A transmissive ionizing-radiation beam monitoring system includes an enclosure structure including an entrance window and an exit window to an incident ionizing-radiation beam, where the entrance window and the exit window are highly transmissive. The system further includes a thin scintillator within the enclosure structure that is directly in an incident ionizing-radiation beam path and transmissive to the incident radiation beam and an ultraviolet (“UV”) illumination source within the enclosure structure facing the scintillator for internal system calibration. Embodiments further include a UV photosensor within the enclosure structure positioned to monitor and calibrate the UV illumination source and a machine vision camera within the enclosure structure that includes a lens which views the scintillator through a close proximity mirror including a folded optical axis system located to a side of the scintillator.
