A variation on dark field illumination adds the element of color contrast to transparent objects that are difficult to image clearly any other way. While dark field gives you a black background and glowing subject, you can increase the visibility even more on difficult to image subjects such as totally clear protozoans or very clear crystals. The idea behind Rheinberg illumination is to give the background and subject contrasting colors, such as a blue background and yellow back illumination. Here are some experiments with granular sugar crystals, which I found to be not only cubic but some are rectangular in shape as well, unlike salt which is always cubes.

Please click on images for the 1290 x 960 images. (Reduced from the 10 megapixel originals)

Graphic from the web site Molecular Expressions showing how Rheinberg illumination works. A bi colored filter is inserted in the condenser and projects two cones of colored light upward. The inner cone will be the background, and the backlighting for dispersion is the ring of light outside the inner cone. Ill show you some actual microscope stage shots to show you how this actually works.

Rheinberg masks are made by using colored markers or cellophane on a sliding plastic base to go into the side of the condensers. They provide a slit for this for both dark field illumination ( upper piece) and several types of Rheinberg illumination. The central disk is the background color, and the ring outside of it will become the back illumination for the transparent subject.

Slid into the condenser and ready to go.

Under the microscope stage we mount the condenser, and you can see the plastic strip with the colored areas inserted.

It is quite hard to get a clear shot of what Rheinberg illumination looks like. Here, the condenser is in focus and concentrates the outer ring to a sharp pin point on the specimen. But you cant see the inner blue cone until you do this....

By lifting the slide (which is frosted for clarity) up, the blue cone becomes visible. THIS is what the microscopes objective lens sees - a blue background but not the yellow ring.

This is a standard dark field shot (using only a black mask for the central disk and a clear ring around to back illuminate) Sugar at 60x shows a variety of shapes, all cubic and many are rectangular.

With Rheinberg Illumination, we here use a blue center surrounded by a red ring. Note the nice color contrast of the crystals now!

Using a green disk and red ring did not work quite as well but is interesting. 60x

Better is the most used colors - blue background (center disk) and yellow back illumination (outer ring). 60x Small clear marine and freshwater invertebrates show up very well with this color set, and look natural too.

The red background and green ring was not as nice.

Now we move up to the 10x objective yielding around 150x on this image. This of course is a dark field shot of the crystals.

A blue background and red ring makes the shot far more exciting and more detail can be seen.

A blue background and yellow back illumination is very clear as well.   In summary, Rheinberg illumination will be come a powerful tool in the future for my imaging work and I am looking forward to trying it out on many other materials and living things!