Photonics Dictionary

phase-contrast microscope

A microscope that has an annular stop in the lower focal plane of the condenser, and a quarter-wave retarding and absorbing ring in the upper focal plane of the objective that just fills the image of the annular substage stop. Direct light passing through a transparent object falls on the ring where it is retarded by a quarter-wave and reduced in intensity. Light diffracted by the phase differences in the transparent object because of variations in refractive index and/or thickness, misses the retardation ring and interferes with the direct light in the manner of an interference microscope. This leads to an image in which variations of phase in the object appear as variations in brightness.
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