Abstract: Mounting source components in a single piece machined housing block enhances stability of operation of a vapor lamp light source. Additionally, the light path providing sample light to a reference detector is matched to have the same numerical aperture as the optical fiber bundle used to output the light from the source. By matching the feedback optical path and the output optical path, more accurate, sensitive feedback control is made possible. Brightness of output of the light source is maximized by placing the vapor lamp as close as possible to the output fiber bundle. An output node provides a signal indicative of operational status of the light source, for use by a process controller.