A particle detector, having a housing defining a chamber; an air stream injector, producing an airstream in said chamber from air taken from outside said chamber; a light source, producing a light beam that crosses the air stream and wherein said light beam is shaped so that a transverse extent of said light beam has a uniform intensity over said transverse extent of said air stream. Also, a photon detection assembly, including an optical train of lenses, is positioned to accept light from said light beam, emitted by the particles, and to focus this light onto a photon detector. A particle detection assembly detects the particles, responsive to the photon detection assembly. Finally, a particle size estimation assembly estimates size for each detected particle, based on number of photons detected by said photon detection assembly from said particle, as it crosses said light beam.