A particle counter for optically detecting an unconstrained particle of less than one micron in size suspended in a flowing liquid includes a sample chamber having a fluid inlet and a fluid outlet; a laser module producing a laser beam; a beam shaping optical system providing a multiple laser beam pattern in the sample chamber, and a CMOS optical detector located to detect light scattered by the particles in the sample chamber. The particle counter has a particle sensing area within the sample chamber in which the intensity of light is at least 10 Watts/mm.sup.2, the sensing area having an area of 0.5 square mm or more. The detector has thirty or more detector array elements. In the preferred embodiment, the laser optical system reflects and refocuses the laser beam to effect multiple passes of the same laser beam through the sensing area.