Also called "post-detection correlation interferometer." It was invented by Robert Hanbury Brown
After running through a high-pass filter to remove the DC component,
which is proportional to the square of the complex visibility. However, this technique throws away the phase information. It has the benefit that it is much less sensitive to atmospheric phase fluctuations because the signals input to the correlator have followed roughly the same path through the atmosphere. Delay tracking and local oscillator stability don't have to be as accurate. However, it requires a very high SNR, so only a few bright sources can be observed. A 20 m separation is also required to get the same SNR as a Michelson interferometer with a 10 cm separation. Unlike a Michelson interferometer, there is no need to find the point at which fringes vanish, so shorter baselines can be used. Intensity interferometry was so counterintuitive that physicists initially believed that it could not work.
