In phase contrast microscopy there are various sources of unwanted stray light. Two that predominate in practice and are associated with the phase contrast technique itself occur when (1) the direct and the diffracted light cannot be completely separated by a phase plate (spatial filter) of a practically reasonable size and (2) the phase plate itself produces some light diffraction/scattering and reflections that diminish the sensitivity and imaging contrast of the phase contrast method. In the past, technological or engineering progress in conventional phase contrast instrumentation was mainly achieved by systematic reduction of stray light. An overview of this progress, supplemented by some unknown research results, is presented with an emphasis on the reduction of light reflections from the phase plate.
"Stray-light problem in phase contrast microscopy or toward highly sensitive phase contrast devices: a review," Optical Engineering 32(12), (1 December 1993). https://doi.org/10.1117/12.148714