You haven't mentioned the name of your disease so I can only give a rough idea.
Narrow-band UVB refers to a specific wavelength of ultraviolet (UV) radiation, 311 to 312 nm. This range has proved to be the most beneficial component of natural sunlight for psoriasis and looks promising in the treatment of some other skin conditions including atopic eczema, vitiligo, pruritus, lichen planus, etc.
Patients attend 2-5 times weekly. The patient is placed in a specially designed cabinet containing fluorescent light tubes. The patient stands in the centre of the cabinet, undressed except for underwear, and wears protective goggles. Usually the whole body is exposed to the UVB for a short time (seconds to minutes). The amount of UV is carefully monitored. A number of protocols exist depending on the individuals skin type, age, skin condition and other factors.
The skin may remain pale or turn slightly pink (the Minimal Erythemal Dose) after each treatment. Let your therapist know if you experience any discomfort.
Patches of psoriasis generally start to become thinner after five to ten treatments. Most patients with psoriasis require 15 to 25 treatments to clear.