The theory of propagation in optical systems developed up to this point was based on the assumption that there exist strictly monochromatic light sources. An ideal monochromatic light wave, generated by such an ideal light source, can only exist if it fills all space at all times. Then it is also called a strictly coherent wave since it has complete correlation of the wave at all points of space-time. Obviously, such idealized waves do not exist in nature. The best one may achieve in practice are the so called quasi-monochromatic waves. While some of the processes discussed in this book are not seriously affected by the nonideal nature of the light sources, others may have to be significantly modified. One example of the latter was already encountered in the two-point resolution of imaging systems (see section 6.1.1). To understand the required modifications and the approximations involved, this chapter is mainly concerned with partially coherent light. An additional subject of this chapter is the propagation of randomly corrupted wavefronts which is closely related to the first subject.
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