An optical particle counter has a gain-apertured laser cavity producing laser
light, an inlet jet providing fluid flow into a particle detecting region within
the laser cavity, the inlet jet having an inlet jet orifice; a detection optics
assembly located to collect light scattered from particles with the detecting region
for producing an output signal indicative of the particles; and an optical barrier
complex located to reduce noise as compared to the gain-apertured system without
the optical barrier complex for fluid flow rates greater than or equal to about
0.1 cubic feet per minute. The optical barrier complex inhibits laser light from
illuminating turbulent eddy currents originating on the interior walls of the inlet
jet. The optical barrier complex includes one or more physical apertures, one or
more optical stops, or both which are located to prevent laser light from illuminating
the eddy currents.
