An Article In Meteorite-Times Magazine
by Tom Phillips and Martin Horejsi


Differential Interference Contrast

DIC (Differential Interference Contrast) Read this segment of a write up on this technique taken from a microscope Web Site and you will see why I just had to try it on meteorites. I am always using equipment and techniques in ways they were never designed for.

Reflected light microscopy is one of the most common techniques applied in the examination of opaque specimens that are usually highly reflective and, therefore, do not absorb or transmit a significant amount of the incident light. Slopes, valleys, and other discontinuities on the surface of the specimen create optical path differences, which are transformed by reflected light DIC microscopy into amplitude or intensity variations that reveal a topographical profile. Unlike the situation with transmitted light and semi-transparent phase specimens, the image created in reflected light DIC can often be interpreted as a true three-dimensional representation of the surface geometry, provided a clear distinction can be realized between raised and lowered regions in the specimen.

A significant difference between differential interference contrast in transmitted and reflected light microscopy is that two Nomarski (or Wollaston) prisms are required for beam shearing and recombination in the former technique, whereas only a single prism is necessary in the reflected light configuration. Light passes through the same Nomarski prism twice, traveling in opposite directions, with reflected light DIC. The shear produced when the light waves pass through the prism on the way to the objective is canceled during their second journey through the prism upon returning from the specimen surface. In this regard, the Nomarski prism and objective serve an identical function for incoming light waves as the first prism and condenser optical system in a transmitted light microscope. Similarly, light reflected from the specimen surface is gathered by the objective and focused into the Nomarski prism interference plane (conjugate to the objective rear focal plane), analogous to the manner in which these components function in transmitted light.

My intent was to capture differences in the reflective properties of various locations on a meteorite slice. Several people emailed me about the October Micro Visions, where I showed two images with what appears to be Widmanstatten Patters in an iron fleck, I used this technique to bring those patterns out. I have no idea if this is in any way useful, but it sure does produce some colorful images.

I have included a before and after combination on two of the images just to show you what a difference this technique makes. The rest are just for fun!

Tom Phillips can be reached by email at:
STARSANDSCOPES@aol.com

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