Displaying a Polarized Future

If you had less trouble getting money out of the bank lately, it may be thanks to a circular polarizer (or was it the machine-gun?). The magic is performed by adding a thin polarizing filter to the screen of an automatic teller machine that increases its contrast as much as 18 times, eliminating 99% of the glare (Polaroid Inc. numbers). A circular polarizer lets the light originating inside the display come out and meet your eyes, but blocks reflected light from the outside.

New display technologies are key to many of the new gadgets transforming our lives. Some of these technologies reflect ambient light (such as LCDs), while others generate their own light. These last "active" displays include LEDs, CRTs, incandescent filament, planar gas discharge, and vacuum fluorescent displays. They all have to compete with reflected light that lowers their contrast. That's when circular polarizers become very handy.

How does it work?

Bounce vertical linearly polarized light from a mirror and you get more of the same (check it out by looking at yourself in a mirror with 3-D polarized glasses: with your left eye you cannot see your right eye, and vice versa). On the other hand, circular polarized light changes from right-handed to left-handed upon reflection from a mirror [and also from most other surfaces when the angle is not too shallow (< Brewster angle)]. Thus, you get what I call the Dracula effect: a circular polarizer that transmits some light without absorption will block the same light after it is reflected on a mirror.

This graph (borrowed from Polaroid) shows how it is used. Most commercial circular polarizers are fabricated by combining a linear polarizer with a "wave-plate" (white sheet on the drawing). This last component doesn't absorb any light but can convert linearly polarized light (aligned along two possible directions) into circularly polarized light. Light originating inside the display is unpolarized, and will remain so after going through the wave-plate. It will leave the screen after traversing the vertical polarizer (yellow sheet). On the other hand, light coming from the outside is first vertically polarized and then converted to right-circularly polarized light. After it is reflected from the screen it becomes leftist, the wave-plate converts it to horizontal and the filter blocks it. Even though the intensity of the display is reduced by half at the polarizer, the contrast in bright conditions will improve dramatically. The end result is that you will clearly see that your account balance is close to nil.

Polarized time?

What about Liquid Crystal Displays (LCD), such as your digital wristwatch? As a rule they don't use circular polarizers (in spite of claims in the advertisement of a well-known optical supplier): they need to reflect the outside light to be visible! However, LCD do use polarization; indeed they are based on its manipulation.

Because of the cover plate, from the outside the display can be linearly polarized, circularly polarized, depolarized or even change polarization from place to place. For example, my wristwatch becomes black with a polarizer aligned horizontally with respect to the digits. Once I performed a rapid check of the polarization axis of a low power laser using my wristwatch while shooting a hologram (Danger here: the faceplate will produce a mirror-like reflection that can blind you if the laser has more than a few mW). Sometimes, when the faceplate is a stressed plastic, colored fringes appear when seen through a polarizer (linear or circular).

There are several types of LCD.  This is a sketch of the most common display (twisted nematic type). Between two linear polarizers (with their transmission axis perpendicular) there is a liquid crystal. The liquid crystal is composed of elongated molecules parallel to the plane of the polarizers. In addition, the alignment of the molecules has a smooth twist of 90 degrees between the first polarizer and the second one. Linearly polarized light will follow the twist and rotate its vibration plane by 90 degrees. Thus, the second polarizer transmits the light that goes through the first polarizer.  A diffusive reflector makes some of the light retrace its path and exit the front: the pixel appears gray. However, when an electrical voltage is applied between the two plates, the molecules tend to align parallel to the field (perpendicular to the plates), loosing the twist. Under those conditions the light does not rotate and  is absorbed by the second polarizer. The pixel becomes black. Although conceptually it has similarities with the previously described circular polarizer application, here light remains linearly polarized and only rotates very slowly with respect to its wavelength.