XN/Pol vs BF

Pardon?

If the cryptic title of today’s post seems bizarre please remember that for most people microscopy is limited to only one sort: light microscopy. Further one could say that for most people light microscopy is limited to one particular sort: bright field. For the last few posts we’ve been getting into polarized light microscopy in a very general way and with that in mind the obscure little title should make perfect sense.

XN is still used in some papers by authors who wish to refer immediately to crossed Nicol prisms, or crossed polaraizer and analyzer. In practice one may rapidly find the point at which their polarizer and analyzer are crossed by turning one or the other until the light which is seen to pass between the two is at its lowest ebb.

XN in Action

The following photomicrographs were all taken with trinocular AO Spencer microscope using a Nikon 1 J1 consumer grade digital camera fitted with a Nikon 1 to c-mount adapter and c-mount to 23.3mm microscope eyepiece tube adapter. The polarizing apparatus was constructed out of two small discs of polarizing film which cost only $5.00 with shipping. If a full sized image is desired one need only click the image but be warned they are several MB in size. First will be shown the specimen in bright field, followed by the object under XN.

A human hair.

A human hair.

The same hair.

The same hair.

A mouse hair.

A mouse hair.

The same hair.

The same hair.

Portion of a fly wing.

Portion of a fly wing.

The same flys wing.

The same flys wing.

A centipedes forcipule.

A centipedes forcipule.

DSC_0998

The forcipule again.

Many natural fibers, everything from cotton to the hair on ones head, are strongly birefringent. Different forms of fiber will show differently under crossed pols. A motivated individual can discern much from a hair without resorting to such destructive methods as scale casting.

Insects may be surprisingly dull subjects for polarized light and the process may do little but reveal how much dust was remaining on the specimen, as in the case of a hastily mounted flies wing which was collected from a disused attic. At times one may find with surprise that small portions are powerfully birefringent, as in the case of the hardened, venomous, forcipule of the garden centipede pictured above. Frequently only the mouth parts of a specimen will show double refraction. This simple fact can be immensely helpful when trying to identify the mouth parts accurately in whole mounts, especially when optical sectioning is insufficient.

One of these days I’ll have to put up a video of some chemical crystals under XN, stay tuned! -K

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Simple Polaroid-based Polarizing Apparatus

The Polaroid

One is going to need a quantity of polarizing film (polaroid) for any easily constructed polarizing apparatus. Fortunately, the material is inexpensive and readily available from any number of sources online. When seeking the material for construction one should purchase linear polarizing polaroid rather than the circularly polarizing filters common in photography. Do not hope to luck out with a bargain by purchasing the sort of polarizing film sold for use with LCD screen repair and refurbishment, it will not prove suitable.

The size of the film purchased will vary depending on the sort of apparatus which is planned but in most cases a small piece of five square centimeters (two square inches) is enough. One shouldn’t feel obligated to purchase expensive polaroid whether that expense is attributed to the supposed quality of the film (the perfectness of the polarization) or its thickness or any protective coating. Very often one may have the option to purchase polaroid in varying thickness, and the thicker film is useful for applications that require a large self-supporting filter, but in many cases the thinner product is preferable simply because it is easier to work with.

The Example

Not one to miss out on a potential market, Bausch & Lomb marketed a simple polarizing apparatus for users who did not require (or have the budget for) the more complex prism-based variety. Below is seen an exceedingly simple set composed of polarizing film set into light metal frames. One portion is a 21mm disc and the other is of 32mm, a split ring retainer is included. The concise instructions on the reverse of the box direct the user to install the smaller disc in a standard eyepiece by separating the components of the eyepiece so that the disc may rest upon the eyepiece diaphragm. The eyepiece itself then becomes the analyzer which is in this instance the rotating component. The 32mm disc is sized to be compatible with filters used in most substages and serves as the polarizer.

Simple commercial example of a type anyone can produce.

Simple commercial example of a type anyone can produce.

Right away one can see that an essentially identical set may be produced for just a few dollars. If one is loath to risk the cleanliness of an ocular by separating the components to insert the analyzer, a cap may be fashioned that holds the polaroid and fits above the microscopes eyepiece. It will work in precisely the same fashion and has the advantage of not requiring an ocular be put aside for polarizing work only. Regrettably, one will recognize very quickly that such a set, whether the analyzer is integrated with an ocular or placed over it, will not work effectively on a binocular or trinocular microscope.

Special Considerations

For microscopes equipped with binocular or trinocular heads, one should place the analyzer in a location such that it acts upon the light prior to that light being sent into the eyepiece or photo tubes. Fortunately it is often a simple matter to remove the microscopes head and place the analyzer within. Once the analyzer is positioned one must look to the way in which the polarizer may be accommodated. In most cases it is not advisable to use a 32mm disc placed in the substage filter holder simply because rotating it once positioned is inconvenient. Very often only a small effort need be expended to create a holder that may be placed in the substage to facilitate rotating the polarizer. In any case one should endeavor to arrange polarizer and analyzer so that both may be quickly removed or installed, and one of the two is rotatable.

Improvised polarizer and analyzer in place on AO Spencer microscope

Improvised polarizer and analyzer in place on AO Spencer microscope

In the photograph at right one can see that a simple disk of polarizing film has been placed intermediate to the objective turret and trinocular head of this AO Spencer Microstar microscope to serve as the analyzer. A rotating polarizer has been constructed from a plastic film canister lid and aluminum screw cap, it fits conveniently in the 32mm filter recess of the microscopes integrated illuminator. By virtue of the microscopes construction only one finger screw needs to be loosened to remove the head and place the analyzer. For ease of handling, and so that it may serve double duty the analyzer was cut to a size of 32mm and may be used as the polarizer when placed in the substage filter holder of a monocular microscope. One should note that the polarizer is of a size that no light may pass out of the integrated illuminator that does not pass through the polarizer.

Next time: eye-candy! A few nice photomicrographs of slides with bright-filed and polarized light. -K

Polarized Light Microscopy Apparatus

The Polarizing Prism

Prior to the advent of thin-film polarizing filters one relied upon specially arranged prisms of a substance known as Iceland spar. This transparent calcite (primarily sourced from Iceland) has the peculiar property of acting as a double refracting filter. There is a certain amount of speculation that Iceland spar is in fact the old Norse sun-stone of legend that permitted navigation based upon the position of the sun even in cloudy conditions. In any case, the ability of the mineral to polarize light, together with the fact that it cleaves easily into rhombs renders it uniquely suitable for the creation of a various forms of prism, two sorts of which were common in polarized light microscopy.

Invented in 1928 by William Nicol, the prism so designated is composed of two portions of a single crystal of Iceland spar cut at precise angles with respect to the axis of their polarization and cemented back together. Once reassembled in accordance with Nicol’s design the double refracting crystal becomes a filter which effectively reduces any light entering it to a single ray of polarized light. A Nicol prism is easily identified because either end of it will show parallel faces of 68°. That the active faces are at an angle makes the Nicol prism less suitable for use as an analyzer and it will be most often found in a polarizer.

Unlike the Nicol prims, a Glan-Thompson prism has both of its active faces at right angles to the axis of polarization. This simple fact makes it well suited to use in an optical system when one needs to maximize the amount of light which will pass through it, and minimize the distance at which it may be placed conveniently over an optical lens. The Glan-Thompson prism acts in much the same way as the Nicol prism, it simply permits a greater percentage of polarized light to pass through.

Antique Apparatus

Historically polarized light microscopy was practiced much as it is today; with either a petrographic microscope constructed specifically for that use, or a pair of accessories that adapt a standard light microscope to the task. Although the precise configuration of the apparatus took may have varied, it generally took one of two forms depending upon the placement of the analyzer. In each form a polarizer was mounted in place of, or beneath the microscopes condenser. One sort used an analyzer that screwed into the objective end of the microscope body after the objective or nosepiece and so introduced the Nicol prism into the optical axis. The second form fit over the end of the microscopes draw tube so that the Nicol prism is introduced over the ocular at the eye-point with a subsequent lens that focuses appropriately. In either form one of the elements will rotate, polarizer or analyzer.

In nearly all forms the rotating component will be inscribed with markings designating the degree of rotation from 0 too 360. Occasionally, the manufacturer may not provide precise or complete markings. Very often the polarizer and analyzer were sold together in a case as either on its own would be of only limited use. Below is an example of a representative Bausch & Lomb polarizing apparatus from the era of the Triple Alliance (1907 – November 1915) first in its case and then fitted to a Bausch & Lomb BH8 dating to c. 1919.

Bausch & Lomb polarizer (left) and analyzer (right).

Bausch & Lomb polarizer (left) and analyzer (right).

Polarizing apparatus on period appropriate microscope.

Polarizing apparatus on period appropriate microscope.

The black surface will face towards the underside of the microscopes stage in use.

The black surface will face towards the underside of the microscopes stage in use.

The Polarizer

This example, which carries a Nicol prism of Iceland spar friction fit into a cork, is designed to fit into the substage in place of the microscopes condenser. One will be quick to note that this means one should employ the concave mirror in order to obtain appropriately converging light for illumination. Later varieties of the apparatus constructed along the same lines would feature filters composed of selenite with which one could control the color of the polarized light.  Once fitted the condenser adjustment is wracked upwards to bring the Nicol prism as close to the specimen as possible, in this way ensuring the entire field is filled with polarized light.

When mounting a polarizer of this sort (that does not rotate) one should take notice of the orientation of the polarizing prism when placed in position for use. It may be desirable to orient it such that the axis of polarization is not at an odd angle. However one may simply place the polarizer as is convenient and then orient the analyzer.

The Analyzer

The left portion contains no optical components and is little more than a mounting collar.

The left portion contains no optical components and is little more than a mounting collar.

Here, the analyzer being the rotating component, is more complex. Of two portions, the first is fit over the eye-tube of the microscope prior to the placement of an eyepiece and is held in position by a knurled set screw. Once the base portion of the analyzer is fitted to the microscope an eyepiece is inserted and the top portion friction fit into a recess in the unit. Graduated marking around the top portion (which bears the polarizing prism) range from 1 too 360 and show the relative orientation as it is about the optical axis. Due to the limited size of the prism it is not possible to obtain a complete image of the normal field of view with a prism set in a fixed position. For that reason the eyepiece of the polarizer is adjustable in the manner of a draw tube so that an optimum field of view may be had for a given ocular.

The primary advantages of an analyzer of this type are the ease with which the orientation of the analyzer may be read, and the retention of the tube length. Where an analyzer which screws into the body of the microscope between the objective and body tube will add to the overall length of the body tube, this apparatus will not, enabling the markings on the draw tube to be used as normal for coverglass accommodation or other similar adjustments.

Notes:

∗Some of the technical information concerning various varieties of polarizing prims may be found excellently presented in the following pdf available from SPIE, the international society for optics and photonics: PM200.pdf

Polarized Light Microscopy

The Background

Very often those introduced to light microscopy, and enthusiastic about it, rapidly become dissatisfied with not being able to work with all of the methods read about in books. From differential interference to phase contrast there are more than a few methods of light microscopy that one may long for, but lack the budget to pursue. Fortunately, one exotic sort of light microscopy has been doing nothing but becoming more accessible for the last one hundred years or so; that method of course is polarized light microscopy. A step beyond simple things like throwing the mirror to one side for oblique lighting, or slipping a patch stop into the filter holder for dark field or even Rheinberg illumination, polarized light microscopy is now more economical than ever.

When light microscopy was just finding its stride among professionals and hobbyists, opticians sold microscopy equipment in much the same was as furniture stores pursue their trade today. One might purchase a central item and then outfit it with a number of accompaniments either immediately or a bit down the line. In many ways this meant that one could pursue the limits of microscopy as an amateur as easily as any professional. Regrettably, it also meant that the cutting edge in microscopy as pursued by professionals was just beyond the financial means of most amateurs. Apparatus for polarized light microscopy was no exception.

With the patent of polaroid (the product as opposed to the company) in 1929 it became possible to produce polarized light microscopy apparatus for far less than had been possible previously. From then, the quality of polarizing films has increased while the cost of manufacture has decreased.  At this very moment one is likely within reach of a number of polarizing filters. Liquid crystal displays, windows, eyeglasses, and sunglasses are all common made with polarizing filters. It’s only a slight stretch to say that polarizing filters are everywhere, and the ability of the internet to connect suppliers with clients has made finding the product simple and fast.

The What & Why of Polarization

As with most specialized methods of illumination, polarized light microscopy has nothing more as its goal than increasing the microscopists understanding of their specimen. With polarized light microscopy visibility of some structures is outright increased and specimens or particular structures glow brilliantly in the dark field of crossed polarizers. Crystals may turn from bland colorless structures into colorful landscapes that provide a key to upstanding their form and composition.

The increased visibility offered by polarized light microscopy is the product of two polarizing filters. One filter, called the polarizer, is placed below the specimen so that the light which passes through it (and illuminates the specimen) is polarized. A second polarizing filter, called the analyzer, is then placed above the specimen so that the light which reaches the eye of the observer has passed through two polarizing filters. A polarizer or analyzer used on its own will not reveal much but with both one can determine the following with a little effort:

  • Whether the object is polarizing (ainsotropic) or isotropic (non-polarizing)
  • Whether it is uniaxial or biaxial
  • Whether it is subject to interference phenomena
  • Whether it rotates the plane of polarization
  • Whether it is pleochroic
Polarization sketch

Polarization sketch

Light can be thought of as vibrating (it’s a particle and a wave) in all planes surrounding an axis of propagation. With the microscope this axis of propagation is (ideally) aligned with the optical axis of the microscope. In the little sketch is shown the optical axis of a microscope with B at the point of illumination and A at the point of observation. C represents the image plane of the specimen. In normal use the light vibrates as in C1, in all planes around the axis. When a single polarizer is introduced into the optical axis all vibration is eliminated save for two planes that vibrate at right angles to each other as in C2. By introducing a second polarizing filter one may orient the two so that one, both, or neither of the two remaining vibrational planes remain. In C3 the filters have been oriented to leave a single plane. Crossed poles, or crossed Nicols (abbreviated in the literature as XN) block out all planes and results in a perfectly dark field, unless a specimen in the image plane is birefringent.

It is the orientation of the polarizer and analyzer to each other that reveals much of this information. Because the polarizing filters act upon light in a very specific and consistent way, we are able to describe the way the specimen acts upon light with as much certainty. Consider polarized light microscopy something akin to optical algebra.

Next time: classic, brass-era, polarized light microscopy apparatus! -K

Notes:

∗A linear polarizer. There are different sorts of polarizing filters. Many used for photography or general glare reduction are circularly polarized and although suitable for a general demonstration will not reveal as much as a linear polarizing filter. Don’t rush out and buy polarizing filter that is not linear.