| Polaroid is the name of a type of synthetic plastic sheet which is used to polarise light.
The original material, patented in 1929 (U.S. Patent 1918848 ) and further developed in 1932 by Edwin H. Land, consists of many microscopic crystals of iodoquinine sulphate (herapathite) embedded in a transparent nitrocellulose polymer film. The needle-like crystals are aligned during manufacture of the film by stretching or by applying electric or magnetic fields. With the crystals aligned, the sheet is dichroic: it tends to absorb light which is polarised perpendicularly to the direction of the crystal alignment, but transmits light which is polarised parallel to it. This allows the material to be used as a light polariser.
This material, known as J-sheet, was later replaced by the improved H-sheet Polaroid, invented in 1938 by Land. H-sheet is a polyvinyl alcohol (PVA) polymer impregnated with iodine. During manufacture, the PVA polymer chains are stretched such that they form an array of aligned, linear molecules in the material. The iodine dopant attaches to the PVA molecules and makes them conducting along the length of the chains. Light polarised parallel to the chains is absorbed, and light polarised perpendicular to the chains is transmitted.
Another type of Polaroid is the K-sheet polariser, which consists of aligned polyvinylene chains. This polariser material is particularly resistant to humidity and heat.
Polaroid sheets are used in liquid crystal displays, optical microscopes and sunglasses.
Polaroid is also used as a trade name for a variety of products sold by licensees of the Polaroid Corporation, including consumer electronics, sunglasses based on Polaroid polarisers, and instant-print photographic film and cameras. | 
