Nomarski Interference Contrast

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Differential Interference Contrast (DIC) is also known as Nomarski Interference Contrast (NIC) or simply Nomarski microscopy. Nomarski Interference Contrast, which was developed by Polish physicist Georges Nomarski, uses a relatively complex optical system to generate visible contrast in unstained specimens from differences in the refractive index of the various points in the specimen. A DIC image not only includes the detail that would be visible in a standard brightfield image of the specimen, but also includes contrast which represents gradients in the refractive index of the specimen.

The condenser in a typical Nomarski-equipped microscope contains a linear polarizer, which produces linearly polarized light from the illumination source, and a Wollaston prism, which splits the linearly polarized light into two mutually coherent yet orthogonally polarized components that are spatially displaced (i.e., sheared) by a slight amount. These two spatially displaced components pass through the specimen and the objective lens in the normal fashion and are then converted to the same polarization and recombined, with the original shear removed, by a second Wollaston prism located between the objectives and the oculars.

The two orthogonally polarized lighting components reaching the second Wollaston prism each contain what is essentially a standard brightfield image, with one spatially displaced from the other. Although these brightfield images, if viewed separately, would appear the same (neglecting the slight shear), each differs from the other in relative phasing, on a point-by-point basis, due to the point-by-point differences in the refractive index of the specimen experienced by the two sheared components.

Although phase differences are not directly visible to the human eye (which is why the two brightfield images carried in the two sheared components would appear identical if you neglect the shear), when the two components are recombined by the second Wollaston prism, constructive and deconstructive interference between the two sheared components creates visible contrast in the resulting image representing the path-length differences seen by the two sheared components (which were caused by differences in the refractive index of various points within the specimen).