In the optical method the cells suspended in the appropriate solution pass through a narrow channel illuminated with focal beams. When the cells pass through the laser beam path, the beam will be diverted (diffraction) or dispersed. Appropriately positioned set of lenses directs the dispersed beam onto the photomultiplier tube or photodiode. Each cell from the focus illuminates the photomultiplier tube, alters its state from dark to bright, which results in an electric impulse. The impulses are being amplified and counted until the specific volume of suspension passes through the channel. The photocell produces a number of electric impulses that are proportional to the number of the cells.

Technicon H1, H2, and H3 instruments use this optical detection system.