  A novel interferometric technique that uses the spectrum of the current
fluctuations of a quadratic detector, a type of detector commonly used in
Astronomy, has recently been introduced. It has major advantages with respect
to classical interferometry. It can be used to observe gravitational lenses
that cannot be detected with standard techniques. It can be used to carry out
very long baseline interferometry. Although the original theoretical analysis,
that uses wave interaction effects, is rigorous, it is not easy to understand.
The present article therefore carries out a simpler analysis, using the
autocorrelation of intensity fluctuations, which is easier to understand. It is
based on published experiments that were carried out to validate the original
theory. The autocorrelation analysis also validates simple numerical
techniques, based on the autocorrelation, to model the angular intensity
distribution of a source. The autocorrelation technique also allows a much
simpler detection of the signal.
  In practice, the gravitational lens applications are the ones that can
readily be done with presently available telescopes. We describe a practical
example that shows that presently available VLBI radio-astronomical data can be
used to observe microlensisng and millilensing in macrolensed Quasars. They may
give information on the dark matter substructures in the lensing galaxies.
