Cavity ring-down spectroscopy (CRDS) is a highly sensitive optical spectroscopic technique that enables measurement of absolute optical extinction by samples that scatter and absorb light. It has been widely used to study gaseous samples which absorb light at specific wavelengths, and in turn to determine mole fractions down to the parts per trillion level.

The University of Bristol Cavity Ring Down Spectrometer (CRDS) is a G2401 Picarro Inc, which measures CO2, CH4 and CO at high frequency (3 Hz).