Method for measuring an amount ( c x) of a gas species ( G x) present in a gas, the gas species being able to absorb light in an absorption spectral band (Δ x), the method comprising the following steps: a) placing the gas between a light source and a measurement photodetector, the light source being able to emit an incident light wave, the incident light wave propagating through the gas to the measurement photodetector, the light source being passed through by an electrical supply current, so as to bring the light source to a temperature value; b) illuminating the gas with the light source; c) measuring, with the measurement photodetector, an intensity ( I ( k)), referred to as the measurement intensity, of a light wave transmitted by the gas, in a measurement spectral band (Δ mes) comprising the absorption spectral band (Δ x); d) measuring, with a reference photodetector (20 ref), an intensity ( I ref ( k)), referred to as the reference intensity, of a reference light wave (12 ref), the reference light wave being emitted by the light source in a reference spectral band (Δ ref); steps b) to d) being implemented at a plurality of measurement times (1... k... K), the method comprising, at each measurement time: e) on the basis of the reference intensity ( I ref ( k)) measured by the reference photodetector, taking into account a correction function ( δ) representative of a variation in an intensity of the incident light wave in the measurement spectral band (Δ mes) relative to an intensity of the incident light wave in the reference spectral band (Δ ref); f) estimating an amount ( c x ( k)) of the gas species ( G x), on the basis of the measurement intensity measured in step c), of the reference intensity measured in step d), and of the correction function taken into account in step e); the method being characterized in that the correction function ( δ) is established beforehand in a calibrating phase, by comparing, at various temperature levels, or at various supply-current levels, the light intensities emitted, by a test light source, in the measurement spectral band and in the reference spectral band, respectively, the test light source being considered to be representative of the light source employed in step b), at each measurement time. a) placing the gas between a light source and a measurement photodetector, the light source being able to emit an incident light wave, the incident light wave propagating through the gas to the measurement photodetector, the light source being passed through by an electrical supply current, so as to bring the light source to a temperature value; b) illuminating the gas with the light source; c) measuring, with the measurement photodetector, an intensity ( I ( k)), referred to as the measurement intensity, of a light wave transmitted by the gas, in a measurement spectral band (Δ mes) comprising the absorption spectral band (Δ x); d) measuring, with a reference photodetector (20 ref), an intensity ( I ref ( k)), referred to as the reference intensity, of a reference light wave (12 ref), the reference light wave being emitted by the light source in a reference spectral band (Δ ref); e) on the basis of the reference intensity ( I ref ( k)) measured by the reference photodetector, taking into account a correction function ( δ) representative of a variation in an intensity of the incident light wave in the measurement spectral band (Δ mes) relative to an intensity of the incident light wave in the reference spectral band (Δ ref); f) estimating an amount ( c x ( k)) of the gas species ( G x), on the basis of the measurement intensity measured in step c), of the reference intensity measured in step d), and of the correction function taken into account in step e);