From the time I was kid to my student days as an undergraduate in physics my abiding passion was light and vision. Since my earliest years I have been interested in creating images and in understanding the role that light plays in image creation. As a student no other part of physics engaged me as much as the study of light.
The study of light, and polarized light in particular, turns out to be of great importance in understanding how the most important stereoscopic moving image systems function. It’s a subject of great interest to people in the field or for those who have an intellectual interest in the making and projecting of 3D movies. This article is about polarized light and how it is applied to image selection for stereoscopic movies. The term “image selection” means: how one gets the left image to the left eye and blocks the unwanted right image from the left eye, and vice versa. If you have a high school education through trigonometry and physics you have the background to understand a lot of what you need to know about polarized light. If you are motivated to know more I recommend that you look at a basic physics text like Fundamentals of Optics by Jenkins and White. On the other hand, you don’t have to know anything about polarized light to enjoy or make 3D movies. You can consider polarized light image selection to be a black box and stop worrying about it. Since you’re reading this, you probably want to know more. This is not going to be a complete description and I am only going to focus on what I need to sketch in the story about how polarized light works for the stereoscopic cinema.
Physicists use the construct that light phenomena can be explained by considering it to be a longitudinal or transverse wave. From the time of Newton, people who have thought about such things have thought that light could be explained as its being either a particle or a wave, but early on experimental evidence pointed in the direction of light being a wave phenomenon. This idea was cemented along the way by the work of various smart people. A lot of work was done after Newton to explain observed phenomenon in terms of waves without understanding their nature but it was Michael Faraday who conceived the idea of electric and magnetic fields. James Maxwell took Faradays’ ideas about fields and used them as the basis for the creation of a set of equations that explains light in terms of it being an electromagnetic phenomenon. He provided a basis for understanding and predicating how light works in terms of it being a combination of electric and magnetic fields and he predicted the existence of radio waves.
To understand what follows you have to accept the fact that light is an electromagnetic phenomenon and that it behaves like a wave. When I wrote earlier that it’s a longitudinal or transverse wave, I’m talking about the kind of wave that you can produce in a string like so: If you tie a string a few feet long to a doorknob and flick your wrist in an up-and-down motion you will produce a longitudinal wave. You’ll observe that the amplitude or the height of the wave is perpendicular to the direction in which the wave travels – toward the doorknob. That is what is meant by a longitudinal wave. It’s also a plane polarized wave because the wave resides within a plane.
Light can be thought of as being made up of a field with longitudinal waves described by electric and magnetic vectors. These two fields are in phase and at right angles to each other. We are going to forget about the magnetic vector because the eye is sensitive to the electric component and it’s simpler to continue this explanation by ignoring the magnetic component of light. The light that you see reflected from most surfaces or emitted by the sun, a candle, or a light bulb is unpolarized. Assuming you could see the structure of the light leaving emissive surfaces or being reflected from many other surfaces, the planes in which the electric vectors reside are randomly oriented so there is no favored direction to their orientation. In plane (sometimes called linear) polarized light (there are other kinds), the wave is restricted to a plane, which is, as noted, exactly what happens when you try the experiment with the string.
Polarized light can be produced by a number of means. The way we are concerned with as used in stereoscopic projection is by means of the kinds of sheet polarizers that Land and Bernauer produced in the 1920s and early 1930s. Sheet polarizer is made of a substrate or base of a stretched sheet of plastic, usually polyvinyl alcohol, into which has been infused a dye like iodine, a kind of polymer that has long chains. These long molecular chains are oriented to follow the stretch pattern. The base is stretched, the dye is introduced into the material, and the long chain molecules of the dye line up and follow the direction of the stress of the plastic. This creates a microscopic or molecular structure that favors the passage of light whose waves are oriented in only one plane. (We are not going to talk about how that is accomplished.) That means that the light that is passing through a sheet polarizing filter will have the electric vectors of its waves all having the same parallel orientation.
Since these electric vectors are aligned in a plane that plane can be marked on the sheet polarizer with a straight line and it’s called an axis, in particular it is called the transmission axis. The other axis, at right angles to the transmission axis, is called the absorption axis. If you have a second polarizing filter just like the first one, and you place it on top of the first polarizer and you rotate it (say they are on a light box), what you will see is that the transmission of light goes through maxima and minima every ninety degrees. When the transmission axes of the polarizers are crossed you get a minimum and little light passes through and when these axes are parallel you get a lot of light passing through. The polarizers don’t have to be in contact in order for this work. You can project a beam of polarized light onto a polarization-conserving projection screen (usually painted with aluminum metal) and observe the same phenomenon when looking through a polarizing analyzer. In physics the second polarizer is called the analyzer so the polarizers in stereoscopic eyewear are analyzers.
There are two kinds of materials that we need to think about: conductors and dielectrics (or insulators). Conductors conduct heat and electricity well, and they do this because they have free electrons. Usually conductors are metals. Non-conductors or dielectrics don’t have free electrons. Polarization-conserving screens have a metallic coating or they’re painted with metal, so they have free electrons on the surface. It is these free electrons which reradiate the polarized light or reflect it back in a way that conserves the properties of polarization. That is why a matte screen, which has a dielectric surface, cannot work for polarization image selection: It doesn’t have free electrons.
If you have two projectors, that have linear polarizers over their lenses. whose axes are at right angles to each other – and you project them overlapping on this metallic screen, and you wear eyewear that have analyzers whose axes are lined up just like the ones on the projectors, one eye will see the reflected beam from one projector and the other eye will see the beam from the other projector, but each eye can only see the beam from its projector. That’s perfect for projecting stereoscopic movies, because we can transmit one perspective for one eye and block the unwanted image for that eye, and so on.