Two-beam interferometers: a classification which takes into account multiple localizations

Two beam interferometers are traditionally classified by the method used to separate the beams. This classification is suitable for considering localizations when the source is incoherent and continuous since an amplitude division interferometer yields the classical localization plane while a wavefront division one yields no fringes. However when the source is incoherent and periodic and the symmetry is plane there can be multiple localization planes and these cannot be easily analysed using this classification. In the present paper these planes are taken into account classifying two beam interferometers as those where the 'effective interfering sources' are the images of the source of light and those where they are not. The latter either yield no fringes or non-localized ones while the former yield several planes with straight, sinusoidal, localized fringes provided certain requirements are fulfilled. One of these is the equivalent sine condition which is here derived for any observation plane. Thus interferometers of the first class are further subdivided in two: those where the equivalent sine condition is verified on every observation plane and those where it is not. Experimental results illustrating the validity of the theoretical predictions are shown.