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The cavity ring‐down spectroscopic (CRDS) technique developed by O’Keefe in 1988 utilizes a sample cell with high reflectivity mirrors to achieve pathlengths on the order of kilometers in conjunction with a laser light source (O’Keefe and Deacon 1988). The off‐axis integrated cavity output spectroscopic (ICOS) method is a similar method; both methods are a breakthrough in trace gas analysis (Figure 4‐15).

Instruments using these methods are able to measure gases at part per trillion levels. The CRDS and ICOS techniques are discussed in Chapter 5.