Abstract: The present invention teaches an apparatus and method for preventing the accretion of opaque material on an optical port of a reaction chamber by using a dual-window optical port to maintain a heated gas flow in contact with the windows of the dual-window optical port. The dual-window optical port can be an end point window, a viewing port, or any optical port for which accretion of an opaque material is unwanted. The opaque material can include reaction byproducts such as polymer or any other chemical species present in the chamber. The gas flow preferably includes an inert gas such as compressed dry air. The apparatus preferably includes a housing for maintaining a spaced pair of windows in optical alignment. Ideally, one of the windows is an existing optical port built into the chamber, and the housing is mounted on the chamber over the built-in optical port to form an enclosure or cavity about the optical port.