Patent ID: 6983639

Claim:
A method for determining the concentration of NO x in an exhaust plume of a vehicle passing through a beam of radiation projected along an optical path between a radiation source having an initial intensity, and a detector for measuring intensities of radiation incident upon the detector over a range of wavelengths, the method comprising: measuring, under ambient conditions, intensities of radiation over a range of wavelengths to determine ambient NO x concentration, wherein the range of wavelengths includes at least one absorption wavelength for NO x ; measuring intensities of radiation, over the range of wavelengths, while the beam of radiation is passing through an exhaust plume of a vehicle after the vehicle passes through the optical path; determining a baseline intensity by interpolating a substantially linear region over an absorption dip in an absorption curve generated from the intensities measured while the beam of radiation is passing through the exhaust plume of the vehicle; calculating the concentration of NO x in the exhaust plume of the vehicle, wherein calculating the concentration of NO x includes using the baseline intensity to compensate for changes in the initial intensity; and subtracting the ambient NO x concentration from the concentration of NO x in the exhaust plume of the vehicle.