  The interference between radiation fields superposed appropriately contains
all available information about the source. This will be recapitulated for
coherent and incoherent fields. We will further analyze a new kind of twisted
3D interferometer which allows us to generate interferograms with high
information content. The physical basis for these devices is the geometric
parallel transport of electric fields along a 3D path in space. This concept
enables us to build very compact 3D interferometers.
