When we talk about Polarization in Lasers we mean placement of the crystal at Brewster angle to block the s-polarization and only the p-polarization will survive. How do we compare the term polarization in Lasers and in liquid crystal displays? So, for example, we know there are polarizers present in the case of LCD. Does that mean we are also using these polarizers to block the back-light s-polarization and want the p-polarization at the output?


Polarization has the same meaning in both cases, as a general optical phenomenon, but it is exploited differently in the two situations.

In lasers the Brewster window (or some other polarizing optic) is used to create losses in the feedback path for one of the polarizations. This ensures that only the other polarization can lase and, therefore, that the output of the laser has a well defined polarization.

In liquid crystal displays, restricting our discussion to single color LCDs such as can be found in alarm clocks and wrist watches, crossed polarizers are used to prevent any light from being transmitted when the liquid crystals have no voltage applied across them. These crossed polarizers are represented as layers 1 and 5 in the image below. In between the two polarizers is a thin layer of liquid crystals (layer 3) sandwiched between two transparent electrodes (layers 2 and 4). The electrodes have the pattern to be displayed etched into them such as the numbers shown on layer 2, or, in the case of a television, individual pixels. When a voltage is applied across the liquid crystals they have the property of rotating the polarization of the light passing through, this allows the polarized light transmitted through layer 5, which would initially be blocked by layer 1 to be transmitted. Finally, layer 6 is either a reflective surface, in the case of a wristwatch, or a backlight, in the case of an alarm clock or TV. For more information on the workings of liquid crystal displays check out the wikipedia article.

Wikimedia Commons illustration of the internals of an LCD screen


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