The present invention relates to a gas measuring system for measuring the presence of a predetermined gas including a light source and a light receiver separated by a gas volume, the light source emitting light within a chosen range of wavelengths including characteristic absorption wavelengths of the gas to be measured and the light receiver being capable of detecting light in said wavelength range and the system including a spectrum analyzer and a storage means for storing the specific absorption wavelengths of the gas thus to recognize a gas absorbing at said specific wavelengths. The storage means also includes specific absorption wavelengths characterizing at least one other material having absorption wavelengths partially overlapping the absorption wavelengths of said gas, and thus detect the material in the volume. The system includes an analyzer unit being adapted to detect said other material from the gas measurements so as to distinguish between said material and gas in the volume, and to provide a measure of said gas.