[1] In astronomy, the most common use of such an astronomical interferometer is to determine the apparent angular diameter of a radio source or star.
If the distance to the object can then be determined by parallax or some other method, the physical diameter of the star can then be inferred.
In quantum optics, some devices which take advantage of correlation and anti-correlation effects in beams of photons might be said to be intensity interferometers, although the term is usually reserved for observatories.
An intensity interferometer is built from two light detectors, typically either radio antenna or optical telescopes with photomultiplier tubes (PMTs), separated by some distance, called the baseline.
Both detectors are pointed at the same astronomical source, and intensity measurements are then transmitted to a central correlator facility.