Microscope Activities, 17: Rheinberg Illumination

In the past, Hooke College of Applied Sciences offered a microscopy workshop for middle school and high school science teachers. We thought that these basic microscope techniques would be of interest not only for science teachers, but also for homeschoolers and amateur microscopists. The activities were originally designed for a Boreal/Motic monocular microscope, but the Discussion and Task sections are transferable to most microscopes. You may complete these 36 activities in consecutive order as presented in the original classroom workshop, or skip around to those you find interesting or helpful. We hope you will find these online microscope activities valuable.

EXPERIMENT 17: Rheinberg Illumination

Goal

To make and use Rheinberg filters to achieve differential color illumination.

Level

Basic

Materials Needed

Transparent color filters made from gelatin or acetate or polypropylene, etc. as used in photography and in theatrical stage lighting and professional studio lighting; even candy wrappers can be used, such as the transparent red wrapper from anise-flavored hard candies. A theatrical gel sample or swatch booklet, such as the Roscolux available from Rosco, would be ideal. You will also need a paper punch (~¼”), and an acetate sheet protector.

Procedure

Cut two or three discs of a variety of colors that have an outside diameter equal to the diameter of your substage filter tray (about 32 mm in our case), and punch out or cut a ¼” hole out of the center of each of the larger discs. The cuts may be made freehand, or coins or a compass may be used for neater versions.

Next, combine different colored large discs with complimentary or contrasting center discs. You may want to stack two or more layers of the same color to deepen the hue. You may also want to try adding waxed paper or neutral density grays to the center stop to cut the intensity way down. Or, you may want to cut the outside disc, or “ring” in half, and substitute another color for the other outside half. Or, make a four-sector outside ring of, say, red-green-red-green, combined with a blue center spot.

Whichever color combinations you wish to start with, place the color combination between the leaves of a folded acetate sheet; about 1½” x 3”, as you did for the darkfield stop. Figure 17-1 illustrates a sample booklet of color filters, several freehand cut discs with centers punched out, several central colored discs, and a color combination mounted in a double layer of acetate, hinged at the narrow end.

Finally, place the color combination on top the lower swing-out ground glass, just as you did with the darkfield stop, and swing the colored filters into the light path. Make sure that the aperture diaphragm is wide open, and adjust the condenser height to optimize the image.

Discussion

Julius Rheinberg was a young man when, in the 1880s, he was utilizing darkfield illumination with darkfield stops of his own making. The sample was, of course, white on a black background. Then he put various color filters in the light path, and obtained red, or green, or blue, etc. sample against a black background. Then he reasoned that there was no compelling reason to make the central stop opaque so as to obtain a black background; the background could, in fact, be of any color you desire! Simply substitute for the black opaque central stop whatever color you wish the background to be. You can control both the hue and intensity of the background by stacking layers of the same color to darken the hue, and control the intensity by adding neutral density filters, or using frosted or waxed paper.
The color of the specimen will be determined by the color(s) of the outside ring. This outside ring can be a simple color, two colors (half and half), four-sector, etc. Using multiple colors, the sample can be lighted with a different color coming from different angles. The possibilities are limitless.

Because of the extreme beauty and unlimited color combinations, Rheinberg Illumination has been popular for the last 130 years, or so, and photomicrographs made with Rheinberg differential color illumination have been prize winners in numerous photomicrography contests over the years.

Although commercial units have been produced to provide these colors—Zeiss made a Rheinberg condenser (Mikropolychromar) in the 1930s, and Bausch & Lomb had a set made with Kodak Wratten gelatin filters—the best results are still achieved by making your own.

Task

Using your 10X objective, focus on a sample of Radiolaria or Foraminifera, or anything else you choose. Make a couple of sets of Rheinberg filters using the Roscolux sample swatch booklet of filters. Try 1, 2, 3, or more layers of the same color to intensify them. Compare light-on-dark, and dark-on-light colors. Make a set of Rheinberg filters using your school colors, or patriotic colors, or pastels.