An optical system for particle detection. The system includes a sample inlet housing; a sample outlet housing; a detection cavity having an axially surrounding wall and disposed between the sample inlet housing and the sample outlet housing; a light source configured to irradiate light through the detection cavity to particles of a sample fluid flowing inside the wall of the detection cavity; a light detector for detecting the light that is scattered by particles of the sample fluid in the detection cavity; an alignment rail having a base and sidewalls which a) extend from the sample inlet housing to the sample outlet housing and b) connect the sample inlet housing to the sample outlet housing; and the alignment rail comprising a channel formed by the base and the sidewalls, the channel having a channel lateral width fitting to a housing width of at least one of the sample inlet housing and the sample outlet housing, whereby the sample inlet housing, the housing, and the sample outlet housing are held in alignment together.
