# The Influence of Sebum on Directional Reflectance of the Skin
## 1 Introduction  
The sebaceous glands are responsible for the secretion of sebum, a greasy substance which is a mixture of free fatty acids, triglycerides, squalene, wax esters, and cholesterol and its esters. Sebum functions are to maintain a proper epidermal barrier, to participate in metabolic processes within the epidermis, to provide antibacterial and moisturizing effects, and to participate in vitamin D synthesis. Overactivity of the sebaceous glands affects the properties of the skin and leads to the development of many dermatoses [1, 2]. Electromagnetic radiation, which induces many negative processes and leads to the development of cancer, is also a threat to the skin [3]. It is important to know about the factors and substances that can protect the skin against a wide range of radiation. The majority of research has focused on protection against ultraviolet light [4, 5], and relatively few studies explore the harmful effects of radiation in the visible light (VL) and infrared (IR) ranges [6]. Currently known filters protect the skin against UV radiation, but they are not sufficient to protect the skin against a light beam with wavelengths in the VL and IR ranges [7, 8]. In the skin, reflectance depends on its properties. The more hydrated the skin, the smoother the surface and the better the reflection of the light beam, thus the higher the reflectance value [9, 10]. Sebum affects skin hydration, so it affects the reflectance value as well. The reflectance/diffuse reflection of the skin depends on its water content but also onthe smoothness of the surface. Moisturizing the skin, on the one hand, causes a decrease in the refractive index (more radiation penetrates the skin), but on the other hand, the increase in hydration causes greater smoothness of the skin. Therefore, it is to be expected that skin hydration increases specular reflectance but decreases diffuse reflectance. With almost normal (nearly perpendicular) radiation on the skin, only a small amount is reflected due to the change in refractive index, which is 1.0 for air and about 1.55 for the stratum corneum. Therefore, in the case of perpendicularly incident radiation in the spectral range of 250-3000 nm, about 4-7% of the radiation is reflected/scattered (regardless of the melanin content in the skin). However, at an angle of incidence other than 90 degrees, as in practice, a much higher percentage of the radiation is reflected/scattered from the skin. Because the surface of the stratum corneum is not smooth and flat, the skin reflectance is not specular. The reflectance that occurs at the surface of the skin can be clinically significant. Significantly, sebum may be an important factor influencing the air-tissue reflectance/dispersion coefficient. Sebum can act not only as a combining factor but also as a \"smoothing\" factor for the stratum corneum [11]. Sebum is a substance that occurs naturally on the surface of the epidermis and is produced by the sebaceous glands. The main functions of sebum are to moisturize, lubricate, and protect the skin and hair, maintaining an effective hydrophobic barrier that prevents water loss and the invasion of microorganisms [12]. The composition and rate of sebum production varies from person to person and depends on where on the body sebum is produced. Sebum is a complex mixture of glycerides, free fatty acids, wax esters, and squalene. The amount and dynamics of sebum secretion change in response to numerous environmental factors, including the seasons [12; 13]. Sebum production increases in summer. In addition, androgens in men stimulate especially the growth of sebum-producing sebaceous glands located primarily on the face, chest, and upper back. Systemic medications that increase sebum secretion include: testosterone, progesterone, and phenothiazazine. Among the drugs that reduce the secretion of sebum, retinoids should be mentioned first [14]. The gold standard in the assessment of sebum content is the use of absorbent tapes whose optical properties change after contact with sebum.