Is phase contrast microscopy just a type of dark-field microscopy?
No, that is a common misconception. Dark-field microscopy blocks the direct light and only lets diffracted light reach the eye, producing a bright image on a dark background. It reveals edges and fine details but loses information about the internal phase structure of the specimen. Phase contrast, by contrast, does not block the direct light; it shifts its phase relative to the diffracted light. This interference creates contrast that is directly proportional to the optical path difference—a quantitative measure, not just a qualitative outline. In my 1942 paper, I emphasized that phase contrast is a linear method, meaning the brightness is a linear function of phase, whereas dark-field is nonlinear and thus less suitable for measurement. Seeing is not believing—measuring is. Phase contrast allows measurement, not just visualization.
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