How could one measure degree of coherence with Fresnel biprism? Hello. I have some questions about the study of partial coherence with a Fresnel biprism. In common text degree of coherence and its relationship to visibility is introduced considering a Young Double Slit experiment. At the same time, in basic texts, the Fresnel biprism is introduced as an equivalent to Young's wave-front split interferometer. HOWEVER, when studying partial coherence one measures visibility of fringes produced by a Young interferometer BECAUSE one is studying to correlation of the fields AT PINHOLES. Therefore giving information about the relationship between to laterally spaced points of the field. I don't see how this could happen with a Fresnel biprism and my question is: 1) Which two points, pinholes, or regions of space am I refering to, if any? Although I am mainly interested in this first question, I pose another one. 2) In a Young experiment if one varies the width, for example, of the source, the pinhole dimensions stay the same. In Fresnel birprism when the width of the source varies so do the virtual sources. Won't this affect meausres of visibility? I am seeing this differences and probably I am not considering/understanding something. Thanks in advance to everyone.