Nomarski Prism Action in Polarized Light
When a Nomarski or modified Wollaston compound differential interference contrast (DIC) prism is sandwiched between two crossed polarizers and examined with light transmitted through both polarizers and the prism, a pattern of parallel interference fringes with a predominant central black band (fringe) can be observed. This interactive tutorial explores how varying prism wedge geometry, utilized for different objective numerical apertures, affects the interference pattern observed between crossed polarizers.
The tutorial initializes with a randomly selected objective prism (10x through 60x) positioned between two polarizers (commonly referred to as a polarizer and an analyzer), which are oriented randomly with respect to the transmission vibration planes. In order to operate the tutorial, use the Analyzer Orientation slider to rotate the upper polarizer (analyzer) through its full 180-degree range of motion. As the slider is translated, the Nomarski prism can be viewed at zero degrees (polarizer and analyzer transmission directions parallel), through 90 degrees (polarizer and analyzer crossed), on to 180 degrees (polarizer and analyzer transmission directions parallel, once again). As the analyzer is rotated, the interference pattern observed in the Nomarski prism window changes from two parallel dark fringes with peripheral colored fringes (polarizers parallel) to a predominant central dark fringe flanked by colored fringes (polarizers crossed). To choose a different Nomarski prism, use the Objective Magnification radio buttons positioned beneath the slider.
The patterns observed from examining the Nomarski prisms with crossed polarizers result from interference between the obliquely pitched ordinary and extraordinary wavefronts emerging across the face of the prism. To the left and right of the central dark interference fringe, the peripheral fringes display increasing orders of the classical polarization interference color spectrum. Prism wedges designed for objectives having different focal lengths and numerical apertures are cut at increasingly shallow angles (as magnification and numerical aperture increase) to yield narrower interference fringe banding. This concept is illustrated on the right-hand side of Figure 1 for a series of fixed Nomarski prisms designed for successively higher objective magnifications (indicated in the figure).
If a first-order compensator (red plate) is added to the crossed polarizer sandwich in a diagonal position (not illustrated in Figure 1), the black fringe is replaced with interference colors showing subtraction (yellow) on one side and addition (blue) on the other side of the original dark fringe position. Adding a second Wollaston or Nomarski prism on top of the first will compensate the phase shifts (and resulting interference fringes) of the first prism across the entire length, resulting in extinction (illustrated in Figure 1; note that this effect can only be observed if the two prisms employed for the experiment have the same shear angle). By translating one of the prisms laterally with respect to the other, a uniform bias, or change in path length, will be introduced and can be observed through the sandwich (Figure 1). Sliding the prism in one direction will darken and then lighten the prisms, while sliding it in the other direction will produce a series of uniform interference colors (starting with first-order yellow).
Douglas B. Murphy - Department of Cell Biology and Anatomy and Microscope Facility, Johns Hopkins University School of Medicine, 725 N. Wolfe Street, 107 WBSB, Baltimore, Maryland 21205.
Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.
Matthew Parry-Hill, Robert T. Sutter, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.