Embodiments described herein generally relate to an in-situ metrology system that can constantly provide an uninterrupted optical access to a substrate disposed within a process chamber. In one embodiment, a metrology system for a substrate processing chamber is provided. The metrology system includes a sensor view pipe coupling to a quartz dome of a substrate processing chamber, a flange extending radially from an outer surface of the sensor view pipe, and a viewport window disposed on the flange, the viewport window having spectral ranges chosen for an optical sensor that is disposed on or adjacent to the viewport window.