Blue light has been shown to release bioactive nitric oxide (NO) from nitrite and nitrosated proteins found in high concentrations in the skin. This bioactive NO has many physiological functions regulating immune responses, proliferation / differentiation as well as local blood Perfusion of the skin. The study will test the PSO-CT02 device, an new investigational medical device emitting blue light with a peak wavelength of 453nm on treating localised mild Psoriasis vulgaris. It can be worn on the Skin above the effected skin area. In this study Treatment (target) and control area as well as intensity of blue light are randomized. The control area will serve as reference. 50 Patients will treat the target area daily (at least 5 times/week) at home for an initial treatment period of 4 weeks. During those 4 weeks, patients will return to the study site for safety and effectiveness assessments twice. After this initiation period patients will treat their plaque for further 8 weeks (3 times/week). This is followed by a 4 week follow up phase without treatment.