Method and apparatus for characterizing a combustion flame

A method of detecting an amount of a contaminant in an enclosure, the method comprising: detecting (S7) a first amplitude of energy within a first wavelength band of a first width centered about an emission wavelength of the contaminant, the method characterized by: detecting (S8) a second amplitude of energy within a second wavelength band of a second width larger than the first width, the second wavelength band also centered about the emission wavelength of the contaminant; determining (218) a ratio of the first amplitude of energy to the second amplitude of energy; and comparing (220) the ratio to a known threshold to determine the amount of contaminant in the enclosure.