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The Polaroid spectra film - The original Polaroid is the name of a type of synthetic plastic sheet which is used to polarise light.
The material 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.
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This material, known as J-sheet, was later replaced by the improved H-sheet Polaroid. Another type of Polaroid is the K-sheet polariser. 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.
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