A method and apparatus for providing a uniform UV radiation irradiance profile across a surface of a substrate is provided. In one embodiment, a substrate processing tool includes a processing chamber defining a processing region, a substrate support for supporting a substrate within the processing region, an ultraviolet (UV) radiation source spaced apart from the substrate support and configured to transmit ultraviolet radiation toward the substrate positioned on the substrate support, and a light transmissive window positioned between the UV radiation source and the substrate support, the light transmissive window having an optical film layer coated thereon. In one example, the optical film layer has a non-uniform thickness profile in a radial direction, wherein a thickness of the optical film layer at the peripheral area of the light transmissive window is relatively thicker than at the center region of the optical film layer.