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An intensity interferometer is an interferometer showing interference based not upon electromagnetic waves interfering (e.g., canceling) but from variation in intensity, a phenomenon termed the Hanbury Brown and Twiss effect (HBT effect). Using one is termed intensity interferometry aka HBT interferometry. The intensity variations are below a megahertz in frequency (a frequency-regime termed radio frequency) so such an optical interferometer can convert the varying intensity to electric signals and use electronics to correlate them, whereas visible-light waves' frequency would be much too high for that. It is not much different than deploying optical telescopes (possibly single pixel each), converting the intensity to an electronic signal and carrying out the same type of correlation as a radio interferometer carries out on amplitude. However, unlike radio amplitude interferometers, no wave phase differences are captured, and the typical aperture synthesis techniques that use the phase aren't an option.
The term stellar intensity interferometer (i.e., that carries out stellar intensity interferometry, SII) was common for the early examples because measurement of stellar diameters was an appropriate application: intensity interferometers inherently require some source brightness and were most practical for bright stars, i.e., nearby giants.
Intensity interferometers were successfully used during the 1950s/1960s after which interest died, disadvantages being the required brightness of the sources, even bright ones requiring very long integration times, the limited observational data it yielded, and that it wasn't something that could be make good use of existing optical telescopes. There has been a recent revival of interest, not only because it does offer data otherwise unavailable, but modern technology can make it substantially more efficient. Experiments have been carried out in 2017 at the Cô d'Azur Observatory in France, and in 2019 at VERITAS in Arizona. The latter is a Cherenkov detector designed to capture visible-light signs of air showers within the atmosphere. Though it is intended as an indirect gamma ray/cosmic ray detector, it can be adapted to carry out visible-light intensity interferometry on astronomical objects: this offers the opportunity to test and advance intensity interferometry techniques and it may be that some such future facilities will be dual-purpose.