Polarization of Light

How do LCD devices use polarized light?

LCD stands for Liquid Crystal Display. A liquid crystal is composed of long, thin molecules that are free to move like a liquid but organize themselves in a regular array like a crystal. In an LCD display the liquid crystal material is in a thin layer between two glass sheets. The bottom sheet is rubbed in one direction so that the molecules in the liquid crystal touching the surface align themselves with the rubbing. The top sheet is rubbed in the perpendicular direction, aligning the molecules touching it in that direction. As a result, over the thickness of the liquid crystal material the direction of the long axis of the molecules rotate through 90°. Polarizers are placed on the outsides of the glass sheets in the same direction as the rubbing. When light enters the back of the display it is polarized. As it passes through the crystal its polarization direction is rotated through 90° so that it passes through the second polarizer. Thus light passes through the display; it appears bright.

Each glass sheet is coated with a thin, electrically conductive layer. If a voltage is placed across the sheets the molecules align themselves with the electric field. The molecules no longer rotate the direction of polarization of the light, so no light passes through the display. It appears dark. By varying the voltage different degrees of darkness can be produced.

The entire display is composed of tiny pixels, each connected to a source of the control voltage. Thus each pixel can be switched between light and dark. Color filters can be placed over each pixel to produce a full color display. In a 1,080 pixel high-definition television display there are 1,920 pixels in the horizontal direction and 1,080 in the vertical, for a total of 2,073,600 pixels.

An LCD television screen uses liquid crystals composed of long, thin molecules between two glass sheets. An electrical field rotates the molecules in the prixels so that polarized light either passes through or is blocked. Color filters are added to the pixels to achieve color.

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