# The Influence of Sebum on Directional Reflectance of the Skin
## 4 Discussion  
Analysis of the correlation between the amount of sebum and directional-hemispherical reflectance was carried out. The obtained results indicate a lack of correlation in the spectral range corresponding to ultraviolet radiation (335-380 nm), while a negative correlation was obtained at the spectral ranges 400-520 nm and 480-600 nm. The lack of correlation in terms of UV radiation indicates that the amount of sebum does not have a statistically significant effect on skin reflectance. The possible reason is that in this spectral range, UV radiation is strongly absorbed by the skin (low reflectance value) and only slightly dispersed. Nevertheless, this is a surprising result if we consider that the longer the wavelength, the easier it is to penetrate biological structures. Therefore, the expected, but unobtained, result would be an increase in reflectance at the lowest wavelength range. UV light is strongly absorbed by melanin as well as blood, so reflectance in this region is significantly affected by their local concentration. An increased concentration of sebum should reduce the reflectance since there is increased absorption in the lower part of the 335-380 nm spectral band. On the other hand, at the wavelength ranges 400-520 nm and 480-600 nm, a negative correlation was obtained between reflectance and amount of sebum. This indicates that increased sebum secretion causes less radiation to be reflected and thus more to be absorbed/transmitted at this spectral range. If the skin more effectively absorbs radiation at the wavelength range 400-600 nm, the remaining parts of visible radiation will be more \"exposed\". As a result, it can cause the optical impression of reddened skin. It can therefore be concluded that an increased amount of skin sebum may subjectively cause the skin to seem more red. For the other spectral ranges, i.e., 590-720 nm, 700-1100 nm, 1000-1700 nm, and 1700-2500 nm, a positive correlation between the amount of sebum and skin reflectance was recorded. This indicates that sebum secretion causes an increase in the radiation energy reflected (diffused) from the surface of the skin. The highest correlation was recorded in the near-infrared range, i.e., 700-1100 nm, and sebum secretion may therefore have a protective effect against infrared radiation. This radiation does not carry as much energy as ultraviolet radiation (radiation wavelength is inversely proportional to energy), but it has a strong negative impact on cells and tissues mainly due to the fact that the depth of skin penetration  
Figure 9: Correlation of sebum and skin reflectance at the wavelength 1700–2500 nm. Red line—trend line. increases along with wavelength. Therefore, ultraviolet radiation, despite the fact that it carries a lot of energy, penetrates the skin much shallower than infrared radiation. A significant difference in the dispersion of results for individual spectral ranges may result from differences in the scattering of radiation through skin unevenness. The longer the wavelength, the less the radiation will be scattered by skin irregularities. The second cause may be the uneven distribution of skin chromophores. The main skin chromophores (hemoglobin and melanin) have absorption maxima in a spectral range between 335 and 720 nm, and their non-homogeneous arrangement may therefore result in the greater dispersion of the results. Thirdly, the penetration of radiation into the skin depends on the wavelength. Higher wavelength radiation penetrates the skin more effectively, which may also contribute to the smaller spread in the reflectance range. The protection of the skin against infrared radiation from the presence of skin sebum is insignificant. Nevertheless, even this modest protection can have significant physiological and clinical importance in long-term exposure. It should also be emphasized that many cosmetic and dermatological procedures are associated with the inhibition of sebocyte activity or sebum removal from the skin surface.