The ideas of partially coherent light were introduced by discussing a two-beam interference experiment in which the two beams were isolated from the field by using two small apertures (Chapters 10 and 11). In all of these discussions, it was assumed that the light was coherent over either of the two apertures when considered independently. Hence, the envelope function for the interference fringes was characterized by the Fraunhofer diffraction pattern of the individual aperture. But, in fact, the light is not always completely coherent across the individual apertures if they are of finite size. One of the questions we wish to investigate in this chapter is the effect on the characteristic Fraunhofer diffraction pattern when the difffracting aperture is not completely coherent. This is an important consideration when using diffraction techniques for measurement purposes, such as in certain techniques for measuring small particles and fiber filaments.
As a further example in the understanding of partially coherent light, we will extend the two-beam interference analysis to multiple-beam interference.
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