Abstract
The intensity distribution of two-beam interference resulting from the presence of a thermal source, in the form of a straight filament placed in the passage of one of the two beams, is calculated. An expression for fringe contrast is deduced from the intensity distribution. From the information of the phase difference, the fringe diameter is correlated to the refractive index of the plasma surrounding the heating source for static and dynamic fringe systems. An example of an application, for a static fringe system, is presented showing the validity of the analysis. A modulated interference pattern showing a certain ellipticity due to the heating source is compared with circular fringes obtained in the absence of the source.