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Differential Interference Contrast (Nomarski, DIC, Hoffman Modulation Contrast)

The thickness of most specimens prevents all parts from coming into focus all at once, limiting the usefulness of higher magnification lenses. D.I.C. and related optics give a specimen a three dimensional appearance that is not unlike the appearance of a specimen in a scanning electron microscope. These methods enhance depth of focus so that thicker specimens can be observed at higher magnifications.

A nice specimen to try is Chaos (Pelomyxa) carolinensis, especially when the Chaos contain ingested Paramecium in their food vacuoles.

Principle

Differential interference microscopy requires several optical components, therefore it can be very expensive to set up. Light from an incandescent source is passed through a polarizer, so that all of the light getting through must vibrate in a single plane. The beam is then passed through a prism that separates it into components that are separated by a very small distance - equal to the resolution of the objective lens. The beams pass through the condenser, then the specimen

In any part of the specimen in which adjacent regions differ in refractive index the two beams are delayed or refracted differently. When they are recombined by a second prism in the objective lens there are differences in brightness corresponding to differences in refractive index or thickness in the specimen. Regions such as the edge of a cell or nucleus are very distinct because the quality of the specimen changes so much over a very short distance.

One or more components of the system are adjustable to obtain the maximum contrast. When the contrast is optimized one can obtain a very distinct image that appears three dimensional. The effect is very much like what you see when a subject is shadowed by a strong light coming from one side, as with craters on the moon near the terminator, namely the boundary between the sunlit portion of the Moon's surface and the dark side.

Mimicking a DIC effect

If you have a condenser that is adjustable, that is, it can be placed off center, you can produce a shadowing effect that is very much like a true DIC image. You simply need to experiment to find the best position for the condenser for each objective lens. Generally you need a lot of light, since the effect is obtained by selective reduction of illumination.