The FACS optical system is the group of components responsible for both generating the laser beam that will intercept the cells, and collecting their emitted signal (Figure 1). There are two main components:
Excitation optics: They consist of the laser and lenses that are used to focus the laser beam onto the cells. The exact point where the laser intercepts the cells is called the interrogation point.
Collection optics: They consist of a set of collection lenses to collect light emitted from the particle-laser beam interaction, and a system of dichroic mirrors and filters to route specified wavelengths of the collected light to designated optical detectors and provide the FACS measurements. Normally, sets of dichroic mirrors are grouped depending on wavelengths and are located in groups of three (trigons) or eight (octagons).
Figure 1. Schematic view of the optics system.