An optical system for a flow cytometer having a flow channel with an interrogation zone and an illumination source that impinges the flow channel in the interrogation zone includes a lens system and a detection system. The lens system preferably includes at least two lens surfaces located on opposite sides of the flow channel and configured to collect and collimate light from the interrogation zone. The detection system, configured to detect light from the lens system, preferably includes first and second detectors, a first filter that passes a first wavelength of light and reflects a second wavelength of light, and a second filter that reflects the first wavelength of light and passes the second wavelength of light, wherein the first and second filters are aligned such that light reflected from the first filter passes into the second detector and light reflected from the second filter passes into the first detector.