Abstract: The specification discloses a mobile transparent window apparatus and method for use in a photochemical vapor deposition system having a reaction chamber with a quartz window integral with the top surface thereof, which prevents undesirable deposits on the quartz window and thereby enhances the efficiency and rate of the deposition process. The apparatus comprises: (a) a film of a predetermined material that is drawn across the internal face of the quartz window within the reaction chamber, the predetermined material being characterized by being transparent to a selected wavelength of radiation, being stable at the elevated temperature required for the photochemical vapor deposition, and possessing sufficient mechanical strength to be moved across the quartz window; and (b) means for moving the film across the internal face of the quartz window.