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 waveguideelement (1210) comprising a radiation exit window (230),wherein the waveguide element (1210) is configured to receive at least part ofthe radiation (221) and to radiate at least part of theradiation (221) to the exterior of the waveguide element (1210) via theradiation exit window (230) and configured to internally reflectpart of the radiation (221) at the radiation exit window (230), (iii) anoptical sensor (310) configured to sense an internal reflectionintensity (I) of the internally reflected radiation (221), and (iv) a controlsystem (300), functionally coupled to the optical sensor, andconfigured to reduce the intensity of the radiation (221) as function ofreaching a predetermined first threshold of a reduction of theinternal reflection intensity (I) over a time.
