A method of measuring the concentration of a substance using signals generated in a measuring sequence, wherein the method comprises the steps of: providing a sensor comprising a luminescent material having a luminescent activity whose characteristics depend on the concentration of the substance, supplying from a light source a pulse of light to the sensor for a period of time sufficient to cause luminescence of the luminescent material, the luminescence rising to a peak and then decaying after the end of the pulse of light, detecting with a detector the light emitted by the luminescence of the luminescent material and generating signals in response thereto, and analysing with a data processing module the signals and providing data indicative of the concentration of the substance; wherein in a measuring sequence the light source supplies a series of pulses of light spaced apart sufficiently to permit the luminescence to decay between pulses, the signals which are analysed by the data processing module to provide the data indicative of the concentration of the substance are those generated after each pulse of light has terminated and the luminescence is decaying, and wherein the data processing module comprises an analogue-to-digital converter, a processor, and a plurality of accumulators (52A-52D) which are controlled to be operable over a corresponding plurality of different periods of time during the decay of the luminescence, each accumulator (52A-52D) accumulating values indicative of the intensity of the luminescence from the series of pulses in the measuring sequence, and the processor reading the accumulated values from each accumulator (52A-52D) via the analogue-to-digital converter. providing a sensor comprising a luminescent material having a luminescent activity whose characteristics depend on the concentration of the substance, supplying from a light source a pulse of light to the sensor for a period of time sufficient to cause luminescence of the luminescent material, the luminescence rising to a peak and then decaying after the end of the pulse of light, detecting with a detector the light emitted by the luminescence of the luminescent material and generating signals in response thereto, and analysing with a data processing module the signals and providing data indicative of the concentration of the substance; wherein in a measuring sequence the light source supplies a series of pulses of light spaced apart sufficiently to permit the luminescence to decay between pulses, the signals which are analysed by the data processing module to provide the data indicative of the concentration of the substance are those generated after each pulse of light has terminated and the luminescence is decaying, and wherein the data processing module comprises an analogue-to-digital converter, a processor, and a plurality of accumulators (52A-52D) which are controlled to be operable over a corresponding plurality of different periods of time during the decay of the luminescence, each accumulator (52A-52D) accumulating values indicative of the intensity of the luminescence from the series of pulses in the measuring sequence, and the processor reading the accumulated values from each accumulator (52A-52D) via the analogue-to-digital converter.