Embodiments provide systems, methods and apparatus for detecting a cleaning endpoint of a cleaning process performed within a processing chamber. Embodiments include a spectrometer adapted to measure a spectrum response over time of a cleaning reaction within a processing chamber during a cleaning process; and a lens system coupled to the spectrometer and disposed to focus on a selected area within the processing chamber via a viewport and to amplify intensity of radiation from the selected area during the cleaning process. The selected area is chosen based on being the expected location of the last cleaning reaction during the cleaning process within the processing chamber (e.g., a corner in a rectangular chamber). Numerous other aspects are provided.