How do polarizing filters control light in a twisted-nematic LCD?
In nematic arrangements the crystals are randomly positioned with all their long axes pointing in essentially the same direction, given by a vector known as the director. In smectic arrangements the crystals are further constrained to parallel planes; in smectic A the director is perpendicular to these planes, while in smectic C the director is at some other angle to the planes.
In a twisted-nematic (TN) liquid crystal display, light control is achieved by sandwiching a layer of liquid crystal between two perpendicular polarizing filters.12
When no voltage is applied, the liquid crystal molecules adopt a 90° twisted configuration, matching the orientation of the crossed polarizers. As light passes through the first polarizer, the twisted liquid crystal layer rotates the light's polarization by 90°. This rotation aligns the light with the axis of the second polarizer, allowing it to pass through and making the cell appear transparent.132
When an electric field is applied, the liquid crystal molecules reorient to align with the field, effectively destroying the twisted state. In this state, the liquid crystal no longer rotates the polarization of the incoming light. Consequently, the light remains blocked by the second, perpendicular polarizer, causing the cell to appear dark. By selectively applying this voltage to specific pixels, the display creates images through a pattern of light and dark areas.132