A system for detecting signal components of light induced by multiple excitation sources, which includes a flow channel having two spatially separated optical interrogation zones; a light illumination subsystem that directs each of two light beams of different wavelengths to a different zone of the optical interrogation zones; a detector subsystem that converts detected light into a total electrical signal having both modulated and unmodulated electrical signals; and a processor that determines signal components from the light detected from each of the optical interrogation zones.