Nomarski prism

A Nomarski prism is a birefringent optical element used in differential interference contrast (DIC) microscopy to split unpolarized light into two orthogonal polarization states with a controlled spatial offset. It is a variant of the Wollaston prism, consisting of two cemented calcite or quartz wedges cut at a specific angle to the optic axis. When light passes through, it emerges as two coherent beams separated by a few micrometers, laterally displaced but traveling in the same direction.

The device takes its name from Nomarsky (often spelled Nomarski), who refined the design for microscopy applications in the 1950s. Unlike a standard Wollaston prism, which produces a larger beam separation suitable for polarimetry, the Nomarski configuration creates a much smaller shear distance, typically 0.1 to 1 micrometer depending on the prism cut and wavelength. This tight spacing is essential for DIC, where the two beams recombine after passing through the specimen and interfere to produce contrast from minute optical path differences.

Nomarski prisms are sensitive to temperature and mechanical stress because their birefringence depends on refractive index, which shifts with thermal fluctuations. High-quality specimens use cemented designs with matched expansion coefficients to minimize drift over the operating range. The prism must be precisely oriented relative to the microscope's condenser aperture and analyzer; misalignment of even a few degrees degrades image contrast. Some systems include a compensator or wave plate to fine-tune the relative phase of the two interfering beams.

Use in microscopy and beyond

In transmitted-light microscopy, the Nomarski prism sits between the light source and condenser, or between specimen and objective, depending on the optical design. It allows visualization of unstained, transparent biological structures and thin films with sharp contrast and excellent optical sectioning. This makes it valuable for live-cell imaging, where fluorescence or staining would be disruptive. The technique also works in material science for inspecting thin coatings, crystal defects, and transparent polymers.

The main limitation is that DIC works best with coherent, collimated light and requires careful specimen preparation. Thick or opaque samples scatter light excessively, destroying the interference signal. Nomarski prisms can be damaged by mechanical shock or immersion in incompatible solvents; they are typically stored dry and handled only by their mount edges. Replacement prisms are expensive and must be matched to the microscope's magnification and wavelength range, so repair often means sending the entire optical module back to the manufacturer.