Method for analysing a gas by means of double illumination

A method for measuring an amount of a gaseous species within a gas, the gaseous species absorbing light in an absorption spectral band, the method comprising:

• a) placing the gas between a light source and a measurement photodetector, the light source being configured to emit an incident light wave;
• b) illuminating the gas with the light source so that the incident light wave propagates through the gas to the measurement photodetector;
• c) measuring, with the measurement photodetector, a measurement intensity of a light wave transmitted by the gas, in a measurement spectral band, containing the absorption spectral band;
• d) measuring, with a reference photodetector, a reference intensity of a reference light wave, the reference light wave being emitted by the light source in a reference spectral band;
• wherein b) to d) are carried out at a plurality of measurement times, the method further comprising, at each measurement time:
• e) estimating an absorption of the incident light wave by the gas, using the reference intensity measured by the reference photodetector, and the measurement intensity, measured by the measurement photodetector;
• f) estimating an amount of the gaseous species, on the basis of the absorption estimated in e);
• wherein e) comprises taking into account a correction function, representative of a temporal variation in an intensity of the incident light wave in the measurement spectral band relative to an intensity of the incident light wave in the reference spectral band;
• wherein the correction function is established beforehand in a calibration phase, comprising:
• cal-i) placing a test light source, facing a measurement test photodetector, and facing a reference test photodetector, the test light source, the measurement test photodetector and the reference test photodetector being representative of the light source, of the measurement photodetector and of the reference photodetector, respectively;
• cal-ii) illuminating the measurement test photodetector and the reference test photodetector with the test light source, during calibration times lying in a calibration period;
• cal-iii) comparing a temporal variation in the intensity detected by the measurement test photodetector in the measurement spectral band, with a temporal variation in the intensity detected by the reference test photodetector in the reference spectral band.