ABB LGR GLA351 Series - Appendix A: about Gas Analyzers and Laser Absorption Spectroscopy; Conventional Laser Absorption Spectroscopy

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Appendix A: About Gas Analyzers and
Laser Absorption Spectroscopy
Conventional Laser Absorption Spectroscopy
For gas measurements based on conventional laser-absorption spectroscopy (Figure 56), a
laser beam is directed through a sample, and the mixing ratio (or mole fraction) of a gas is
determined from the measured absorption using Beer’s Law, which may be expressed:
Where:
 I
v
is the transmitted intensity through the sample at frequency
 I
o
is the (reference) laser intensity prior to entering the cell
 S is the absorption line strength of the probed transition
 L is the optical path length of the laser beam through the sample
  is the mole fraction
 P is the gas pressure
 Φ
v
is the line-shape function of the transition at frequency 󰄶
In this case,
If the laser line width is much narrower than the width of the absorption feature, high-
resolution absorption spectra may be recorded by tuning the laser wavelength over the
probed feature.
Figure 56: Typical Laser Absorption Spectroscopy Setup