The disclosure involves a particle sensor having a mirror cavity unobstructed by masks and the like. A light detector is at the mirror secondary focal point and well outside the mirror cavity. A variation includes a beam splitter and a secondary light detector to improve detection of larger particles. A second embodiment includes a pair of elliptical mirrors offset along the light beam. Light reflected by the second mirror represents only changes in laser power and light scattered by gas molecules. The resulting signal is subtracted from that produced by the first mirror to obtain a relatively "clean" signal useful to assay very small particles.