A method and apparatus for enhancing the accuracy of spectroscopic measurements using a cavity ringdown spectrometer (CRDS) is provided. A first aspect of the invention consists of a novel algorithm for the processing of ring-down data that significantly reduces the amplitude of an exponential fitting artifact, and thereby gives a better estimate of the actual loss. The primary cause of the artifact is the presence of an unwanted backwards-traveling wave that counter-propagates within the ringdown cavity. Scattering due to small imperfections at the cavity mirrors produces this wave and its intensity may be minimized by adjustment of the mirror positions during cavity construction. A second aspect of the invention consists of an apparatus for measuring the backscattered wave within a cavity to allow such cavity mirror adjustments to be made.