The invention relates to an in-column back-scattered electron detector, the detector placed in a combined electrostatic/magnetic objective lens for a SEM. The detector is formed as a charged particle sensitive surface, preferably a scintillator disk that acts as one of the electrode faces forming the electrostatic focusing field. The photons generated in the scintillator are detected by a photon detector, such as a photo-diode or a multi-pixel photon detector. The objective lens may be equipped with another electron detector for detecting secondary electrons that are kept closer to the axis. A light guide may be used to offer electrical insulation between the photon detector and the scintillator.