A compact particle flux monitor is formed with an enclosure through which a laser beam is directed by a lens. An aperture in the enclosure allows free particles which are to be detected to pass through a sensing area at a limiting acceptance angle thereby providing an indication of direction of particle flow. Photodiodes mounted at the sensing area detect the particles, including relatively small particles, by means of the high intensity beam portion at the region of the focal point of the light beam. The response region along the diverging beam is relatively long so that the response as a function of particle size is above background noise level.