George Nomarski invented the method of differential interference contrast for the microscopic observation of phase objects in 1953. The features on a phase object typically modulate the phase of an incident beam without significantly affecting the beam's amplitude. Examples include unstained biological samples having differing refractive indices from their surroundings, and reflective (as well as transmissive) surfaces containing digs, scratches, bumps, pits, or other surface-relief features that are smooth enough to reflect specularly the incident rays of light. A conventional microscope image of a phase object is usually faint, showing at best the effects of diffraction near the corners and sharp edges but revealing little information about the detailed structure of the sample.

Nomarski's method creates two slightly shifted, overlapping images of the same surface. The two images, being temporally coherent with respect to one another, optically interfere, producing contrast variations that contain useful information about the phase gradients across the sample's surface. In particular, a feature that has a slope in the direction of the imposed shear appears with a specific level of brightness that is distinct from other, differently sloping regions of the same sample.

The Nomarski microscope uses a Wollaston prism in the illumination path to produce two orthogonally polarized, slightly shifted bright spots at the sample's surface. Upon reflection from (or transmission through) the sample, the two beams are collected by the objective lens, then sent through the same (or, in the case of a transmission microscope, a similar) Wollaston prism, which recombines the two beams by sliding them back over each other.