An optical hollow waveguide assembly (1) includes an optical hollow waveguide (2) for guiding illumination light (3). The hollow waveguide (2) has a tubular main body (6) with a continuous waveguide cavity (7). The waveguide cavity has an illumination light inlet (8) and an illumination light outlet (9). A cavity inner wall (10) of the waveguide cavity (7) is configured to be highly reflective for the illumination light (3) under grazing incidence. A gas source (12) has a fluid connection (13) to the waveguide cavity (7). The resulting hollow waveguide assembly exhibits a reduced risk of contamination of the hollow waveguide.