Patent ID: 7333184

Claim:
A method for determining concentration path length of a target molecule, the method comprising the steps of: a) transmitting at least three pulsed laser beams substantially collinearly through a region of interest onto a surface, the at least three pulsed laser beams including: an online laser beam having an online peak wavelength within an optical absorption band of the target molecule; and a plurality of offline laser beams, each of the plurality of offline laser beams having one of a corresponding set of offline peak wavelengths, each of the set of offline peak wavelengths being outside of the optical absorption band of the target molecule; b) measuring a transmitted pulse energy of each of the at least three pulsed laser beams; c) reflecting each of the at least three pulsed laser beams from the surface and back through the region of interest to be incident on an array of optical sensors; d) measuring a received pulse energy of a portion of each of the at least three pulsed laser beams that is incident on the array of optical sensors; e) determining a scaled received pulse energy of each of the plurality of offline laser beams using the transmitted pulse energy of each of the plurality of offline laser beams measured in step (b) and the received pulse energy of the portion of each of the plurality of offline laser beams measured in step (d); f) estimating a reflectivity ratio of the online peak wavelength to one of the set of offline peak wavelengths for the surface based on the scaled received pulse energy of each of the plurality of offline laser beams, the online peak wavelength, and the set of offline peak wavelengths; and g) calculating the concentration path length for the target molecule using the transmitted pulse energy of the online laser beam measured in step (b), the received pulse energy of the online laser beam measured in step (d), the reflectivity ratio estimated in step (f), the transmitted pulse energy of the offline laser beam corresponding to the one of the set of offline peak wavelengths measured in step (b), and the received pulse energy of the offline laser beam corresponding to the one of the set of offline peak wavelengths measured in step (d); wherein the plurality of offline laser beams includes first and second offline laser beams having first and second offline peak wavelengths, respectively; and step (f) includes estimating the reflectivity ratio, using linear interpolation among (i) the first offline peak wavelength, (ii) the online peak wavelength and (iii) the second offline peak wavelength.