Cavity ringdown spectroscopy is a technique used to detect gases and liquids of very low concentrations. Light is introduced in the cavity (space) between two highly reflective mirrors (>99.98%), typically separated by 10-100 cm. The light "leaks" out by ricocheting between the two mirrors until all the light is gone. The rate of leakage is proportional to the reflectivity of the mirrors and the absorption of the gas in the cell. A typical “ringdown” decay time is 10µS. Since light travels at a velocity of 300m/µS, the effective path length is 3km. A 0.5m cell, therefore, is effectively transformed into a 3km cell. A weak absorption can be amplified by extending the effective path length.