Introduction

The idea of using measurements of the correlation between temporal fluctuations in light intensity at different field points was proposed by R. Hanbury Brown as an alternative to interferometry for measuring the spatial coherence function and therefore obtaining stellar data with high resolution. He called it intensity interferometry. Basically, in terms which should by now be familiar to readers of this book, an extended body of angular diameter α, consisting of many incoherently emitting sources, produces a speckled wavefront at the observer in which the speckles have typical size λ/α and typical lifetime τc. A pair of observers separated by a distance considerably less than λ/α are in the same speckle and therefore see the same intensity fluctuations. Observers separated by larger distances are likely to be in different speckles and see fluctuations with lesser correlation. The method was originally used for radio astronomy, in order to overcome the problem of providing identical phase references at two receivers separated by a very long distance (Hanbury Brown et al. 1952). It was then noticed that the measured correlations were immune to severe fluctuations produced by ionospheric instabilities, since these were in a frequency range very different from those of the intensity fluctuations being correlated. This provided the incentive to extend the method to the optical region. One should remember that at that time, the Michelson stellar interferometer was the only interferometric instrument which had provided resolution exceeding the atmospherically limited seeing, having successfully measured the diameters of six stars, and Pease's attempts to extend the baseline from 6 to 15 meters had proved impractical because of problems of atmospheric turbulence and mechanical stability.