DIC-Differential Interference Contrast
Differential interference contrast (DIC) imaging, also known as Nomarski imaging, is one of the fundamental contrast-enhancement techniques used in the examination of highly reflective and opaque specimens. Slopes, valleys, and other discontinuities on the surface of the specimen create optical path differences, which are transformed by the reflected light DIC image-forming system into intensity variations that reveal a topographical profile. The image created in reflected light DIC can often be interpreted as a true three-dimensional representation of the surface geometry.
The key optical train components in a reflected light differential interference contrast system is presented in the figure below. The illumination light beam first passes through a linear polarizer, the linearly polarized light exiting the polarizer is reflected from the surface of a half-mirror placed at a 45-degree angle to the incident beam. The deflected light waves, which are now traveling along the optical axis, enter a Nomarski prism where they are separated into polarized orthogonal components and sheared according to the geometry of the birefringent prism. Then the objective lens focuses the sheared orthogonal wavefronts onto the surface of an opaque specimen. The reflected wavefronts, which experience varying optical path differences as a function of specimen surface topography, are gathered by the objective lens and focused on the interference plane of the Nomarski prism where they are recombined to eliminate shear. After exiting the Nomarski prism, the wavefronts pass through the half-mirror on a straight trajectory, encountering the analyzer. Components of the orthogonal wavefronts that are parallel to the analyzer transmission vector are able to pass through in a common azimuth, and subsequently undergo interference in the image plane of the camera to generate amplitude fluctuations and form the DIC image. Formation of the final image is the result of interference between two distinct wavefronts that reach the image plane slightly out of phase with each other.