An apparatus for detecting and measuring an electrical discharge having a discharge magnitude, wherein the electrical discharge causes a corresponding emission of optical radiation, the apparatus comprising: an optical receiver arrangement configured to receive optical radiation from a scene comprising a source of a potential electrical discharge; a first image forming means configured to form an image based on the optical radiation received by the optical receiver arrangement; a measurement system (20, 50, 80) comprising: an optical detector optically coupled to the optical receiver arrangement to receive and process the optical radiation from the optical receiver arrangement to generate a detector output; a display device for displaying images formed by the first image forming means; characterized by the measurement system (20,50,80) further comprising: a memory device storing pre-determined calibration data comprising calibrated quantitative measurement values associated with magnitudes of electrical discharge and detector parameters corresponding thereto, the detector parameters being electrical parameters associated with operation of the optical detector; and a quantitative measurement module configured to receive and process a particular detector parameter with the stored calibration data thereby to detect an electrical discharge, and determine a quantitative measurement associated with the magnitude of the detected electrical discharge; the apparatus further comprising an image processor configured to overlay the determined quantitative measurement onto the image displayed by the display device. an optical receiver arrangement configured to receive optical radiation from a scene comprising a source of a potential electrical discharge; a first image forming means configured to form an image based on the optical radiation received by the optical receiver arrangement; a measurement system (20, 50, 80) comprising: an optical detector optically coupled to the optical receiver arrangement to receive and process the optical radiation from the optical receiver arrangement to generate a detector output; a display device for displaying images formed by the first image forming means; characterized by the measurement system (20,50,80) further comprising: a memory device storing pre-determined calibration data comprising calibrated quantitative measurement values associated with magnitudes of electrical discharge and detector parameters corresponding thereto, the detector parameters being electrical parameters associated with operation of the optical detector; and a quantitative measurement module configured to receive and process a particular detector parameter with the stored calibration data thereby to detect an electrical discharge, and determine a quantitative measurement associated with the magnitude of the detected electrical discharge; the apparatus further comprising a memory device storing pre-determined calibration data comprising calibrated quantitative measurement values associated with magnitudes of electrical discharge and detector parameters corresponding thereto, the detector parameters being electrical parameters associated with operation of the optical detector; and a quantitative measurement module configured to receive and process a particular detector parameter with the stored calibration data thereby to detect an electrical discharge, and determine a quantitative measurement associated with the magnitude of the detected electrical discharge; the apparatus further comprising an image processor configured to overlay the determined quantitative measurement onto the image displayed by the display device.