Phase Contrast technique: a broader approach
  The treatment found on most general optics textbooks related to the phase contrast technique imposes limitations on the filter phase and object phase variations in order to mathematically explain it in a simple manner. We consider that this simplified treatment can be misleading in terms of the concept the student may develop and also about the potential applications of the phase contrast technique. In this paper we describe a broader and yet simple explanation of the phase contrast process, creating a parallelism between optical image processing and interferometry. 
David Sanchez-de-la-Llave, M. David Iturbe-Castillo, Carlos G   Trevino-Palacios