The invention relates to an assembly for sterilizing surfaces and lumens of a medical device with a light source. The light source can emit light that reduces the number of or removes micro organisms from the lumen and/or surfaces of the medical device. The invention relates to an assembly comprising&mdasha medical device for transporting fluids having a lumen and a first connector part, and&mdashat least one light source configured to emit light having bactericidal effect which light source has a corresponding second connector part, and comprises an optical window being transparent for light from the light source allowing the light to the inlet of the lumen of the medical device and protecting the light source from liquid gaining access to the light source,&mdasha separate non-transparent unit being provided with a first coupling part and a second coupling part, where the first coupling part of the separate non-transparent unit during use is joined to the corresponding connector part (5) of the light source and the second coupling part during use is joined to the connector part of the medical device. The first connector part of the medical device and the second corresponding connector part of the light source via the non-transparent unit can be joined such that the light source is placed outside and in extension of the lumen of the medical device before exposure to light.
