light in skin tissue and reviewed the typical methods for its estimation. A recent approach was also presented for detecting the change of hemoglobin content using a Monte Carlo based spectral reflectance database. We discuss the potential usefulness and problems of this method. Key Words: Skin

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Many dermatological studies have had limited success in revealing skin functions as conventional dermatological methods are known to affect the components in the skin. Recent progress in non-invasive optical imaging has enabled us to visualize the structure of each skin layer non-invasively. However, it is still difficult to identify individual skin components. On the other hand, we can obtain the molecular vibrational signature using spontaneous Raman scattering spectroscopy. However, this technique requires long acquisition times and is rarely applied to skin imaging. Therefore, we have also applied non-linear spectroscopy, coherent anti-Stokes Raman scattering (CARS), and stimulated Raman scattering (SRS) to acquire more rapid molecular imaging of the skin. In this paper, the applications of spontaneous Raman spectroscopy to human skin evaluation in vivo and the latest results of 3D intracellular morphologies in the epidermis in humans during differentiation using coherent Raman scattering microscopy are introduced.

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Fluorescence microscopy can visualize the distribution of fluorescent molecules of interest in living tissues with high spatial resolution and high contrast. However, with increasing the observation depth, it becomes difficult to visualize molecules accurately because of light scattering in living tissues. As one of the imaging to improve the observation depth, photoacoustic microscopy (PAM) has been attracting attention. Because the thickness of skin is several mm, PAM is useful to visualize structures in whole skin with high contrast and high spatial resolution. In this paper, we have demonstrated the visualization of blood vessels in mouse ear using PAM with adaptive optics. In addition, we have introduced the combination of PAM and nonlinear optics to improve depth resolution without using high-frequency components of generated photoacoustic waves.

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Second-harmonic-generation (SHG) microscopy is an attractive imaging modality for the in situ visualizations of collagen fiber in tissues. For example, collagen fibers in dermis can be visualized as the optical sectioning of 3D images with high selectivity, good image contrast, high spatial resolution, and moderate penetration without staining. Furthermore, the image analysis of SHG images enables us to obtain the quantitative parameters of collagen synthesis, assembly, and distribution; polarization analysis gives additional information in terms of microscopic fiber orientation. In this paper, we introduce our recent research progress and discuss the advantages of SHG microscopy in skin measurements.

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A method is needed for measuring the water content in the stratum corneum (SC), the outermost layer of human skin, in a way that is less affected by external formulations on the skin, such as cosmetics. Here, we evaluated continuous-wave terahertz attenuated total reflection (THz-ATR) spectroscopy for measuring the water content in the SC with external formulations applied. THz waves were selected because they are strongly absorbed by water but are transmitted through various materials. We found that the water content in the SC sample was strongly correlated with the ATR signal, which represents THz wave absorption by the sample, and there was little THz wave absorption by ingredients in the external formulations. Moreover, THz-ATR spectroscopy of moisturized skin provided results consistent with the conventional method. Taken together, THz-ATR spectroscopy is expected to enable measurement of the water content in the SC with less influence from materials on the skin.

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