A mains electricity (AC-powered) photoelectric device that noninvasively measures the brain tissue blood oxygen saturation and venous oxygen saturation in the brain. It is typically used as an adjunct monitor for the regional haemoglobin oxygen saturation of blood in the brain of a paediatric or adult patient. It uses a cerebral sensor(s) having a light source and photodiode detector that is/are placed on the scalp/head. Position-1 detector detects infrared light absorption of extracranial blood and position-2 detector detects infrared light absorption of cerebral blood. Cerebral oxygenation is calculated by subtracting the absorption measured at site 1 from that measured at site 2.