The preferred embodiments of the invention is an optical system for a flow cytometer including a flow channel with an interrogation zone, and an illumination source that impinges the flow channel in the interrogation zone from a particular direction. The optical system preferably includes a lens system and a detection system. The lens system preferably includes multiple lens surfaces arranged around the flow channel and adapted to collect and collimate light from the interrogation zone. The detection system preferably includes multiple detectors adapted to detect light from the lens system. Each detector preferably includes a local filter that independently filters for specific wavelengths. Thus, the user may easily swap the filters in any order to achieve the same detection parameters.