a method of producing images of microscopic objects whose structural elements have refractive indexes and optical absorptivities that differ by such small amounts that the elements are indistinguishable in other methods of observing and producing images in a microscope. However, such structural elements introduce light-wave phase shifts that may differ appreciably from one another, producing variations in phase that cannot be detected by either the eye or a photographic emulsion.

Phase contrast consists in the conversion of the phase variations, by means of an optical accessory, into variations in the intensities—that is, in the amplitudes—of the light waves. Such amplitude variations can be detected by a photodetector. Phase contrast was developed by F. Zernike in 1935. (SeeMICROSCOPE: Methods of illumination and observation [microscopy].)