A light source is disclosed that is well-suited for materials photo-processing of low-absorption media. The present invention utilizes optical filters of very high effective finesse in an optical cavity arrangement, wherein the material medium to be processed traverses a central process space of the cavity. In its preferred embodiment, the disclosed optical system is used for ultraviolet treatment of low-pressure vapors in semiconductor processes such as sputter deposition and etching. The cavity irradiates the vapor with a sufficiently high optical flux to enact significant modification, while avoiding interaction with other components of the materials process. A sputtering apparatus for the depositing or etching of insulating, conducting, or semiconducting thin films is also disclosed, in which the sputtering plasma is irradiated with transverse ultraviolet emission produced by the disclosed light source.