A correction system 1 for a detector of a fire system, the correction system 1 comprising: a calculation unit 2 operable to calculate the n + 1 th order differential (wherein n is a whole number and n ≥ 1 ) of an output of a sensor 5 of the detector with respect to time; a measurement unit 3 operable to measure the n + 1 th order differential to determine the extent of an initial curvature of a line of the output of a sensor 5 of the detector with respect to time and a correction unit 4 operable to correct an output of the sensor 5 based on the measurement. The correction system 1 can compensate for impairment of the sensor 5 and results in more accurate sensor 5 readings (and accordingly a more useful fire system).