Abstract: A process of exposing a fluid medium, such as machine oil, to radiant energy and a device to carry out that process include providing at least two juxtaposed, elongated radiant energy source, such as ultraviolet germicidal radiation sources, providing a passage between the sources that is typically less than an inch thick and that has walls capable of transmitting the radiant energy, providing for fluid flow in a direction substantially parallel to the radiant energy sources, passing a fluid medium in a thin film through the passage, and simultaneously exposing the fluid medium to the radiant energy by irradiating the fluid medium with the radiant energy sources. Preferably, the passage is in the configuration of an annulus surrounding one of the radiant energy sources, and a plurality of radiant energy sources is arrayed around the passage.