What is meant by non-localized fringe?

Answer 1

Waves traveling through space interfere and produce visible fringes if the

conditions are right. In particular, the waves must have some degree of spatial and

temporal coherence over a region of space. Fringe localization defines the region

of space where interference occurs and fringes with reasonably good contrast are

observed. The location of this region relative to the components of the interferometer depends on properties of the source and geometry of the interferometer. In this section, basic properties of fringe localization are illustrated for various types of sources and interferometers.

one can divide the different degree of localization in the following way:

1) Localized everywhere: The fringes have high visibility everywhere in the

observation space. (Also called non-localized or unlocalized.)

2) Localized: The fringes have high visibility over some surface in the

observation space. The fringe localization surface can be curved.

3) Localized at infinity: A lens is used to transform the angular distribution of

fringes into a spatial distribution of fringes on an observation plane with

high visibility. The observation plane is usually at the back focus of the

lens. Otherwise, fringe visibility is too low for observation. (Also called

Haidinger's fringes or fringes of equal inclination.)

4) Fringes of equal thickness: Localized fringes, where fringes correspond to

contours of constant thickness between two surfaces.

Answer 2

Fringes are basically interference pattern of alternate dark and bright. The fringes that are observed only in a certain plane with good visibility or contrast are called localised fringes. Example fringes produced between thin air film (Newton's ring, fringes produced in Michelson interferometer etc)