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
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
∗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.