Optical path switching based differential absorption radiometry for substance detection

A method of detecting a substance, comprising the steps of:

• receiving radiation passing along a sample path of interest;
• in dividing said radiation into a time series of alternating first polarized components and second polarized components orthogonal to said first polarized components;
• routing said first polarized components along a first optical path and said second polarized components along a second optical path;
• reflecting all wavelengths of said first polarized components except at least a first wavelength band wherein first filtered radiation excluding said at least first wavelength band is generated;
• reflecting all wavelengths of said second polarized components except at least a second wavelength band wherein second filtered radiation excluding said second wavelength band is generated, wherein said first wavelength band and said second wavelength band are unique and wherein spectral absorption of a substance of interest is different at said first wavelength band as compared to said second wavelength band;
• combining said first filtered radiation and said second filtered radiation to form a combined beam of radiation;
• and
• monitoring magnitude of said combined beam alternately at said first wavelength band and said second wavelength band as an indication of the concentration of said substance in said sample path.

A method of detecting a substance, comprising the steps of:

• receiving radiation passing along a sample path of interest;
• dividing said radiation into a time series of alternating first polarized components and second polarized components orthogonal to said first polarized components;
• routing said first polarized components along a first optical path and said second polarized components along a second optical path;
• reflecting all wavelengths of said first polarized components except at least a first wavelength band wherein first filtered radiation excluding said at least first wavelength band is generated;
• reflecting all wavelengths of said second polarized components except at least a second wavelength band wherein second filtered radiation excluding said second wavelength band is generated, wherein said first wavelength band and said second wavelength band are unique and wherein spectral absorption of a substance of interest is different at said first wavelength band as compared to said second wavelength band;
• combining said first filtered radiation and said second filtered radiation to form a combined beam of radiation;
• transmitting the combined beam to a filter having a band pass characteristic that spans said excluded first and second wavelength bands;
• and
• monitoring magnitude of said combined beam received from the filter alternately at said first wavelength band and said second wavelength band as an indication of the concentration of said substance in said sample path.