Cross-Polarized Crystal Photography
Part I: Introduction and a Layman's Explanation of the Physics
Text and photography copyright Thomas Webster 2003. All rights reserved.
Sulphur crystals produced from a "melt". Copyright Thomas L. Webster 2004
Introduction...I have viewed many wonderful sights through my microscope but few sights have thrilled me more than viewing microscopic crystals under crossed-polarized lighting. This is, in fact, one of the easiest types of lighting to accomplish for an amateur microscopist. All that is really required for basic crossed-polarized lighting are a couple of polarizing filters. These filters may be cut from flat sheets of polarizing material or may be purchased as round, mounted photographic filters. For my setup I purchased two linear polarizing filters from my local camera store for a total of around $30.00. Surplus Shed sells very reasonably priced polarizing filter sheets and circular filters that are suitable for crossed-polarizing filters. Sheet material is more versatile as it may be cut to size to fit filter holders or to fit in any other suitable position within the optical train of the microscope.
Aspirin crystals evaporated from ethol alcohol solution. Copyright Thomas L. Webster 2004

The physics made very simple... Light rays are composed of waves of light that vibrate in all planes. When light rays pass through a polarizing filter only those light rays that are vibrating parallel to the privileged direction of the polarizing filter are able to pass through the filter. This results in light rays that are all vibrating in the same plane of travel. The more oblique the light rays that strike the front surface of the filter the more these oblique light rays are filtered out. Perhaps you have noticed in your own photography that polarizing filters have their greatest effect on a landscape photograph when the scene being photographed is oriented 90° to the sun. In this orientation, the polarizing filter is blocking out the majority of oblique light rays that contribute to reflections and flare and thereby increases the saturation of the photograph.

Polarizing filters come in rotating mounts that allow the photographer to orient the privileged direction of the polarizing filter to obtain the highest degree of polarization. If the photographer were to "stack" two polarizing filters together, view a strong light source through the stacked filters, and then rotate the filters, the photographer would see that the light source would become dimmer and dimmer as the privileged direction of the two polarizing filters approach 90° in relationship to each other. Theoretically, when the privileged direction of the two filters cross at 90°, all light passing through the filters will blocked (extinguished) and the photographer would no longer be able to see the light source through the filters. In this orientation, the polarizing filters are said to be crossed, hence the term crossed-polarization. In reality, however, due to the quantum nature of light, a faintly visible and blue-shifted light source can still be seen. For photographic purposes, though, the extinction of the light is complete enough.

The splitting of light into two components (an ordinary light ray and an extraordinary light ray) by a crystaline substance is known as birefringence. Birefringence is also known as "double refraction". Birefringence is the result of the crystal material having two indices of refraction. One light ray is slowed down and color shifted compared to another light ray. Birefringence is caused by the atoms in a crystal having stronger bonds with one another in one direction and weaker bonds with one another in a second direction.

If all of the transmitted light is being extinguished by the crossed-polarizing filters, then how can a photographer photograph crystals under the microscope? As regards polarized light, birefringent crystals come in two "flavors". When struck by strongly polarized light, isotropic crystals allow transmitted light to pass through the crystal relatively unaltered. Since the second polarizing filter is crossed 90° in regards to the first polarizing filter, a photographer would not be able to view these crystals with crossed-polarizing filters. Anisotropic crystals, on the other hand, act as tiny prisms that break up transmitted white light into its constituent wavelengths of red, blue, green, yellow, and violet light. Uniquely, anisotropic crystals not only break up white light into its constituent wavelengths but anisotropic crystals rotate the constituent wavelengths 90° to plane of the polarized light transmitted through the crystals. The constituent light rays are then able to pass through the second polarizing filter and expose the film in the camera. The results are truly kaleidoscopic! (Continue to Part II...)

Front Page Articles Forums & Galleries Links About Us
Website design and graphics copyrighted Reasonable Expectations Productions 2004. All rights reserved. All images are copyrighted by the original artists/photographers. No content, neither written nor graphic, may be reproduced without expressed written permission of the copyright holders. Copyrights are filed accordingly with the Library of Congress. Infractions of the copyright laws are actively and aggressively litigated and may subject the defendant to actual and punitive damages as well as reimbursement of court and attorney costs. No exceptions! Content on the Internet may be free for public viewing. However, content on the Internet is not free for public use. Let's all work together to protect copyrighted works displayed on the Internet. These sites are best viewed with Microsoft Internet Explorer® version 5.5 or later. Web layout and design produced with Macromedia Dreamweaver® 6.01. Image preparation's accomplished with Adobe Photoshop® 6.01. Interactive content produced with Macromedia Flash® 5.0.