Tutorial on polarization.

by Donald E. Simanek

Your tyrannical editor wants a short intro to polarization. That's not possible, but he still wants this page filled. So here goes.

Screwy Light.

If you've seen a 3-d movie, I hope you saved those glasses you paid for. If not, ask at the box office if you could have a few that have been used and will be discarded. The Real-D 3-d movie process uses circular polarizatation, unlike the 3-d movies of the 50s that were presented using linear polarization. If you are into 3-d photography and project your pictures on a screen, you probably have used linear polarizing glasses of the older kind. Both types of glasses also have other uses, as we shall see.

If you have a film camera, you may have used linear polarizers with it to darken skies, reduce specular reflections, and increase the saturation of colors of foliage. If you have converted to digital cameras, you have no doubt learned that these usually require circular polarizing filters to accomplish the same results. Why? Inquiring minds want to know.

Modern linear polarizers consist of a plastic layer that has its long chain molecules aligned along one direction, called its polarization axis. 3-d movie and slide shows project two images on the screen, one intended for the left eye and one for the right eye. The trick is to ensure that these reach the correct eyes of each audience member. The two images are projected onto a metallic coated screen through polarizers with their axes aligned perpendicular to each other. The standard that has evolved is to orient these axes at 45° to the vertical. The metalic screen reflects the light without altering the direction of its polarization. People in the audience wear glasses with a similar arrangement of polarizeres, and the net result is that each eye sees only the picture intended for it, but not the one intended for the other eye.

But the process is angle-critical. If you tilt your head the alignment of your glasses with those of the projector is wrong, and each eye sees a bit of the other eye's picture, resulting in "ghost images", a headache inducing phenomenon.

Digital 3d movies today use glasses with circular polarizers. They have one layer of linear polarizer laminated with plastic layers that separate that linear polarized light into two components, and retard (slow down) one of these components with respect to the other. If it slows one component so that it emerges just 1/4 wavelength, the resulting light's electric vector moves through space tracing out a spiral or helix. Now spirals can have two orientations, right- or left-handed, just as a bolt can be threaded right- or left-handed. The good Dr. T. tells me he likes this analogy with a scre thread, for a right-handed bolt won't thread into a left handed bolt. Likewise light from a right-circular polarizing sheet will not pass through a left-circular polarizing sheet, and vice versa.

Right- and left-circular polarizers differ in one respect. One uses a 1/4 wave retardation layer, and the other uses a 3/4 wave retardation layer.

But there's an interesting gotcha, that has even been known to trip up folks who know some physics. When right-circular polarized light reflects from a mirror, or from a metallic surface, it bounces back as left-circular polarized light. This is no big deal, so long as you manufacture the glasses with the polarizers on the correct side to achieve the desired result.

Difference between plane and circular polarization.	Circularly polarized light. Two oscillating electric fields, one retarded by 1/4 wavelength, combine to produce a wave in which the electric vector traces a helical path.
Diagrams from the HyperPhysics web site, by Rod Nave. Used with permission.

Polarzers for Photography?

Why do polarizing filters on cameras darken skies, and reduce reflections? Sky light is partially polarized because the scattered sunlight from atoms and molecules has been absorbed and re-emitted. The re-emitted light is polarized, the relative intensity of polarized to unpolarized light being dependent on the scattering angle. Sky light is most strongly polarized when it scatters at 90° so a polarizer will darken the sky most when the camera is aimed at 90° to the position of the sun. The polarizing filter absorbs this the most if its axis is aligned toward the sun.

So why can't you use these linear polarizing filters from the good old days of film on your new digital SLR camera? Because light reflected from mirror surfaces is polarized. That's why the polarizer can reduce reflections from shiny surfaces. The camera's internal mirrors in the autofocus and auto exposure systems would be confused if plane polarized light coming in from the camera's lens was not correctly aligned with respect to the mirror's polarizing axis. The camera would compute incorrect exposures.

But we can get around that. A circular polarizer consists of a linear polarizer laminated with a retardation plate that converts the linear polarized light to circular polarized light. Put one of these in front of the camera lens, with the linear polarizing part in front (nearest the subject) and the retardation sheet behind (nearest the camera). The linear polarizer layer does its job in photography just as the older linear polarizers did, but the emergent circular polarized light has no directional bias, and cannot confuse the automatic systems of the camera.

Inexpensive polarizers for your digital camera.

Rather than pay $30 and up for a glass-mounted circular polarizer at a camera store, you can get the same results with the circular polarizing material from a pair of Real-D 3-d cinema glasses. Remember, that for this application the retardation plate must be nearest the camera, while it must be nearest the screen for viewing 3-d movies. If you have it on the cammera correctly, a clear sky will darken as you rotate the polarizer. If it is incorrectly placed, you will see no change in the viewfinder scene when rotating the polarizer. This is a good way to experiment with polarization before you spend money on a professional glass-mounted polarizer for your camera.

George: Put that pictue of the projector wearing glasses in here somewhere.

Converting your old stereo projector.

You can experience the advantage of circular polarization with your existing stereo projector, for your next slide show. See if you can get some Real-D glasses from a theater. They usually collect the used ones to ship back to the distributor for recycling, but they may be willing to give you a box of them if you ask nicely. Fortunately the linear polarizers in these glasses are aligned just as they were in the old days of strictly linear polarizing glasses, so you can simply break off the earpieces of a pair of glasses and put them in front of your projector's lenses, with negligible loss of light. Then be sure your audience uses circular polarizing glasses. So what's to gain from this? Just one thing: Reduction of ghost images due to audience members' head tilting as they fight off the urge to go to sleep. In all other respects I can think of, circular and linear polarizers are about the same in light efficiency, cost, and clarity. But the Real-D glasses are very well-designed to be sturdy and comfortable, every bit as good as a quality pair of sunglasses.

If your editor gives me more space and more warning, I may tell you next month about some other interesting and creative things you can do with these circularly polarizing Real-D glasses. So don't throw them away.

You can find out more about polarization, with experiments you can easily do, at:
http://www.lhup.edu/~dsimanek/scenario/labman3/polarize.htm.

For a mathematical treatment of the theory, and lots of good experiments you can do at home, see http://instructor.physics.lsa.umich.edu/int-labs/Chapter4.pdf.