Abstract: Infrared or visible light radiation is arranged to traverse substantially the entire cross sectional area of a passageway as a wide, flat beam having a thickness not substantially exceeding the minimum thickness of particles to be counted and a width substantially greater than the thickness, and is detected as a field of radiation of substantially constant intensity. When a particle falls by gravity through any portion of the field, the detected change in intensity functions to activate an electronic counter which provides a direct digital readout of the number of particles passing through the field in a given period of time, distance, or area.