A treatment apparatus (10) is used for treatment of parts of a skin (5). The apparatus comprises a radiation source (1) emitting radiation, and a radiator (2) for guiding the emitted radiation to the parts of the skin (5). The parts of the skin (5) comprise skin cells affected by psoriasis. The radiation source (1) emits radiation in a first wavelength range of 400-460 nm, in a second wavelength range of 600-700 nm, or in a first wavelength range of 400-460 nm and a second wavelength range of 600-700 nm.