… subjects that often appear to be well understood and perhaps even a little old-fashioned have frequently some surprises in store for us.
—E. Wolf 
The mutual coherence of the optical field is
where E(r, t) is the electric field at spatial position r and time t. For simplicity, we ignore the polarization of the field, which could be accounted for by a tensor-valued mutual coherence. The angular brackets signify the expected value of the terms contained over an ensemble of identical physical systems. The mutual coherence and related functions described in this section are of interest because