Some Properties of Coherent Light*

The present paper is concerned with the study of some general properties of coherent light. A clear definition of coherence is given, which appears to be preferable to definitions previously proposed by other authors. Several new theorems relating to correlation functions and the spectral density functions of coherent light are derived. The results are used to establish the Huygens-Fresnel principle for a coherent optical field. This principle has previously been freely applied to such a field, although the validity of the principle has only been justified for the much more idealized (and physically unrealizable) case of a field which is strictly monochromatic. The present formulation of the Huygens-Fresenel principle involves only observable quantities and not the instantaneous amplitudes and the instantaneous phases of the light vibrations.