Re-entrant illumination system for particle measuring device

Small, mobile particles in a fluid are optically detected by an apparatus having spaced apart reflectors bordering and defining an optical cavity into which a light beam is introduced. The reflectors are arranged along axes which are slightly misaligned such that the light beam is introduced at a low angle, barely over the edge of one of the reflectors, and is repeatedly reflected between the two reflectors, overlapping itself many times in a view volume located within the optical cavity, thereby increasing the intensity of detected particles. A fluid flow is introduced into the view volume and the light scattered by particles as they pass through the intensely illuminated view volume is received by a detector which observes the view volume. The detector produces a signal which is indicative of characteristics such as the number or size of particles.