An irradiation device, for example a UV irradiation device for
disinfecting waste water, is arranged to emit radiation from a source and
through an outer wall which is permeable to fluid. Means are provided for
varying the fluid pressure behind the inner surface of the outer wall, to
cause fluid to permeate through it, either inwardly or outwardly. The
arrangement serves to maintain the outer surface of the permeable wall
clear of contaminants.