The effect of the size of a light source on spatial coherence. The interferometer fringe pattern produced by a second point source (point B) of light is shifted relative to the first point source (point A). Superimposing both patterns results in a new pattern with decreased fringe contrast. A small light source has high spatial coherence and produces high-contrast fringes. If a light source is too large (ie, consists of numerous point sources), no fringes will be seen.