Closely related to darkfield illumination is the Rheinberg illumination. The light stop is replaced by colored filter material. Surrounding this filter are filters of one or more colors that contrast with the color of the central filter. The central filter will be the same size as a darkfield stop would be for the objective in use. One must first work out darkfield setup for each objective before trying to work out the corresponding Rheinberg setup. Rheinberg illumination combines normal brightfield illumination in one color with darkfield illumination in another color. When light from the center filter strikes the condenser, it will throw a small, solid cone of light upward after focusing in the plane of specimen. Surrounding this cone will be a hollow cone of light from the surrounding filters.

At the focus in the plane of the specimen, the light from the center filter will be moving roughly straight upward, while light from the surrounding filters will arrive at an angle. The background will be lit by the center filter while three-dimensional objects will pick up light from the peripheral filters. The central filter must be quite dark or the background will not contrast with the objects in the specimen. In fact, a very useful hybrid of Rheinberg and darkfield uses an opaque central light stop bounded by two or more colors. The colored filter material can be assembled into a Rheinberg filter by using small drops of cyanoacrylate glue. Or, the components can be sandwiched between sheets of plastic or glass. Each filter can be built to work with a particular objective and specimen color. Because of this, your collection of Rheinberg filters may grow quite large. Rheinberg illumination rarely provides any more real information than is supplied by darkfield illumination. Rheinberg does, however, make beautiful color effects and spectacular photographs possible. The central disk of a Rheinberg filter can be made of polarizing material instead of a color filter. The specimen is viewed through an analyzer. The only light that will get to the eye by transmitting straight up through the specimen will be light that passes through birefringent particles in the specimen. Any other light reaching the eye will be from the colored outer filters. Centrally stopped illumination setups are often enhanced if the slide is oiled to the condenser. Use type B slide oil, which is thicker than type A. The oil makes the background as dark as possible; otherwise some fogging and loss of light will occur as light bounces around between the condenser and the slide. There are, however, disadvantages to oiling the slide to the condenser. Because the light will be traveling at different angles, you may have to reconfigure the illumination setup. Also, it is inconvenient to clean oil from the stage when changing illumination setups, and it is inconvenient to clean the oil from slides when changing specimens. A finishing point needs to be made about both darkfield and Rheinberg illumination. They are aesthetically pleasing to work with. For instance, the tiny protists that inhabit pond water look slimy under brightfield illumination. Under darkfield illumination they are transformed into animate jewels, swimming about in a glittery field of plankton and bacteria. When using one of these lighting techniques, it is easy to lose yourself for hours watching the antics of these tiny creatures. No novice microscopist should miss the experience. There is one grave disadvantage to darkfield illumination. There is little undiffracted light to interact with the diffracted light. This results in decreased resolution. Annular brightfield illumination can be set up in much the same way as darkfield illumination, except that the size of the stop is reduced so that a few of the directly transmitted rays can reach the objective. The startling color and value contrasts of darkfield illumination are traded for increased resolution. Since the light does not need to be as steeply angled as with darkfield illumination, you may even be able to improvise annular brightfield illumination with the 100X objective.