  The modern definition of optical coherence highlights a frequency dependent
function based on a matrix of spectra and cross-spectra. Due to general
properties of matrices, such a function is invariant in changes of basis. In
this article, we attempt to measure the proximity of two stationary fields by a
real and positive number between 0 and 1. The extremal values will correspond
to uncorrelation and linear dependence, similar to a correlation coefficient
which measures linear links between random variables. We show that these
"indices of coherence" are generally not symmetric, and not unique. We study
and we illustrate this problem together for one-dimensional and two-dimensional
fields in the framework of stationary processes.
