Nomarski Prisms for DIC-Microscopy

When the refraction index is changed (between the cemented quartz wedges), an angular splitting or shear of the wavefronts occurs, as a result of which the waves are spatially separated by an angle termed the shear angle. Moreover, the wavefront are interchanged at the prism wedge boundary - the ordinary wave becomes the extraordinary and vice versa. The shear angle and the separation distance in the prism are constant for all incident wavefronts, regardless of the entry point, and the direction of wavefront shear is defined by the prism shear axis. The mentioned axis lies in the longitudinal plane of a WP and is parallel to the direction of the optical axis of the lower quartz wedge. In general, Nomarski prism produces the shear distance which is less than one micrometer so that a beam of light emerging from the prism has no observable separation.

As a result of the refractive index difference of the ordinary and extraordinary wavefronts, propagation velocities for the two waves as they pass through the lower and upper sections of a WP are also different. In the case where the geometrical paths through the lower and upper halves of a prism are identical, a phase shift of the wavefronts in the lower section of a prism is exactly compensated for in its upper half. However, wavefronts incident on a prism at a distance from its  center pass a longer way either through the lower or through the upper half of the prism, which results in a constant phase shift per unit length in the direction of the shear axis. The phase shift for the ordinary and the extraordinary wavefront is equal in magnitude but opposite in direction. As a result, at one end of the prism the extraordinary wavefront emerges before the ordinary one, whereas at the corresponding point of the opposite end, the ordinary wavefront emerges before the extraordinary one. The focal, or interference, plane of a WP is located in the center of the prism where the optical paths between the lower and the upper quartz wedges are equal. For this reason, it is often difficult (or even impossible) to use a WP with standard microscope objectives, for their rear focal plane (which must physically match the interference plane of a prism) is often located deep inside a lens.

Similar to a traditional WP, the construction of a basic Nomarski prism also consists of two quartz wedges which are cemented together at the hypotenuse. The upper wedge is identical to the upper wedge of a traditional WP, however the lower wedge is cut from a quartz crystal so that its optical axis is directed at an oblique angle to the flat face of a prism. When the wedges are combined to form a birefringent prism, the focal (interference) plane is several millimeters outside the prism plate. This effect occurs due to the shear that takes place at the "air-quartz" interface in the lower section of the wedge prism, unlike the cemented boundary, as in a WP. As a result of refraction at the interface between the quartz wedges in a Nomarski prism, the shear wavefronts converge at an intersection point outside the prism. The actual position of the focal plane of a Nomarski prism can be adjusted at a range of several millimeters by means of altering the oblique angle of the optical axis in the lower quartz wedge of a prism.

To avoid the aperture clearance problem of the objective, modern DIC-microscopes are often equipped with Nomarski prisms. In fact, modified prisms are commonly used for shearing and recombination of both beams in the condenser and objective focal planes. Notwithstanding fewer special constraints for condenser prisms, which can often be positioned precisely in the aperture plane, a Nomarski prism is still used in most cases. Nomarski prisms for condensers can be constructed in a specific way so as to form an interference plane located much closer to the prism than in prisms used with objectives. As a result, apart from the fact that two Nomarski prisms are installed inframes with different geometries in a modern microscope, they are often cut in a different way and, consequently, are not interchangeable.