## 4 Discussion  
Directional hemispherical reflectance enables the determination of the reflectance ratio in a wide spectral range. The reflectance assessment may prove useful in monitoring the course of therapy in many dermatoses, including psoriasis. The skin's reflectance value is influenced by the chromophores it contains, mainly melanin and hemoglobin, as well as structural protein. The penetration of the radiation into the tissue increases with wavelength from the UV to the visible light range and decreases in the IR range in accordance with the selected optical properties. The depth increases with the decrease in the thickness of the stratum corneum [13]. Penetration depths of UV radiation in human skin depend also on anatomical location and content of chromophores. [PERSON] et al. showed that regardless of phototype and location, radiation at a wavelength of 311 nm does not penetrate deeper than 60 \(\mu\)m [14]. [PERSON] et al. showed that in psoriatic lesions there was an increased number of capillary vessels per dermal papilla present as compared with skin without psoriatic lesions. A statistically significantly larger Feret's diameter and perimeter of dermal capillaries in psoriatic skin was also observed compared to healthy skin [15]. The epidermis within the psoriatic lesion is thicker than the epidermis in the non-poseriatric area [16]. In our research, a decrease in reflectance values over the course of psoriatic therapy was observed for both affected and unchanged skin for each of the seven wavelength ranges. At the wavelengths of 335-380 nm, reflectance of psoriatic skin decreases, which  
Figure 2: psoriatic skin before the therapy (**A**) and the ultrasound image corresponding to the lesion marked in the photograph (**C**). Psoriatic skin after 20 irradiations (**B**) and the ultrasound image corresponding to the lesion marked (white circle) in the photograph (**D**). may indicate the reduction of the plaque as a result of the therapy. The thickness of the epidermis also decreased, which means that radiation in this wavelength range could penetrate deeper and melanin content could increase. Reflectance in non-lesional skin also decreased at the wavelengths of 335-380 nm, while the thickness of the epidermis did not change, which may indicate an increase in melanin content. In the skin without psoriatic lesion, the decrease in reflectance at the wavelength of 400-540 and 480-600 nm may be related to the increase in melanin component. At the same wavelength range, reflectance of psoriatic skin did not statistically change after 20 irradiations. Lack of changes in reflectance in this wavelength range may be the result of a decrease in the thickness of the epidermis and a possible increase in melanin content with a simultaneous decrease in hemoglobin content. Thinning of the SLEB layer after the end of phototherapy suggests a decrease in inflammation. [PERSON] et al. used reflectance confocal microscopy (RCM) to visualize capillary blood flow in the dermal papillae of six patient with psoriasis to monitor the therapeutic response of UVB phototherapy. The research shows that the number of dermal papillae with blood flow decreased from 84% at baseline to 40%, 12 weeks after clearance. The size and diameter of blood vessels markedly decreased and capillary blood flow was minimal or absent [17]. No statistically significant differences were found between skin without psoriatic lesions and healthy skin before the start of phototherapy in the wavelength range of 1700-2500 nm, despite differences in epidermal thickness and skin density. In this wavelength range, the main components of the skin responsible for absorption are collagen and water [18; 19]. After phototherapy at the wavelengths of 1700-2500 nm, the reflectance of skin without psoriatic lesion decreased, skin density increased and no changes in epidermal thickness were observed. Reflectance of psoriatic skin also decreased, skin density increased, whereas epidermal thickness decreased.