Differential Interference Contrast

1.     A plane-polarized wavefront created by the action of the condenser and polarizer passes through the Wollaston prism (WP1).

2. The Wollaston prism splits the polarized wave front into two orthoginally vibrating, spatially diverging plane-polarized wavefronts. This prism also acts to "compensate" for off-axis light as compared to optical axis illumination so that only sample phasing contributes to image contrast. (i.e., WP1 acts like a "universal Slit" illuminator).

3. The angular divergence, e1, is transformed into a spatial separation (delta; shear) by the Köhler condenser. The separation is less than the diameter of the Airy Disc at that magnification. The separation of wavefronts by WPI is called "Shear" (d).

4.     Local optical differences within the specimen, caused by differences in optical path (n x l), create a pair of phase-distorted wavefronts, that are imaged by the objective lens. These two wavefronts ("difference images") show a phase separation that was induced by local variations in the sample. This is called "Sample Phasing" (Greek letter Delta).

5. The upper Wollaston prism, which is oriented inverted from the lower Wollaston prism, merges the two sheared and phase-distorted wavefronts. These two are out of phase due to the phase distortion induced by the sample (Delta), but they do not interfere because they are vibrating orthogonally.

6.     If the upper prism is shifted left or right from the center position, one wavefront will be additionally advanced or retarded relative to the other (phase offset caused by light passing through different axial portions of WP2). The change of the relative phase distortion caused by WP2 (Wollaston Prism 2) is called "bias" (Greek Gamma), because it biases the phase offset of the difference images, either retarding one relative to the other or vise versa).

7.     The wavefronts then pass through the analyzer (Ana, 45° to WPs) where the two vibrational wavefronts are are recombined into one vibrational plane. Constructive or destructive wave interference occurs at this point. This causes optical path differences within the sample to be manifested as light or dark areas in the image.

8. In accentuating the slight amount of sample phasing very slight OPL gradients in the sample can Interfere to create visible sample contrast. This contrast can be modified by changing the translational (left/right) position of the upper Wollaston prism.

9.     Where there is no phase difference between ray pairs (i.e., the flat surface of a sample, or the background), interference of equally phased rays produces a uniform gray background.

 Thus it is the interference of two difference images that results in the contrast of the image visualized. This imaging method is therefore appropriately called differential interference contrast.

Pluta (1989) uses the term differential to refer to the small changes in optical path (phase change plus specimen thickness) over an infinitesimally small variation of sample lateral distance. When measured across a phase object of thickness t, refractive index n, and immersed in a medium of refractive index nM, the change in phase of the specimen (d) is given as dd = (nMn)dt. The term dt refers to the object thickness differential corresponding to its lateral dimension: dx. The optical path difference between the two interacting wavefronts is directly proportional to the specimen optical path difference (Pluta, 1989, pp. 153–170).

Schematic diagram of a Differential Interference Microscope. Note that four (4) matched components are required for DIC:

Lower and Upper Polarizer
Lower and Upper Wollaston Prism ("compensator" and "W")

From: Handbook of Optics, V1. Edited by: Bass, Michael; Mahajan, Virendra N. © 2010 McGraw-Hill