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The Herriott cell, named after Donald Herriott (Herriott and Schulte 1965), consists of two opposing concave spherical mirrors, with one mirror having a hole where the light beam enters and exits (Figure 4‐14). The cell can also be designed with an exit hole in the opposite mirror. In the Herriott cell, the number of reflections is changed by changing the separation distance between the two mirrors. More than one light source can be used in the Herriott cell by drilling more holes in the mirrors.

Figure 4‐13 A White multipath gas cell.

Figure 4‐14 A Herriott multipath gas cell.

Figure 4‐15 Integrated cavity output spectrometer (ICOS).