Coating.
1. A thin layer of antireflective material, often magnesium fluoride, applied to the surface of a lens, filter, viewfinder, etc.
The technology was pioneered by English lensmaker H. Dennis Taylor (1862-1943) of Cooke triplet fame, who patented an acid coating process in 1904. However it was not until 1935 that lens coating would become a viable process. In that year Alexander (Oleksander) Smakula (1900-1983), working for German manufacturer Zeiss, devised a method of evaporating either magnesium or calcium fluoride onto glass lenses and used the technology for binoculars.
Most modern cameras have coated lenses, since the coatings result in greater resistance to lens flare. Lenses and filters can be single-coated or multicoated, though multicoating processes are the norm these days for most lenses.
Coated lenses are easy to distinguish from non-coated lenses. It’s the lens coating which gives the distinctive coloured sheen to modern lenses - usually greenish or red-purplish. Coatings unfortunately must be kept scrupulously clean, as skin oils and so on result in shimmering patches on the lens surface.
Ideally each glass surface exposed to air should be coated, since glass-air surfaces typically reflect about 4% of incoming light, and coatings can cut this back considerably - often to below 1%. This is good because stray reflected light bounces around inside the lens and camera, causing ghost images and reducing image contrast.
Coatings work by introducing optical interference. The thickness of each coating is carefully calculated so that light beams reflected by the top part and lower part of the coating layer are 180° out of phase. The two beams of light then cancel each other out through destructive interference and thus do not reflect back.
cf. light interference, flare, multicoating, SMC, SSC, vacuum deposition.
2. A coating applied to paper stock. See coated stock.
Entry last updated 2002-05-06. Term 230 of 1487.
Previous term: coated stock.
Next term: coherent light.