A laser sensor module measures a particle density of particles with a size of less than 20 μm. The laser sensor module includes: a laser configured to emit a laser beam; a detector; and an optical arrangement. The optical arrangement is configured to focus the laser beam to a focus region. The laser is configured to emit the laser beam through the optical arrangement to the focus region. The optical arrangement has an emission window. The detector is configured to determine an interference signal of an interference of reflected laser light with emitted later light of the laser beam. The laser sensor module is configured to provide an indication signal of a soiling of the emission window based on the interference signal determined during a mechanical excitation of the emission window.

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 a photon detector. A particle detection assembly detects the particles, responsive 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.