What is spillover in flow cytometry?

When more than one fluorochromes are used to stain cells, one fluorochrome may add brightness to the others, creating significant background noise and affecting the accuracy of the signal. This phenomenon is called spillover. Spillover is due to the physical overlap among the emission spectra of fluorochromes, which can activate different detectors other than the ones intended for the given wavelength. It derives from the nature of fluorescence, but is also a function of the optical filters' ability to separate these emissions. Spillover cannot be removed by the optical system and has to be corrected electronically, i.e. by compensation.