Hybrid Polarizing Microscopes
Michael Reese Much
Bethlehem, Pennsylvania, USA
Cross-Polarization microscopy has intrigued many amateur microscopists because it opens new avenues of pictorial realization. Brian Johnston’s articles featuring stunning images of chemical melt and evaporative slides inspired me to explore cross-polarization techniques. Unfortunately, the prohibitive cost of a true polarizing microscope demanded that I find alternative approaches using the equipment already on hand. In this article I’ll demonstrate how I set up a standard trinocular compound microscope and a stereo microscope for cross-polarization photomicroscopy.
Trinocular Compound Microscope
Basically, in cross-polarization microscopy the light illuminating the specimen slide is polarized below the slide and then the image of the specimen is polarized again after it exits the objective. In a true polarizing microscope, the polarizer is incorporated in the sub-stage condenser to polarize the light source. The second polarizer—the analyzer—is positioned in the optical path before the eyepiece and/or the camera.
To set up a standard microscope for cross-polarization microscopy you will need two polarizing filters, which can be purchased at any camera store. Polarizing filters come in two types—circular and non-circular.
“Circular” when applied to polarizing filters does not refer to the shape of the filter. A circular polarizer has a quarter-wave filter incorporated into its construction. This type of filter is required for use with autofocus and auto-exposure cameras that may have beam splitters or semi-silvered mirrors in the autofocus and metering systems of the camera that rely to some degree on reflections to function correctly. The quarter-wave filter built into a circular polarizer will allow the camera systems to function normally. A non-circular polarizer in such a camera may cause autofocus and metering errors because it will reduce the effectiveness of the beam splitters and semi-silvered mirrors by reducing the reflections needed much as a polarizer does when taking the reflections out of a store window.
Using circular polarizers may reproduce some of the effects produced when using lambda plates in a polarizing microscope because of the quarter wave filter, and a circular polarizer polarizer combined with a circular polarizer analyzer might have a cumulative half-wave filtering effect, but I haven’t pursued this investigation as yet.
In the setup on my trinocular, I simply place the polarizer on the light source. It is not screwed onto a thread or otherwise secured to the microscope. The polarization of the light source is changed by rotating the filter by hand and visually assessing the effect on the camera’s monitor or through its viewfinder. An inexpensive polarizing filter can be used on the light source because the optical quality of the filter is not that vital to image quality. Not all polarizers are the same—some may have a more profound polarization effect and some may produce variations in the colors produced depending on the brand. I was in a camera shop the other day that had a bin each of used circular and non-circular polarizers at very affordable prices—a very good opportunity to stock up.
In my trinocular setup, the analyzer is located in the photo port—past the binocular eyepieces and before the camera. This means the binocular eyepieces can only be used for finding the subject area on the slide—the polarization effect cannot be seen through the binocular eyepieces; only through the camera viewfinder system.
When I first started using my hybrid cross-polarization setup, I was using an Olympus EVOLT E-500 with a Hoodman right angle viewfinder adapter. I found fine-focusing using the Hoodman finder at 1x very difficult and at 2.5x totally impossible. I switched to the Olympus EVOLT E-330 for the Live View on the LCD screen and the 10x magnification on the LCD in Live View Mode B and have found the viewing and fine focus superb. Plus, the E-330 has an articulated LCD screen which further enhances image assessment. The Olympus E-3 also has an articulated LCD screen which offers the same benefits.
The camera mount is custom-made from old Nikon and Pentax extension tubes. I have found that every time I fabricate a microscope camera mount I am starting from scratch. I recommend that anytime you see used extension tubes in a camera shop, snap them up. They are very handy to have around for building camera adapters. The eyepiece is an Olympus 2.5x photo eyepiece.
One surprising benefit of the Four-Thirds Standard’s sensor size is that using the 2.5x photo eyepiece provides full-frame images—not a circular image that requires severe cropping to make a standard print.
The analyzer—a 30mm Hoya non-circular polarizer—is simply set on top of the eyepiece. Because the image exits the eyepiece through the analyzer to the camera sensor, it is vital to buy a top-quality polarizer for this component. I have tested rotating the analyzer to different rotation points, and have found no difference in effect in relation to the rotation of the polarizer at the light source, so the analyzer can remain stationary.
The camera adapter is topped with an Olympus MF-1 OM Adaptor to mount the Olympus EVOLT E-330 or Olympus E-3 DSLR bodies to the microscope for photomicroscopy. I have built the rings from a punched-out polarizing filter into the mount so that I can rotate the camera freely while it is mounted to the microscope. This facilitates connecting cables and changing batteries without removing the camera. In addition it allows me to rotate the camera for compositional purposes in pictorial photomicroscopy.
I have read some discussion of the practicality of mounting a digital SLR to a microscope because of the vibration caused by the mirror/shutter cycle during exposure negatively affecting image sharpness—particularly at higher magnifications. I am getting very satisfactory results with my setup.
I use the ANTI-SHOCK option provided in Olympus E-System cameras which enables from 1 to 30 seconds of mirror lockup before the shutter trips. I use the RM-1 infrared remote control to trip the E-330 “hands off” and the E-3 also allows the use of the RM-CB1 Remote cable for remote shutter tripping. The cameras can also be remotely tripped and controlled using Olympus Studio Camera Control, which also saves the images on the computer automatically.
Below are two cross-polarized images of Ascorbic Acid shot at high magnification which can attest to the sharpness when shooting with a directly-mounted DSLR.
Stereo microscopes can be very easily be set up as polarizing microscopes—all that needs to be added is an analyzer and a polarizer. On the microscope shown, an Olympus C-4040Z is mounted on the right eyepiece. A wireless color CCD television camera is mounted on the photo port in a custom mount fitted with a 10x eyepiece. The video signal is sent to a receiver that is connected to the VIDEO IN jack on a combination TV/VCR. I found the photo port on this model microscope wasn’t giving me the quality I was looking for when I mounted the C-4040Z or an Olympus SP-320. I found mounting the camera on the eyepiece yielded better quality, though I don’t know why.
Stereo microscopes may be more suitable for larger specimens than a high-magnification compound microscope. I found that some of the larger clusters of chemical crystals offer pictorial images of great beauty when cross-polarized. Shown below are cross-polarized images of Ascorbic Acid:
The setup I am using uses a rather large polarizing filter (77 mm) removed from the rotation rings used to mount the filter on a camera lens. I did this for two reasons:
In addition, I use a small plastic stage on which I can put a slide or small glass plate to hold a specimen. The gap introduced by the stage results in any dust on the Plexiglas or polarizer to be out of focus, meaning less clean-up in post production.
This polarizer is not rotated. Also, this need not be a high quality filter since the image is not passing through it.
The Analyzer on the stereo microscope setup is positioned just below the objectives. I glued a filter ring to the opening of the objective cone on the scope. When I choose to use the stereo microscope for cross-polarized microscopy, I mount a polarizer in the filter ring.
The Analyzer is rotated to control the cross-polarization effect. Since the image passes through this filter it should be a high-quality polarizer.
Since users microscope setups tend to be unique, this article doesn’t presume to be universally applicable. My intention is to demonstrate solutions I have developed for my own cross-polarization techniques. Hopefully, I may have presented some ideas for your own pursuits in this fascinating microscopy technique.
Michael Reese Much is a senior digital technical support representative for a major manufacturer of digital products. He is also a fine art photographer and amateur microscopist.
All comments to the author Michael Much are welcomed.
Microscopy UK Front Page
Published in the March 2008 edition of
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