It is often necessary to measure the phase difference between two periodic signals of the
same frequency. In power distribution systems, the power factor angle between the AC
supply voltage and the load current is of great economic and practical importance to the
power generator and the end user. We have seen that several magneto-optical electric current
sensing systems have AC outputs in which the phase difference is proportional to an
optical polarization angle change induced by the magnetic field produced by the current
under measurement. Also, phase information is important in the description of feedback
system performance and filter behavior.