Patent ID: 7400398

Claim:
A system for remotely sensing a concentration of a gas in an exhaust plume emitted by a vehicle, wherein the gas absorbs electromagnetic radiation with a wavelength that falls within a wavelength absorption band, the system comprising: a spectrometer that comprises a sensor made up of an array of sensor pixels, wherein the spectrometer receives electromagnetic radiation that has traveled along an optical path, the electromagnetic radiation including electromagnetic radiation with a wavelength that falls within a wavelength absorption band of a gas present in an exhaust plume of a passing vehicle, the spectrometer spatially arranging the electromagnetic radiation by wavelength into a spectrum that is incident on the sensor, and wherein each sensor pixel generates an output signal based on an intensity of a portion the electromagnetic radiation that is incident thereon; a storage module that stores a scale that associates each sensor pixel with a wavelength range, wherein the wavelength range associated with each sensor pixel is a prediction of the wavelength range of the spectrum of electromagnetic radiation that will be incident on that particular sensor pixel; and a scale module that (i) that determines an offset amount of wavelength drift in the spectrometer based on spectral data derived from the output signals generated by the sensor pixels while an exhaust plume of a passing vehicle is in the optical path, and (ii) adjusts for the determined wavelength drift by shifting the scale by the offset amount, wherein the offset amount includes a fractional pixel offset amount.