The output of a simple multiplier of the two element interferometer after
delay compensation can be written as:

(9.2.7) |

(9.2.8) |

(9.2.9) | |||

From these two measurement we get

(9.2.10) | |||

(9.2.11) |

Alternatively, for mathematical convenience, the two measurements can be considered as the real and imaginary part of a complex number, i.e.

(9.2.12) |

In the above analysis a narrow band signal (quasi monochromatic) is considered. In an actual interferometer the observations are made over a finite bandwidth and one requires the complex visibilities to be measured as a function of frequencies within . This can be achieved in one of the two ways described below.