Patent ID: 8649835

Claim:
A method of operating a wavelength modulated differential photothermal radiometry system for detecting the presence of an analyte within a substance, said method comprising the steps of: providing a first optical beam and a second optical beam, wherein said first optical beam is characterized by a first wavelength and said second optical beam is characterized by a second wavelength, and wherein said first wavelength is selected so that analyte concentrations within a concentration range of interest will cause sufficient absorption of said first optical beam to produce detectable thermal-wave emission, and wherein an absorption coefficient of said analyte at said first wavelength exceeds that at said second wavelength; producing a first modulated beam and a second modulated beam by modulating an intensity of said first optical beam and an intensity of said second optical beam, respectively; wherein said first modulated beam and said second modulated beam are modulated at a substantially equal modulation frequency, and wherein a phase difference between said first modulated beam and said second modulated beam is approximately 180 degrees; providing a reference sample substantially free of analyte; directing and substantially overlapping said first modulated beam and said second modulated beams onto said reference sample; measuring a first photothermal signal produced by said first modulated beam; measuring a second photothermal signal produced by said second modulated beam; obtaining a relative photothermal phase between said first photothermal signal and said second photothermal signal; controlling a relative intensity of said first modulated beam and said second modulated beam such that a ratio of said first photothermal signal amplitude and said second photothermal signal amplitude is approximately unity; and controlling a relative phase of said first modulated beam and said second modulated beam so that said relative photothermal phase is approximately 180 degrees.