The invention relates to an optical hollow waveguide assembly.  Furthermore, the invention relates to an illumination optical unit comprising such a hollow waveguide assembly, an illumination system comprising such an illumination optical unit, and an inspection apparatus comprising such an illumination system. 
A hollow waveguide of the type set forth at the outset is known from U.S. Pat.  No.  6,552,846 B1. 
Such a hollow waveguide can be one of the guiding reflecting elements in the illumination beam path of the illumination light.  The hollow waveguide can therefore be exposed to a contamination by an illumination light source and/or by components of the further illumination optical unit.