An apparatus for processing a semiconductor substrate, including a process chamber having a plurality of walls and a substrate support to support the substrate within the process chamber. A radiative heat source is positioned outside the process chamber to heat the substrate through the walls when the substrate is positioned on the substrate support. In some embodiments, lenses are positioned between the heat source and the substrate to focus or diffuse radiation from the heat source and thereby selectively alter the radiation intensity incident on certain portions of the substrate. In other embodiments, diffusing surfaces are positioned between the heat source and the substrate to diffuse radiation from the heat source and thereby selectively reduce the radiation intensity incident on certain portions of the substrate.