Micro-World Gems & Gemology, Fall 2015, Vol. 51, No. 3

Modified Rheinberg Illumination

Painted glass slides create filter.
Figure 1. In one method of applying Rheinberg-type illumination, glass slides are fitted to the end of two fiber-optic illuminators and painted with red and blue ink (left) to create a filter that is easily positioned by moving the colored portion of the glass slide into the light path (right). Photos by Danny J. Sanchez.

Lighting control is one of the most important considerations for maximizing the use of the gemological microscope; with greater control over illumination sources, more information may be gathered from observing a specimen. An interesting technique that gives the microscopist another lighting tool is modified Rheinberg illumination, also known as differential color illumination (M. Pluta, Advanced Light Microscopy: Specialized Methods, Vol. 2, PWN-Polish Scientific Publishers, Warsaw, 1989, pp. 113). This method, as modified for gemological microscopy, employs the use of a contrasting color filter between each illumination source and the subject (figure 1) to achieve an “optical staining” effect (figure 2). When viewing crystallographically aligned subjects such as negative crystals or inclusions with well-defined, reflective crystal faces, each illumination source highlights areas that have the same crystallographic orientation (figure 3). This provides dramatic false-color contrast to an otherwise low-contrast subject. This enhanced contrast makes it easier to observe the relationship between areas in an inclusion scene with identical and also differing crystallographic orientations. 

Sulfide inclusion in fluorite seen with Rhineberg illumination.
Figure 2. This silvery metal sulfide inclusion in fluorite shows angular crystal faces, but the low contrast in the image on the left makes it difficult to resolve the orientation of the faces from different sections of the inclusion. Rheinberg illumination provides stark contrast between differently oriented regions by using two colored illumination sources that reflect their independent color off of crystallographically aligned crystal faces, as seen in the center and right images. This allows the microscopist to easily observe the different orientations of the crystal faces in the metal sulfide inclusion, as evidenced by the dramatic boundary of the two contrasting colors. Photomicrographs by Danny J. Sanchez; field of view 1.66 mm.
Crystallographically-aligned negative crystals in quartz.
Figure 3. With modified Rheinberg illumination to impart contrasting color on some of the crystal faces, the negative crystals in the rock crystal quartz (left) prove to be crystallographically aligned (right). Photomicrographs by Nathan Renfro; field of view 7.34 mm.

Nathan Renfro is the analytical manager of the gem identification department and analytical microscopist in the inclusion research department at GIA in Carlsbad, California. Danny J. Sanchez is an independent photomicrographer based in Los Angeles.