The invention provides an system (200) comprising (i) a light source (220) configured to provide radiation (221), wherein the radiation (221) at least comprises UV radiation； (ii) a waveguide element (1210) comprising a radiation exit window (230), wherein the waveguide element (1210) is configured to receive at least part of the radiation (221) and to radiate at least part of the radiation (221) to the exterior of the waveguide element (1210) via the radiation exit window (230) and configured to internally reflect part of the radiation (221) at the radiation exit window (230)； (iii) an optical sensor (310) configured to sense an internal reflection intensity (I) of the internally reflected radiation (221)； and (iv) a control system (300), functionally coupled to the optical sensor, and configured to reduce the intensity of the radiation (221) as function of reaching a predetermined first threshold of a reduction of the internal reflection intensity (I) over a time.
