An optical path switch divides sample path radiation into a time series of alternating
first polarized components and second polarized components. The first polarized
components are transmitted along a first optical path and the second polarized
components along a second optical path. A first gasless optical filter train filters
the first polarized components to isolate at least a first wavelength band thereby
generating first filtered radiation. A second gasless optical filter train filters
the second polarized components to isolate at least a second wavelength band thereby
generating second filtered radiation. A beam combiner combines the first and second
filtered radiation to form a combined beam of radiation. A detector is disposed
to monitor magnitude of at least a portion of the combined beam alternately at
the first wavelength band and the second wavelength band as an indication of the
concentration of the substance in the sample path.
