Polarizer

A polarizer is a filter that passes only light waves vibrating in one direction and absorbs the rest. An LCD needs two, one on the back of the panel and one on the front, set at right angles to each other.

The heart of it is a film of polyvinyl alcohol. The film is soaked in iodine and stretched to about five or six times its length. Stretching lines up the long polymer chains, and the iodine forms chains of its own along them. Those iodine chains absorb light vibrating along their length and let through light vibrating across it. Boric acid locks the structure in place. Because the dyed film is fragile and sensitive to water, it's sandwiched between two clear films of cellulose triacetate. A layer of acrylate resin adhesive lets it be pressed onto the glass.

In the LCD panel, the rear polarizer passes light of one orientation. The liquid crystal turns that orientation by an amount set by each subpixel's voltage, and the front polarizer passes only the part that lines up with it. The price is light: the rear one alone absorbs more than half the backlight. Edwin Land's company Polaroid introduced this iodine-PVA type in 1938.