FACS signal measurement
FACS equipment can measure the amount of emitted fluorescence by the cells after the fluorophores in or attached to them were excited by the laser beam. Furthermore, it also measures the light scattered by the cells when passing through the laser beam. There are two different types of light scattering (Figure 1):
Forward scattering (FSC): It is a measurement of mostly diffracted light and it is detected just off the axis of the incident laser beam in the forward direction by a photodiode. It provides the cell size, and it is a suitable method of detecting particles greater than a given size independent of their fluorescence.
Side scattering (SSC): It is a measurement of mostly refracted and reflected light that occurs at any interface within the cell where there is a change in refractive index (Figure 3-1). SSC is collected at approximately 90 degrees to the laser beam by a collection lens and then redirected by a beam splitter to the appropriate detector. It is proportional to cell granularity or internal complexity.
Figure 1. Light scattering detected by FACS equipment.