Nomarski

Nomarski refers to a method of differential interference contrast (DIC) microscopy developed by Georges Nomarski in the 1950s. It is designed to enhance the contrast of transparent or nearly transparent specimens, revealing fine details that are difficult to see with standard brightfield illumination. The technique is often called Nomarski interference contrast or simply Nomarski DIC.

Principle and operation: Nomarski DIC uses polarized light and a pair of Wollaston prisms (often referred to as Nomarski prisms) to split a beam into two shear-separated beams with orthogonal polarization. As these beams pass through the specimen, any optical path differences caused by refractive index variations or thickness gradients introduce phase differences between the beams. The beams are then recombined and passed through an analyzer, converting phase differences into intensity differences. The result is an image with enhanced edge contrast and a characteristic three-dimensional, shadowed appearance that emphasizes gradients in optical path length rather than absolute brightness.

Instrumentation and setup: A Nomarski DIC system requires polarized illumination, a pair of prisms to split and shear the beams, and an analyzer to convert interference into intensity. High numerical aperture objectives and proper Köhler illumination are typically used to maximize performance. The technique is commonly integrated into modern research microscopes as a standard contrast method alongside brightfield, fluorescence, and phase-contrast modes.

Applications and limitations: Nomarski DIC is widely used in biology and materials science to examine living, unstained cells, thin tissues, and surface features of transparent specimens. It provides rich topographic information and is non-destructive. Limitations include sensitivity to illumination and alignment, potential orientation-dependent artifacts, and the fact that it reveals optical path gradient rather than true optical thickness, which can complicate interpretation in some samples.