An apparatus for processing substrates includes a continuum radiation source, a source manifold optically coupled to the continuum radiation source and comprising: a plurality of beam guides, each having a first end that optically couples the beam guide to the continuum radiation source; and a second end. The apparatus also includes a detector manifold to detect radiation originating from the source manifold and transmitted through a processing area, and one or more transmission pyrometers configured to analyze the source radiation and the transmitted radiation to determine an inferred temperature proximate the processing area.