A retrofit device and method adapt an existing, conventional lighting fixture for resisting bio-fouling of a primary window of the lighting fixture when submerged and exposed to biological organisms within a surrounding environment. The primary window is isolated from the surrounding environment by an auxiliary window member, and ultraviolet radiation is directed to the outer surface of the auxiliary window member from a source within the retrofit device, enabling transmission of ultraviolet radiation to the outer surface of the auxiliary window member of an intensity limited to essentially that which is effective in rendering the outer surface of the auxiliary window member immune to permanent adherence of biological organisms to which that outer surface is exposed when submerged, thereby allowing illumination from a source behind the primary window to be projected effectively through the auxiliary window member into the surrounding environment.
