Measurement of the Degree of Spectral Coherence

Interference phenomenon, regarded as a manifestation of the correlation between light beams at two or more space-time points is most often used to study the coherence properties of light. The correlation function is obtained from the measurement of the interference fringes formed when light from pairs of space-time separated points in the field are allowed to interfere. This idea was first used in the pioneering work of Michelson in the reconstruction of the image of a distant, incoherent source from the optical correlation function (Michelson stellar interferometer) [1] and was later more formally derived by van Cittert and Zemike [2,3]. According to the fundamental relationship stated quantitatively by the van Cittert and Zernike theorem [3], the spatial intensity distribution of an unknown incoherent light source can be reconstructed by taking the Fourier transform of the complex degree of coherence in the far field. Later, a study of the correlation existing in the optical field was more elegantly treated using the theory of partial coherence formulated by Wolf [3,4] and others [5] which gives the statistical description of light in terms of observable quantities. For a better understanding and use of the theory of partial coherence in describing the behavior of optical systems, it is useful to measure the degree of coherence in an arbitrary plane for quasi-monochromatic illumination.