ABSTRACT
The problems of coherence appear if the sources are extended sources or are not monochromatic. This chapter discusses spatial coherence and temporal coherence. Suppose that all points on the source radiate the same wavelength, we will obtain a monochromatic beam of light composed of rays with initial phase randomly distributed. In the case of temporal coherence, the chapter assumes a point source but not monochromatic. This case encompasses a number of sources (e.g., sources sufficiently small and located in the foci of a lens radiating a wide flat spectra). It also includes sources which emit only one wavelength but which has a certain spectral width which is large enough to degrade substantially the contrast. White light is composed of all the wavelengths belonging to the visible spectrum. This spectrum is assumed to be flat, that is to say the intensity is identical for all the wavelengths.
