applied sciences Article  
Effect of Narrow-Band Ultraviolet B Therapy of Psoriasis Vulgaris on Skin Directional Reflectance, Skin Density and Epidermal Thickness  
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Footnote  
Narrow-band UVB phototherapy is associated with the irradiation of the patient's entire body and contributes to a large amount of cumulative UVB exposure. During the treatments, the skin with psoriatic lesions is irradiated, as well as the skin without psoriatic lesions [7].  
Microvascular abnormalities are a characteristic feature of psoriasis. Within psoriatic lesions, there are microvascular changes of superficial vessels. Patients have an increased number and size of blood vessels in the skin, as well as increased infiltration of inflammatory cells and the formation of Munro micro-abscesses. The blood vessels are twisted, woody, and their endothelium has gaps through which the migration of inflammatory cells is facilitated [8].  
The aim of the study was to assess the effect of narrow-band UVB 311 nm phototherapy on skin covered with psoriatic scales and without lesions in patients with psoriasis vulgaris on the total skin directional reflectance, skin density and epidermis thickness.  
Skin reflectance measurement determines the qualitative and quantitative interaction with electromagnetic radiation in the spectral range of 335-2500 nm. It allows us to determine what range of radiation is reflected and what is absorbed/transmitted, which is important in phototherapy.  
Skin reflectance depends on many factors. Light propagating in the skin was absorbed mostly by chromophores. Melanin is one of the skin pigments mainly located in the basal layer of the epidermis. The volume fraction of melanin in the epidermis depends on sunlight exposition and hormonal regulations. There are two forms of melanin--brown-black eumelanin and yellow-red pheomelanin. Absorption coefficients of both types of melanin are similar in the visible light range; however, they can influence the diffuse reflectance slightly. In the dermis, the dominant chromophore is hemoglobin. Depending on whether it is oxygenated or not, it shows different absorption spectra. Hemoglobin has absorption peaks in the green-yellow region of the light spectrum and absorbs little light in the red range, while melanin's absorption decreases linearly in the red range. The contents of \(\beta\)-carotene, bilirubin and water also have an impact of DHR. The structure of the collagen fibers affects scattering in the skin. Their arrangement and size influences the type of scattering that will occur in the skin tissue--Rayleigh scattering or Mie scattering, which also affects the HDR measurement [9; 10; 11].  
Thickness of the epidermis and the structure of its surface have an influence on HDR. Psoriatic lesions are clearly demarcated from the surrounding skin and are characterized by a rough surface. A hallmark of psoriasis is hyperkeratosis, which contributes to an increase in the thickness of the stratum corneum. Abnormal proliferation, incomplete maturation of keratinocytes and retention of nuclei in the stratum corneum can affect the light propagation in the skin. The inflammatory process, the larger diameter of the capillaries within the psoriatic lesions and greater blood perfusion affect the content of the volume fraction of hemoglobin.