We review some new results about recently proposed intrinsic degrees of coherence of partially polarized light. We show that they can be derived from an invariance principle equivalent to that used to derive the standard degree of coherence of scalar fields. We compare them with the degree of coherence recently proposed by Wolf. We illustrate the difference and the complementarity between these two definitions on a simple example of optical experiment.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.