differential display PCRmedical dictionary

Method of image formation in the light microscope based on the method proposed by Nomarski (though strictly speaking all forms of optical microscopy rely to a greater or lesser extent on differential interference). The light beam is split by a Wollaston prism in the condenser, to form slightly divergent beams polarized at right angles. One passes through the specimen (and is retarded if the refractive index is greater) and one through the background nearby: the two are recombined in a second Wollaston prism in the objective and interfere to form an image. The image is spuriously three dimensional the nucleus, for example: appears to stand out above the cell (or be hollowed out) because it has a higher refractive index than the cytoplasm. The Nomarski system has the advantage that there is no phase halo, but the contrast is low and image formation with crowded cells is poor because the background does not differ from the specimen.

This entry appears with permission from the Dictionary of Cell and Molecular Biology

(11 Mar 2008)