Cross-Polarized Crystal Photography
Part II...Equipment Setup
Text and photography copyright Thomas Webster 2003. All rights reserved.

Equipment Setup...If you have a spare few hundred dollars to a spare few thousand dollars laying around you can purchase some very fine polarizing microscopes. If, on the other hand, you are as economically challenged as I am you can make your current microscope into a limited polarizing microscope with the addition of a couple of inexpensive polarizing filters.

Vintage 1960s Nikon SK-e Microscope. Copyright Thomas L. Webster 2003

As mentioned above, polarizing filter material may be purchased in flat sheets and cut to fit where necessary. In my setup (to the right) I was able to purchase 2 linear polarizing filters designed to be used on photographic camera lenses. Both filters together cost around $30.00 at my local camera store and are optically flat having little effect on the image transmitted to the film. Polarizing filters cut from sheets may not be totally optically flat but are still useable for observation purposes at least.

The important consideration here is that the specimen must lie between two polarizing filters and at least one of the filters must be accessible to be rotated. In my setup one of the polarizing filters (#1) simply rests atop the centering condenser and can be easily rotated. The #1 polarizing filter is the one I rotate to produce the maximum crossed-polarizing effects. The second polarizing filter (#2) sits within the macro bellows where indicated in the illustration to the right. The macro bellows comes apart easily at that location to insert the filter. The second polarizing filter actually sits above the projection ocular (hidden by the microscope adapter). Since my interest is in photomicrography it was of particular importance that this second polarizing filter be optically flat so as to not degrade the image projected onto the film in the camera.

Unfortunately, the position of the second polarizing filter does not allow me to preview the polarizing effects through the microscope's binocular eyepieces. To determine the effects of the crossed-polarizing filters I must preview the image through the right-angle finder attached to the camera. I have learned, however, to recognize areas of strong polarization when viewing the sample through the binocular eyepieces. Areas of strong polarization will look strongly 3-dimensional compared to the appearance of the rest of the sample. Also, areas of strong color will show weak colors when viewed through the binocular eyepieces. I scan the sample for these strongly 3-dimensional and colored areas and then switch to the right-angle finder on the camera to view the final image that will be projected on the film.

Black backgrounds when viewing or photographing your sample result from the near total extinction of background light between the two polarizing filters. It is therefore important where in the optical path you place additional filters. For viewing purposes a blue filter is generally used to cool down the light from the illuminator so that the light looks more white. For photographic purposes I insert an 80 A, B, or C filter so that the light striking the film is the proper temperature (in °K) to record colors properly on color film. The plastic material these filters are made of can cause the light from the illuminator to become depolarized. This results in gray backgrounds rather than black. Sometimes this can be attractive (as seen in the photo to the right) but at other times this can be very distracting and disappointing. It is important, then, to place any additional color correction filters before the first polarizing filter. In my setup I place the color correction filters below the centering condenser. Thus, light passing through the centering condenser is already color corrected before the light passes through the first polarizing filter. (Continue to Part III...)

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