## 4 Discussion  
The stratum corneum, as the subject of this research, and more precisely its structure and thickness, affects how the solar radiation hitting the skin is scattered/reflected. The correlation between microdermabrasion and directional reflectance of the skin indicates the effect of the treatment on increasing the reflection/scattering of electromagnetic radiation in the studied solar spectral range. It should be emphasized that the impact of solar radiation on the skin largely depends on the wavelength. As the wavelength increases, the radiation energy decreases, and the depth of penetration of radiation into the skin increases. Studies on full-thickness skin fragments from the breasts of a 36-year-old woman (n = 3) have shown that, under normal conditions, UVA radiation (315-400 nm) can penetrate even into the dermis, while UVB (280-315 nm) and UVC radiation (100-280 nm) are absorbed almost entirely by s. corneum [7]. The microdermabrasion treatment has a double effect. First, reducing the thickness of the stratum corneum [8; 9] causes the deeper penetration of the radiation, more absorption by skin chromophores and a decrease in reflectance. On the other hand, the microdermabrasion treatment immediately smooths the skin surface. A smooth surface reflects/diffuses radiation better than a surface with a high degree of roughness. In 2016, [PERSON] et al. [10] used corundum microdermabrasion, 50% lactobionic acid and the mixed method to demonstrate the effect of these treatments on reduction in sebum secretion. A similar study, but using a diamond microdermabrasion device on oily skin ([PERSON] and [PERSON]) [3], confirmed the effect of reducing sebum secretion, and also indicated the effect of increasing skin-hydration level, and reducing acne scars and open comedones. [PERSON] and [PERSON] [11] studied the effect of microdermabrasion on thick skin with dilated pores and a tendency to excessive sebum secretion. The results were also satisfactory considering the subjective opinion of patients, nurses and doctors. A biopsy, performed after a series of treatments, showed an improvement in the polarity of keratinocytes in the epidermis and a decrease in the fluidity of S. basale cells. Moreover, a decrease in the level of solar elastosis in the papillary layer and a slight reduction in the number of telangiectasias, inflammations and edema were noted [11]. After a 2-3 microdermabrasion sessions, it is possible to observe an increase in the number of ceramides in the epidermis, and thus improvement in its barrier function. However, it may be related to the previous damaging of the barrier as a result of S. corneum abrasion, which inducted the compensatory effect in the form of lipid level increase [12]. Microdermabrasion affects the thickening of the epidermis and connective tissue of the skin. [PERSON] et al. [13] also indicated an increase in the amount of collagen in the reticulate and papillary layers, which might reduce fine wrinkles on the skin surface. Figure 14: Median and quartile range of reflectance at the wavelength of 1700–2500 nm in the group of women undergoing microdermabrasion, before and after the procedure. After microdermabrasion treatments, a slight decrease in the amount of elastin fibers in the papillary and upper reticular layers was observed. However, [PERSON] et al. [14] noted opposite results: an increase in elastin levels in two out of three cases, and no significant changes in collagen levels during the observations. In other studies, in which one of the study groups consisted of people suffered from melasma, decreased melanin, and a more regular distribution of melanosomes in the epidermis were observed histopathologicalally [15]. In research by [PERSON] et al. [14], the same changes were shown using the Fontan-Masson stain. However, the effect of the treatment on hyperpigmentation changes was questionable.