Geologists like me teach students and conduct research with microscopes called "polarizing" or "pol" for short. After development in the late 19th century, pol scopes became critical tools for identifying the minerals and structures of rocks (our Earth), and for determining much about how they formed. Such users are petrologists, who are doing petrography. This type of microscope is also used by mineralogists, geophysicists, soil scientists, crystal chemists, materials engineers, medical researchers, asbestos consultants, and many others including biologists. The method that can be employed with these instruments is called PLM (polarized light microscopy). "Simple polarizing" scopes are usually just biological microscopes with pol filters, one inserted under the head and another in a substage filter holder. Any scope can be so modified, including stereo microscopes (but the common style is a compound microscope). Those that are designed for geological work are "petrographic" (="rock imaging") microscopes, which have accessories to assist mineral studies in rock sections.In my research lab, an Olympus BH-2 BHSP polarizing petrographic microscope (below) is serving me well, and I am convinced that it is the best of the 20th century "benchtop" pol microscopes.
You might disagree with this assessment, but in my research and business I have used more than two dozen different petrographic microscopes of many brands and models. Actually, I liked them all, but some were better than others in design, function, and quality. Six of the best in my experience are the subject of my article in the internet journal called Micscape, also linked below.
There is nothing too mysterious or strange about the polarized light accessories used in a petrographic microscope. Its brightfield capabilities are the same as for any good biological microscope, with the addition of polarizing filters placed above and below the sample, at least one of which can be easily moved in and out of the light path. The circular stage rotates to show how different orientations of the sample affect polarized light, but it can also be fixed in place for other uses. A Bertrand lens (sometimes only a pinhole with a magnifier) is available to observe polarized light patterns on the back of a higher-power objective. A slot allows the insertion of a filter that adds or subtracts portions of wavelengths of light, called a compensator or wave plate. Many crystalline materials (organic as well as inorganic) produce birefringence effects with both polarizers in place, and some biological samples can show useful changes of contrast.
Polarized Light (PLM)
So, what happens to light in a pol scope? We can think of light as moving in linear rays, with photons vibrating in a plane along the ray, like waves on a shaking rope. Different wave lengths within these vibration planes appear as different colors. Raw light is comprised of many wave lengths and vibration orientations, but a polarizing filter squeezes light into a single plane. Essentially the filter allows through the portions of light that can be oriented into its vibration plane, and blocks the light that cannot fit that direction. The second filter (called the analyzer) has its vibration plane oriented perpendicular to the first, and so no light will pass through it. That is, no light if only air, liquid, glass, or other isotropic material is present between the filters.
Minerals also polarize light, due to the layers or planes of atoms in their crystal structures. Most actually split and polarize light into two different vibration planes (two refractions, or birefringent). So, if you place a birefringent mineral between the two pol filters, the polarized light coming to it from the first filter is converted into one or two new directions of vibration planes, and some of that light can get through the second filter to reach your eye. Only part of the original light makes it all the way, thus the need for a strong light source.
Light slows down as it moves into the mineral, and the two new polarizing vibration planes conduct light at different rates in the mineral. Each light ray also bends (refracts) as it passes in and out of the mineral, with an angle proportional to its change in velocity. Then they accelerate back to the same speed in the air out the other side. However, the light waves of the rays are now out-of-sync because they moved at different velocities within the mineral. When the wave planes are combined into one plane by the second filter, the waves "interfere" with each other, producing new wavelengths with startling new "interference colors." This amazing feature of birefringence (two refractions) is caused by the two mineral light rays. Birefringence and interference colors vary according to the particular crystal structure unique to each mineral, and so we can interpret important crystallographic properties and identify microscopic minerals, and often infer some of their chemical compositions.
Tools such as compensator wave plates can also provide clues to identification, while reference books provide data tables, optical descriptions, and methodology. There are many written for students and professionals, and you should have one or more. If the book is a few years or decades old, not a problem, as the technique has not changed much, and the minerals not at all! Among my favorites are:
Optical Mineralogy by Paul Francis Kerr (1977, McGraw Hill)
Optical Mineralogy by Ernest Ehlers (in two volumes, 1987, Blackwell Scientific)
Petrography of Igneous and Metamorphic Rocks by Anthony Philpotts (2003, Prentiss Hall)
Atlas of Rock-Forming Minerals in Thin Section by Mackenzie and Guilford (1984, Longman)
Any crystalline material might reveal features of its structure and chemistry via PLM, including wonderful new colors. Not just inorganic rocks and minerals, but also hard parts of plants and animals, and organic crystals formed by diseases. Interesting and beautiful effects might appear as well as new details, so it is worth a look for almost any sample you are studying. And that is easy to do with a pol scope.
A few images from my basalt slides are below, showing interference colors typical of common birefringent minerals. See this Idaho State University page about thin sections of rocks, which since the late 1800s have provided the evidence for much of what we know about the Earth. Where would science be without microscopes?
Although all the major makers produce models of polarizing microscopes (see below), there has been some decline during recent years in their use in geological education and research. Considering that the Earth is made of rocks, and rocks cannot be studied or understood without petrographic microscopes, this is not good! See this article by Mickey Gunter about the problem.
Greg's Leitz Microscope Page
Here is a separate page about Leitz Wetzlar microscopes, including photos, parts, accessories, documents, and links. All types, not just pol scopes. Other brands (including Leica) will stay on this page for now. I am not a Leitz expert, so please feel free to send me corrections and additions to make these pages grow.
Several important features common to petrographic microscopes are illustrated in this photo of the middle section of my Olympus BH-2 pol scope. Between the head and the arm is an intermediate tube with a Bertrand lens, an analyzer filter, and 6x20 mm slot for a compensating wave plate. The Bertrand lens has a dial to swing it in and out of the light path, with a second dial to focus it. The analyzer (upper polarizing filter) can be moved in and out via a sliding plate, and its part of the tube is marked in degrees for rotation, with a locking screw. Wave plates for the open slot have notches to allow precise stops in the light path (an open-hole stop and a wave filter stop).
The nose turret slides on a dovetail for easy replacement, and holds four DPlan objectives (here 4x, 10x, 20x, 40x) designated PO for their strain-free construction. Three of the turret holes can be centered via side screws, using special small wrenches, or because these are easily lost and hard to replace, small screwdrivers. The same wrenches fit two centering screws for the circular stage, which is graduated in degrees with a lever to engage a "click" every 45 degrees, and a stop screw to lock the stage in place. A special x-y mechanical stage (slide holder) is made with low control knobs, so as to not interfere with the objectives during rotation.
Beneath the stage is a Pol (strain-free) condenser with a flip-up top lens. This top lens concentrates light for the higher-power objectives, needed when the Bertrand lens is used to make a "conoscopic" view of an "interference figure" (a diagnostic light pattern formed on the the back lens of the objective). The lower part of the condenser holds a polarizing filter that can be rotated, with a click for 90-degree "crossed polarizers." On my Olympus, all of these special pieces are satisfyingly well made, and I take good care of them because they are expensive!
Glass can become slightly polarizing under uneven pressure or if it is not cooled evenly, leaving residual "strain." This can cause unwanted birefringent effects in your pol scope, so its optical parts are constructed (or carefully selected from normal production) to minimize strain. Such objectives and condensers might be marked P, Po, Pol or SF (strain free), or have red lettering. However, many "regular" optical parts might work OK in your pol scope as well, so it is worth a test. A bigger problem is reflection polarization by mirrors and prisms in binocular and trinocular heads, which is why many older pol scopes use monocular heads. Two-eye viewing is worth some extra cost if you can find a binocular pol head, or if not, you might be willing to put up with slight induced birefringence from a standard biological head. Of course, there will be none if you can swing out or remove both of the pol filters when viewing in "plane" light.
Because they were made in relatively small numbers, or their modular parts could be rather specialized, petrographic microscopes both new and used command premium prices -- often from double to triple the price for a biological version of the same model, depending on how complete it is. If you are outfitting one with additional optical parts, you might find that less expensive objectives, eyepieces, and even condensers from biological (i.e. not certified as strain free) versions work just fine, or at least are so close that their strain effects are hardly noticeable. But if you are buying one at its higher petrographic price, it should already have the proper strain-free versions of optics to get your money's worth (and to preserve its higher resale vale).
Thanks for permission to use these images and documents:
American Optical Spencer and Polarstar manual -- William Gasser
AO-Spencer documents on Steve Neeley's website
Cooke polarizing microscopes brochure -- Mark Glusker
Gillett & Sibert student scope -- Richard Morris (bluecerise400)
Swift and Son lab scope, and Wild M21 objectives -- Ray Sloss
Leitz Laborlux and SM Lux Pol scopes -- Ridge Equipment
Leitz Ortholux Pol scope and pol parts -- Tamagno
Leitz Ortholux camera and light meter -- William Day
Leitz Aristomet research scope -- Chip Sanders (mr-keyboard)
Leitz AM Pol and SM Pol scopes -- Raymond Hummelink
Leitz HM Lux pol scope and brochures -- Allen Carpenter
Lomo Min-8 benchtop scope -- Dirk Marel (eapoecistron)
Microscope documents from Gordon Couger's website
Nikon Ske pol scope -- Jay Stanley
Olympus POM and accessories -- Mike Symons
Reichert Zetopan -- electrowise
Zeiss Standard Pol brochures and pricelist -- Paul V. Heinrich
Zeiss and Leitz price list catalogs -- Jason Weinstein
Zeiss aus Jena benchtop scope -- BIGGERBY2002
Below are galleries of polarizing microscopes both new and old, some interesting accessories, links to useful websites, and documents including pdf files of scanned instruction manuals. Eventually, I will add a section on my adventures with buying, renovating, repairing, adjusting, and selling microscopes. Also see these notes about adapting Nikon digital cameras to my Olympus BH-2 microscope. On eBay are my Guides to Buying Microscopes and Caring for Microscopes. If you find them to be useful, please check the response boxes so I can stay in the top 1000!
Documents, files, links, and images on these pages are for your own information and many are copyright protected. They are not for commercial use and may not be altered, reproduced, sold, or used without specific permission from me or the good people who are credited.
Images of Polarizing Microscopes
Models on the Current Market (Spring 2008)
Hinotek polarizing microscopes
JNOEC XS-213A-P laboratory scope
LEAD Optical PBM-P1 and PBM-P2
Lomo microscopes for geology/mineralogy
Models Produced in the Past
American Optical Spencer P45 student scope (1960s)
American Optical (Reichert) Series 110 (Polstar)
Cooke, Troughton, & Simms lab scope (1950s)
Gillett & Sibert student scope (1970s)
Leitz Wetzlar Microscopes (separate page link)
Lomo Min-1 packed in its lunch box
Reichert Austria Neopan binocular
Reichert Austria small monocular
Reichert Austria benchtop scope
Reichert Zetopan research scope
Swift and Son (England) lab scope
Vickers large monocular w/incident light accessory
Vickers M70 monocular w/incident light accessory
Wild Heerbrugg M5 Stereo Photo Viewer
Zeiss aus Jena Amplival benchtop scope
Zeiss Standard binocular (gray)
Zeiss Standard monocular (gray)
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Parts and Accessories
AO Spencer compensator plates and micrometer slide
CTS incident light attachment w/objectives clip
McCrone dispersion staining objective
Nikon Coolpix 990 with Leitz Periplan relay lens
Nikon pol objectives (as per Labophot)
Nikon S Pol condenser and turret
Olympus POS objectives and accessories
Olympus POM objectives and accessories
Olympus BH-2 Berek compensator
Olympus BH-2 Pol objectives and condenser
Olympus BH-2 pol intermediate tube
Olympus BHS-LSH 100 watt lamphouse
Olympus BH "simple polarizing" filters
Reichert Zetopan analyzer and compensator plates
Vickers compensating wave plates
Wild M21 pol objectives (centering barrels)
Wild M21 pol stage X-Y slide holder
Zeiss Universal rotary stage w/centering wrench
Zeiss pol objectives, 2-ring centering
Zeiss Standard analyzer plate in intermediate tube
Zeiss Standard stage with X-Y slide holder
Zeiss 4x12 mm 1/4 wave (mica) compensator plate
Pol Scope Instruction Manuals and Documents
AO Polarstar Series 2300 reference manual
AO 110 Polstar instruction manual
B&L Polarizing Microscopes catalog, 1970
B&L Polarizing Microscopes price list, July 1970
Leica Polarization Use brochure
Leitz 1971 Pol Scopes Catalog Pages 1-41
Leitz 1971 Pol Scopes Catalog Pages 41-83
Leitz SM-LUX Pol instructions
Leitz Orthoplan-Pol instructions (older gray version)
Leitz Orthoplan-Pol brochure (newer white version)
Leitz HM-POL instructions
Leitz Laborlux Pol instructions
Leitz Ortholux-2 Pol instructions
Polarized Light Microscopy (Leitz, 3rd edition, 1985)
Meiji ML-POL instruction manual
Meiji ML-9000 series instruction manual
Olympus POS instruction manual
Olympus POM instruction manual
Olympus BH-A-P instruction manual
Olympus BH-2 BHT-P instruction manual
Olympus BH-2 BHS-P instruction manual
Olympus SZ30-SZ40-SZ60-SZ11 instruction manual
Spencer 1943 'The Use of Polarizing Microscopes' on Neeley's website
Unitron Bio-Pol instruction manual
Wild M21 Polarising Microscope Instructions for Use (larger version)
Wild M21 instruction manual (smaller version)
Zeiss Axioskop 40 Pol brochure
Zeiss Axiolab Pol operating manual
Zeiss polarizing microscopes brochure
Zeiss Standard Pol Operating Instructions
Zeiss Standard Intermediate Tube
Zeiss Standard Pol Accessories
Zeiss Polarizing Microscope Price List (1982)
Zeiss Polarizing Microscopes Catalog (1979)
Zeiss Student Polarizing Microscopes Catalog (1978)
Zeiss Jena Amplival-Pol brochure
Microscopy from the Very Beginning
How to Make Rock Thin Sections
Asbestos identification Methodology by PLM - NIOSH 9002
Internet Articles and PLM Websites
Micscape article on "The Six Best Polarizing Microscopes"
Microscopy UK's Micscape Magazine
Gordon Couger's website for microscope documents
Molecular Expressions Optical Microscopy Primer
Olympus Polarized Light Microscopy Primer
Olympus Applications of Polarising Microscopy
Olympus Basics of Polarising Microscopy
Olympus Polarized Light Image Gallery
J. M Derochette's Microscopy and Minerals web site
DMOZ directory of microscope dealers and suppliers
Lightscapes Webring for PLM imagery
Nikon's MicroscopyU Introduction to Polarized Light
The Quekett Microscopical Club
Jay Stanley's Wild Heerbrugg M20 site
P. S. Neeley's AO Spenser microscope page
Links for Mineralogy and Petrography
Olympus BH-2 BHT-P at its "station" and with a Nikon Coolpix 990 digital camera mounted for quick photography. Note the PM-10 automated 35 mm camera controller at right, which is connected to a film camera back that can be easily set in place of the digital camera. The table is a heavy mdf laminate top screwed onto a frame base made of 2x4 lumber, making a rigid and very solid work bench free of tremors.
Corporate Websites