Flux spectrometer was a type of spectrometer used aboard Federation starships in the 24th century.
In 2368, the USS Enterprise-D was performing luminosity studies of the Typhon Expanse. Normally, the flux spectrometers were the preferred tool. However, during this mission, the flux spectrometers were down for re-alignment and the completion of this task was delayed as the stellar dynamics lab's request for the installation of new science modules took precedence. In their place, gravitron polarimeters were used for the luminosity studies. (TNG: "Cause And Effect")