UV Absorbing Filters in Photography

UV Absorbing Filters in Photography


Ultraviolet (or UV) radiation is that portion of the electromagnetic spectrum abutting the visible blue wavelengths, that extends well beyond the limits of human vision. Although most of the UV range is absorbed by glass, those longest wavelengths next to the shortest visible blue are transmitted by glass lenses and are recorded by photographic films. Thus, the near-UV, as it is called, grades imperceptibly into the visible blue in photography, with an area of ambiguity present.


The near-UV is of particular concern to outdoor photographers because sunlight is rich in ultraviolet. Although the human eye cannot see it, color films record the near-UV either as unwanted additional blue or as an overlay of magenta, a purplish red. (Black-and-white films can record well into the UV, but it is seldom of use or concern to any but scientific photographers.) Filters designed to drop out ultraviolet have several uses in outdoor photography.


High-Altitude Photography


At low altitude excess ultraviolet is absorbed by the atmosphere, but in the thinner air of the mountains much more gets through. At altitudes commencing as low as 7000 to 8000 feet, excess ultraviolet radiation may become visible in color photographs as a magenta tint on some types of neutral gray rocks. An ultraviolet absorbing filter such as a Wratten 1A, or in more extreme cases a 2C, filter will absorb the ultraviolet that is causing the problem. The 1A is the familiar Skylight filter.


Some high altitude distance haze is caused by the atmospheric scattering of near ultraviolet radiation. While it is not visible to the eye, it is registered by the film as an additional and usually undesirable blue haze. With color films, the 1A is normal correction and the 2C may also be appropriate. In black and white work, any yellow filter absorbs ultraviolet along with the visible blue.


Color Effects


Color photography in open shade under a blue sky, or on lightly clouded days, results in an unpleasant blue color cast in the pictures, because the light has an excessively high color temperature. The ultraviolet absorbing Wratten 1A Skylight filter will correct this effect as well as take out any UV that may be present.

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