A rapid thermal processing system is described incorporating features that enable in situ optical measurements. In particular, a system incorporating an in situ spectroscopic ellipsometer is described highlighting some of the unusual features necessary for ellipsometry measurements. These features include independent optical, vacuum, and heating modules, optical and heating window design, reflector design, and sample manipulation to enable proper positioning for measurement. Although specifically designed with ellipsometry in mind, many of the same principles used in the design of this system will apply to systems for other optical measurements. © 1996 American Institute of Physics.