Particle counter

A particle counter comprising:

• a light source that emits light;
• a light superimposition unit configured to superimpose two lights in a space;
• an irradiation optical system configured to irradiate a fluid flowing in a flow passage with one light among a plurality of lights obtained by branching the light from the light source to form a detection area;
• a detection optical system configured to make a scattered light in a direction different from an optical axis of the irradiation optical system, among scattered lights from particles contained in the fluid in the detection area, enter the light superimposition unit;
• a reference optical system configured to make another one light among the plurality of lights enter the light superimposition unit as a reference light;
• a detector configured to receive an interference light by the scattered light and the reference light with a light receiving element, the interference light being obtained by the light superimposition unit, the detector being configured to generate a detection signal corresponding to the interference light;
• a filter configured to perform, with respect to the detection signal generated by the detector, a filtering process for passing a frequency component corresponding to an intensity change of the interference light;
• a determination unit configured to determine, from a peak level of the detection signal before filtering and a peak level of the detection signal after filtering, whether the detection signal is due to the particle, according to a predetermined calculation formula;
• and
• a counting unit configured to perform, if it is determined by the determination unit that the detection signal is due to the particle, particle counting based on the detection signal after filtering, and not to perform, if it is not determined by the determination unit that the detection signal is due to the particle, the particle counting based on the detection signal after filtering,
• wherein:
• the predetermined calculation formula is (Vp1−Vp2)/Vp1 for determining an attenuation rate from the peak level Vp1 of the detection signal before filtering to the peak level Vp2 of the detection signal after filtering.