Angles Of View
The fourth volume of this series will undertake to examine various issues related to the concept and nature of Information - any display system’s best product. Anyone can see, of course, that light is the medium by which information is borne in a visual display. Yet, some of the exact ways this is accomplished are not always obvious. One particularly interesting property of light is the orientation of its waves as they propagate through space. The manipulation of this orientation attribute can be extremely useful in creating visual information and thus this first article will consider
Polarization - Learning to Ride the WavesLet’s begin, as always, with a simple light source. Since it’s a projector, the light from its lamp is transmitted through a lens. If we measure the brightest light we can get from this projector, we will discover that it will be the "white" light from the lamp that will the most intense.
If, however, we cause the projection beam to contain some information, we’ll immediately discover that we have lost energy. Some of that admixture of wavelengths we call white light has been taken away in order to make room for the information. Just as the text on this page (or screen) takes away from the overall whiteness (or brightness) of its background, so whatever content we choose to transmit through our projector must similarly reduce its available lumen output.
To elaborate this concept just a little farther, recall that each color that we are able to see is distinguishable from every other color solely by virtue of its unique wavelength. White light has no specific wavelength but is a superposition of many wavelengths. To make any portion of an illuminated image other than white requires the suppression of some number of its constituent wavelengths. And when we pay this energy cost in some sort of organized way, the benefit is that the resultant collection of wavelengths becomes suddenly interesting because it now has something to tell us (or show us) which, of course, is really what we mean by information.
Another facet of projected images and of the information they contain is that the organization of the light rays filling them up is spatial. Each little packet of light rays must be modulated exactly to fulfill the instructions for the pixel it will illuminate. And the position of that pixel within the image must be maintained rigorously. The techniques for accomplishing this are numerous and varied, but they always require a lens to preserve this organization throughout the image area. And even when we look at kinetic imagery, movies, for instance, we really are not looking at content that’s continuously changing in time, but instead are given the illusion of movement by a series of quite static, spatially well organized, frames.
It’s perfectly possible, of course, to make light contain information that is actually organized temporally - semaphores come to mind, but those devices are not what our industry would comfortably call a projector and displaying their content upon a screen in no way enhances its comprehension. Conversely, when we project light that has been spatially organized and converged onto a screen, we suddenly have an extremely efficient surface from which we may assimilate the displayed information.
Thus far the optical vocabulary we have for getting information into light rays has been limited to two terms: wavelength and amplitude. Powerful as we know the combinations of those two can certainly be, there is a third property of light which, when added to the optical tool kit, makes the transmission of visual information even more effective. This, of course, is polarization.
To understand what polarization is and, at least coarsely, how it works, we have to shift the discussion from what projected information is to a description of how the light containing it moves.
Light travels in waves. More particularly, it travels in waves which oscillate side to side and perpendicular to the direction of the wave’s propagation. Because they are at right angles to the axis of propagation, waves of this type are called transverse.
Figure 1 diagrams the three, orthogonal components of an electromagnetic wave. The drawing assumes that the light wave is traveling left to right across the page and hence (in Cartesian terms) along the x-axis. The y-axis extends up and down the page and the z-axis pierces the page perpendicularly. The oscillation (the wiggles) of the wave can rotate in any and all orientations about the x-axis. Its undulations move back and forth between being parallel with the y-axis and parallel with the z-axis and everywhere in between. We have seen already that the distance between each of the waves peaks (or valleys) is its wavelength. And the height (or depth) of each wave from the x-axis defines its amplitude. To these two variables let us now add a third, which is going to be the orientation of the light ray as it fluctuates around that x-axis. That orientation at any given infinitesimal moment of time is the wave’s polarization. We take advantage of this attribute by forcing the light to adopt only one of those infinite possible orientations and then we make it stay that way .
Figure 2 schematically illustrates how this works. The light is again traveling from left to right and we see that its orientations are always perpendicular to the direction of its propagation but are otherwise random and numerous. But when we insert into its path an object called a polarizer only the light which matches the orientation of that device gets through.
As the human eye is largely insensitive to polarization, changing its orientation doesn’t change what we see. That’s the good news. The bad news is that every time we polarize a beam of light we throw away 50% of its energy. (The reasons that this is so are complex, but suffice it to say here that they are imposed by the laws of physics and not by any human failure to be ingenious.) Now, we are often willing to pay that very high cost because the benefits we can derive are also considerable. One of them is the ability to employ liquid crystal displays.
LCDs require light from their lamps to be polarized so that molecules of the liquid crystal making up their panels can be made to rotate in ways which modulate the amount of light they will transmit. The utility of this technology to almost everybody in the projector world has become unassailable. The intriguing thing about this technology to some of us in the screen world has been that the light emanating from an LCD projector is polarized. Always.
In principle, therefore, it is possible to create a screen surface which could have a polarizer built right in to its surface. If that polarizer were to have the same orientation as the polarizers in the projectors pointing at it, then all projected light would be returned from the screen undisturbed and undiminished.
The fate of light from all other sources falling on the screen, however, wouldn't be nearly as benign. Returning to Figure 2, we can see that when unpolarized light is passed through a polarizer, only one of its orientations emerges and we now know that this output light can at best be only 50% as bright as the original input was.
Therefore, and again in principle, a screen sensitive to polarization could exhibit 100% of the projected light while simultaneously absorbing (and thus not exhibit) fully 50% of the competing ambient light.
Attractive as this idea is theoretically, it tends to be impracticable because there is no compulsion for the manufacturers of LCD projectors to all orient their polarization schemes in the same, identical way. That being so, a screen designed to have a vertical polarizer would work fine in front of an North/South projector, but wouldn't mate at all well with one that was East/West. And since a lot of projectors are East/West and still others are even oriented diagonally (e.g. Northeast/Southwest), such a screen just isn't statistically useful enough. But it is a bright idea, isn't it?