A system for increasing light throughput in cavity enhanced spectrometry, and a model for cavity enhanced absorption measurements are presented. The cavity has an entrance mirror, an opposed exit mirror and a detector positioned adjacent the exit mirror. An input aperture is defined in the entrance mirror to allow light from a source to enter the cavity. The input aperture improves light throughput without significant departure from the theoretically predicted amplification of absorbance. This results in improvement of detection limits, even with mirrors of modest reflectivity and inexpensive detectors.