Page 174 - Fundamentals of Light Microscopy and Electronic Imaging

THE DIC OPTICAL SYSTEM 157

The DIC Prism

The DIC prism, known technically as a Wollaston prism, is a beam splitter made of two
wedge-shaped slabs of quartz (Fig. 10-4). Since quartz is birefringent, incident rays of
linearly polarized light are split or sheared into two separate O- and E-ray components.
The shear axis (direction of shear) and the separation distance between the resultant O
and E rays are the same for all O- and E-ray pairs across the face of the prism. The E
vectors of the resultant O and E rays vibrate in mutually perpendicular planes as they do
for other birefringent materials.
In a standard Wollaston prism the optic axes of two cemented wedges of calcite or
quartz are oriented parallel to the outer surfaces of the prism and perpendicular to each
other. If such a prism is placed between two crossed polars and examined face-on, a pat-
tern of parallel interference fringes is observed due to interference between obliquely
pitched wavefronts of the O- and E-ray components. The interference fringes appear to
lie inside the prism at a location termed the interference plane. This makes it difficult to
use a conventional Wollaston prism for certain objective lenses, where the interference
plane of the prism must lie within the back focal plane (the diffraction plane) of the lens.
This is especially problematic when the diffraction plane lies within the lens itself. Note,
however, that the Smith T system of Leica incorporates Wollaston prisms into specially
modified objectives. Therefore, modified Wollaston prisms are generally used for the
objective lens, since the interference plane is displaced from the center of the prism to a
distance several millimeters away from the prism itself (refer to the next demonstration).
Such a prism does not have to be physically located in the aperture plane of the objec-
tive and is much easier to employ. Interference fringes in the field of view are also
avoided, and uniform contrasting is achieved.

Conventional Wollaston Modified Wollaston
prism (Nomarski) prism

Interference
planes

Figure 10-4
Design and action of a Wollaston prism. Wollaston and modified Wollaston (or Nomarski)
prisms are used in DIC microscopy to generate and recombine pairs of O and E rays. The
optic axes of the two quartz wedges comprising an objective prism are indicated (↔, ). The
oblique orientation of the optic axis of one of the wedges in a modified prism displaces the
interference plane to a site outside the prism. Interference and visualization of the
interference fringes require the action of the analyzer.