Why do most LCDs need a backlight?
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.
Most liquid crystal displays (LCDs) require a backlight because they do not emit their own light. Instead, an LCD functions as a light-modulating device that uses liquid crystals to control the passage of light through the display.123
In a standard LCD, the liquid crystal layer acts as an electro-optical shutter placed between two polarizers. By applying an electric field, the orientation of the liquid crystal molecules changes, which determines whether light from an external source can pass through the assembly or be blocked. Because the display relies on manipulating this external light to create images, a backlight—typically a fluorescent tube or similar illumination source—is necessary to provide the light that the pixels then modulate.341
While some specialized "reflective displays" use ambient light or mirrors to eliminate the need for a backlight, most conventional LCDs require one to ensure visibility and image quality.4