A method of measuring a particle density of particles includes emitting, by a laser, a laser beam directed to a mirror, redirecting the laser beam by the mirror with a predetermined periodic movement, and focusing the laser beam to a detection volume by an optical imaging device. The method further includes determining a self mixing interference signal of an optical wave within a laser cavity if the self mixing interference signal is generated by laser light of the laser beam reflected by at least one of the particles and suppressing a false self mixing interference signal for particle detection if the self mixing interference signal is caused by a disturbance in an optical path of the laser beam. The false self mixing signal caused by the disturbance in the optical path of the laser beam is suppressed in a defined range of angles of the mirror during the periodic movement.