A measurement apparatus for measuring at least one property of an electron bunch or other group of charged particles travelling through a cavity (310), comprises a plurality of electrodes (302-308) arranged around the cavity, a plurality of optical sensors (322-328), wherein the plurality of electrodes are configured to provide signals to the optical sensors thereby to modulate at least one optical property of the optical sensors. The apparatus further comprises at least one laser source (330) for providing a laser beam comprising a series of laser pulses to the plurality of optical sensors to obtain measurements representative of said at least one optical property of the optical sensors, and a processing resource (320) configured to process at least a first measurement signal from a first one of the optical sensors and a second measurement signal from a second one of the optical sensors, thereby to determine at least one property of the electron bunch or other group of charged particles, wherein the at least one property comprises charge and/or lateral position.